unit 3 thermochemistry specific curriculum outcomes

Unit 3

THERMOCHEMISTRY

Specific Curriculum Outcomes

Suggested Time: 30 Hours

CHEMISTRY 3202 CURRICULUM: GUIDE90

THERMOCHEMISTRY

Thermochemistry

Introduction Energy is the essence of our existence as individuals and as a society. Anabundance of fossil fuels has led to a world-wide appetite for energy. Thereare pros and cons to using fossil fuels. The relationship between energy andchemistry needs to be explored to help us find alternative fuels.Thermochemistry includes energy changes that occur with physical andchemical processes. The study of energy production and the application ofchemical change related to practical situations has helped society to progress.

Focus and Context Thermochemistry focuses on energy in various systems. Skills involvingplanning, recording, analyzing, and evaluating energy changes will bedeveloped. Fuels for energy provide the context for student research andprojects. These fuels could include energy for industry, energy from foods, orany other relevant context. This unit will help students to develop an interestin global energy issues and to appreciate the idea of possible solutions to aproblem. Doing lab work and performing calculations allow students todiscuss their evidence and problem-solving in order to consolidate theirunderstanding of energy changes.

Science

Curriculum Links

Science 1206 included balancing chemical equations. Heat and temperaturewere discussed in the weather unit. Chemistry 2202 outcomes useful for thisunit include measuring amounts of moles as well as the energytransformations associated with bond breaking and forming.

CHEMISTRY 3202 CURRICULUM: GUIDE 91

THERMOCHEMISTRY

STSE Skills Knowledge

Students will be expected to Students will be expected to Students will be expected to

Curriculum Outcomes

Nature of Science and Technology

114-5 describe the importance of peerreview in the development of scientificknowledge

Relationships Between

Science and Technology

116-4 analyse and describe exampleswhere technologies were developedbased on scientific understanding

Social and

Environmental Contexts of Science

and Technology

117-6 analyse why scientific andtechnological activities take place in avariety of individual and group settings

117-7 identify and describe science- andtechnology-based careers related to thescience they are studying

117-9 analyse the knowledge and skillsacquired in their study of science toidentify areas of further study related toscience and technology

118-2 analyse from a variety ofperspectives the risks and benefits tosociety and the environment of applyingscientific knowledge or introducing aparticular technology

118-8 distinguish between questionsthat can be answered by science andthose that cannot, and betweenproblems that can be solved bytechnology and those that cannot

118-10 propose courses of action onsocial issues related to science andtechnology, taking into account an arrayof perspectives, including that ofsustainability

Initiating and Planning

212-3 design an experimentidentifying and controlling majorvariables

212-8 evaluate and select appropriateinstruments for collecting evidence andappropriate processes for problemsolving, inquiring, and decisionmaking

Performing and Recording

213-6 use library and electronic researchtools to collect information on a giventopic

213-7 select and integrate informationfrom various print and electronic sourcesor from several parts of the same source

213-8 select and use apparatus andmaterials safely

Analysing and Interpreting

214-3 compile and display evidenceand information, by hand orcomputer, in a variety of formats,including diagrams, flow charts, tables,graphs, and scatter plots

214-6 apply and assess methods ofpredicting heats of reaction

214-15 propose alternative solutionsto a given practical problem, identifythe potential strengths and weaknessesof each, and select one as the basis for aplan

Communication and Teamwork

215-4 identify multiple perspectivesthat influence a science-relateddecision or issue

215-6 work cooperatively with teammembers to develop and carry out a plan,and troubleshoot problems as theyarise

308-2 explain temperature using theconcept of kinetic energy and theparticle model of matter

324-1 write and balance chemicalequations for combustion reactions ofalkanes

324-2 define endothermic reaction,exothermic reaction, specific heat,enthalpy, bond energy, heat ofreaction, and molar enthalpy

324-3 calculate and compare theenergy involved in changes of state andthat in chemical reactions

324-4 calculate the changes in energyof various chemical reactions usingbond energy, heats of formation andHess’s law

324-5 illustrate changes in energy ofvarious chemical reactions, usingpotential energy diagrams

324-6 determine experimentally thechanges in energy of various chemicalreactions

324-7 compare the molar enthalpies ofseveral combustion reactions involvingorganic compounds

ACC-8 define, calculate and comparefuel values

92 CHEMISTRY 3202 CURRICULUM: GUIDE

Outcomes

THERMOCHEMISTRY

Elaborations—Strategies for Learning and Teaching

Students will be expected to

Temperature and Kinetic Energy

• explain temperature and heatusing the concept of kineticenergy and the particle modelof matter (308-2)

– define temperature as ameasure of the averagekinetic energy of theparticles of the system

– identify and describe thechanges to particlemovement in systems inwhich the energy change isaccompanied by a change intemperature of the system

– describe heat as a transfer ofkinetic energy from a systemof higher temperature(higher average kineticenergy) to a system of lowertemperature (lower averagekinetic energy) when each isin thermal contact with theother

• calculate and compare theenergy involved in changes oftemperature (324-3)

– define the terms: joule, heatcapacity and specific heatcapacity

After defining temperature, the teacher could use temperature differences inthe classroom to illustrate differences in particle movement and heat. Forexample, ask students to place a hand on their desk. The desk will feel cool.What’s happening? The temperature of the hand is higher than thetemperature of the desk, therefore the kinetic energy (E

K or KE) (particle

movement) in the hand is higher than the EK (particle movement) in the

desk. The result is a transfer of EK from the particles in the hand to particles

in the desk as collisions occur in the interface. This is heat transfer fromthe hand to the desk.

Questioning could be used to lead students to the above explanation.

Whenever a temperature change occurs, the change in EK involved can be

determined from experimentally obtained data or from data provided, usingthe First Law of Thermodynamics.

93CHEMISTRY 3202 CURRICULUM: GUIDE

Tasks for Instruction and/or Assessment Resources/Notes

THERMOCHEMISTRY

Temperature and Kinetic Energy

Paper and Pencil

• Students could distinguish between temperature and heat byrelating these to the energy of the atoms and molecules. (324-3)

Journal

• In their science log, students could respond to the followingquestions:

– Explain why, on a hot, summer day at the beach, the sandcan be unbearably hot on bare feet yet the water is very cold.

– What happens when direct sunlight is blocked by a cloud?How does this affect the temperature of the sand versus thatof the water ? (308-2, 117-7, 324-7)

• Students could be told they can use a balance to find the mass of theirsubstance only when it is at room temperature. They should explainthis statement. (116-4, 308-2)

• Students could be asked, “Why is it helpful to fill your thermosbottle with hot water before filling it with a hot beverage?” (114-5,215-4, 308-2)

• A Scottish chemist, Joseph Black (1728–1799), differentiatedbetween temperature and thermal energy. Students could discuss howthese are different. They should give an example of an experiment toshow when two objects at the same temperature do not necessarilyhave the same thermal energy. (308-2, 324-2, 212-3)

www.gov.nl.ca/edu/science_ref/main.htm

MGH Chemistry, pp. 628-629


MGH Chemistry, pp. 628, 632,pp. 636-637


Outcomes

THERMOCHEMISTRY



Calculating Heat

• calculate and compare theenergy involved in changes oftemperature (324-3) (Cont’d)

– identify that the amount ofheat lost or gained by anobject is dependent upon;type of material, change intemperature of material, andmass of material

– perform calculationsinvolving heat capacity, C,and specific heat capacity, c

– distinguish between asystem and surroundingsand the relationship ofeach to the universe

– define open, closed andisolated systems

– identify and explain thatenergy can be exchangedbetween a system and itssurroundings through heatlost or gained by a system

( system surroundingsq q= − )

– state the First Law ofThermodynamics andapply it to determine theamount of heat an objectcontains

Students need to understand that the change in temperature depends on thetype of material and the mass which should be illustrated through theconcepts of heat capacity (C) and specific heat (c). For example; you wishto boil water for a cup of tea. Why would it be faster to place 250 mL ofwater in the kettle rather than 2000 mL of water? This reinforces theconcept of the importance of mass to energy lost or gained.

For example, it requires more energy to raise the temperature of a 1 kgaluminium frying pan by 200oC than it does to raise the temperature of a1 kg iron frying pan 200oC. Why? This reinforces the concept of theimportance of the type of matter to energy lost or gained. This leads intothe explanation of specific heat and related calculations.

For example, a 20.0 g piece of iron has a heat capacity of 8.88 J/oC.Calculate the specific heat of iron. For example: The specific heat ofaluminium is 0.890 J/goC. Calculate the heat capacity of a 500 galuminium saucepan. Refer to the table of specific heat values forcomparison. The previous calculation can quickly lead to calculation of

heat lost or gained using the formula q mc T= ∆ . Water and its specific

heat capacity can be used to introduce the above formula through simpleproblems.

Some sample problems could include;

(i) How much energy (q) is required to raise the temperature of 1.00 gof water 1.00 oC? Students will quickly answer 4.18 J because this is thevalue of the specific heat capacity of water.

(ii) How much energy (q) is required to raise the temperature of 2.00 gof water by 1.00 oC? Student answer: 8.36 J..... Why?

(iii) How much energy (q) is required to raise the temperature of 2.00 gof water by 2.00 oC? Student answer: 16.7 J..... Why?

(iv) How much energy (q) is required to raise the temperature of 511 gof water by 26.3 oC? Now students need a formula to calculate theexact answer. What is the formula?

Energy = (mass) x (specific heat capacity) x (change in temperature)

q mc T= ∆

Practise using this formula for calculating one of q, m, c, or T∆ given theother three variables. Teachers should note that math skills in rearrangingformulas may be weak for some students. For example, how much energy islost when of an aluminium mailbox, with a mass of 400.0 g, cools from26.2 oC to 12.4 oC ?



THERMOCHEMISTRY

Calculating Heat

Performance

• Students could calculate which has more heat: 250.0 mL of water at90°C or 20 000.0 mL of water at 30°C? They should qualitativelyand quantitatively explain their result. (324-3)

Paper and Pencil

• Heat lost equals heat gained. Students could explain thisassumption. (324-3)

Presentation

• For a selected reaction, students could make the case that the Lawof Conservation of Energy has been “upheld.” (324-5, 214-3)

• Students could investigate industrial applications of heat capacity.For example; refrigerator, household heating systems and cookwaredesign. (324-3)

MGH Chemistry, pp. 632-633,pp. 831-832




Outcomes

THERMOCHEMISTRY



Calculating Heat (continued)

• calculate and compare theenergy involved in changes oftemperature (324-3) (Cont’d)

– define the calorimeter andidentify it as the basicinstrument for measuringheat transfer

Once students understand the concepts of C, c, T∆ , and the calculation ofenergy (q) lost or gained, the idea that heat lost by a substance(s) is gainedby some other substance(s) must also be understood, since energy is alwaysconserved in all processes (First Law of Thermodynamics). This leads to theequation, q

system = -q

surroundings which can be applied to collected

experimental data and to problems that determine specific heat (c) or heatcapacity (C) of specific substances or objects. The experimental process isreferred to as calorimetry and the insulated container used in suchexperiments is a calorimeter.

For example, 10.0 g of metal X at 280.0 oC is dropped into 200.0 mL ofwater at 20.0 oC in a coffee cup calorimeter. Metal X and water reachthermal equilibrium (same temperature) at 25.0 oC. Calculate the specificheat capacity of X.

Solution:

( )

( ) ( )

25.0 280.0 255.0

25.0 20.0 5.0

(200.0 )(4.18 )(5.0 )( )( ) (10.0 )( 255.0 )

1.

o

system surroundings

x water

o o ox f i

o o owater f i

oJg Cwater

x ox

x

heat lost heat gained q qmc T mc T

T T T C C C

T T T C C C

g Cmc TCm T g C

C

= = −

∆ = − ∆

∆ = − = − = −

∆ = − = − = +

−− ∆= =∆ −

= 6 oJg C

Simple temperature versus time graphs are often useful in solving suchproblems.

T(oC)

280.0

20.0

25.0

Time

(mc)T)X

(mc)T)water

XWater

Students should perform problems involving solutions for any of the 6variables m,c, T∆ of X, or m, c, T∆ of water. Heat capacities can also beincorporated into such problems.

– calculate the heat gained orlost from a system using theformulas q mc T= ∆ orq C T= ∆ where c is thespecific heat capacity, C is

the heat capacity and T∆ isthe change in temperature



THERMOCHEMISTRY

Calculating Heat (continued)

Journal

• The calorimeter is a basic instrument for measuring heat transfer.Students could explain what a calorimeter is, how it measures heattransfer, and how it works. (324-3)

Paper and Pencil

• Students could explain what is meant by the following terms: specificheat, heat of reaction, and molar enthalpy. (324-2, 324-3)

• As students plan one of their labs for this unit, they should list theskills and knowledge needed to perform the lab properly. (324-3,117-9)

• Students could calculate the heat gained or lost from the followingsystem:

– A piece of metal having a mass of 100.0 g, originally at-30.0°C, was dropped in 300.0 g of water at 35°C. Thetemperature of the water went down to 32.0°C. Studentscould calculate the specific heat of that metal. (324-2,324-3)

• HCl(aq)

+ NaOH(aq)

→ NaCl(aq)

+ H2O

(l)

When 50.0 mL of 1.00 mol/L HCl(aq)

and 50.0 mL of 1.00 mol/LNaOH

(aq) are mixed in a Styrofoam cup calorimeter, the temperature

of the resulting solution increases from 21.0°C to 27.5°C. Studentscould calculate the heat of this reaction (measured in kilojoules permole of HCl

(aq)). (324-2, 324-3)


MGH Chemistry, pp. 664-665,p. 832


Outcomes

THERMOCHEMISTRY



Enthalpy Change

• define enthalpy, endothermicprocess, exothermic process,and molar enthalpy (324-2)

• calculate and compare theenergy involved in changes ofstate and that in chemicalreactions (324-3)

– use and interpret change inenthalpy ( H∆ ) notation forcommunicating energychanges

– identify and explain thatchemical changes andphase changes involvechanges in potentialenergy only

– explain that energy isrequired by a systemwhenever attractive forcesbetween particles arebroken and that energy isreleased from a systemwhenever new attractiveforces form betweenparticles (bond-breaking vs.bond-forming)

Enthalpy change ( H∆ ) is the energy lost or gained during a process(chemical change or phase change) at constant pressure. For comparisonpurposes H∆ values for specific processes are reported in units ofkJ/mol.

Students should write thermochemical equations to represent enthalpynotation. For example; combH∆ and fH∆ .

H∆ values for processes can be indicated by writing the H∆ valueimmediately after the equation ( H∆ notation) or by including it inequations which represent these processes.

For example:

H2 (g)

+ ½ O2

(g) → H

2O

(l)H∆ = -286 kJ/mol

The sign of H∆ is negative for exothermic processes (energy is aproduct in the reaction) and positive for endothermic processes (energyis a reactant in the reaction).

H2

(g) + ½ O

2

(g)→ H

2O

(l) + 286 kJ/mol

Enthalpy diagrams can also be used to represent H∆ of reactions. Herethe difference between potential energy (E

P or PE) of reactants and

products is related to H∆ .

H2(g) + ½O2(g)

H2O(l)

)H = -286 kJ/mol = PEproducts - PEreactants

Reaction Coordinate



THERMOCHEMISTRY

Enthalpy Change

Journal

• Students could answer, “What sources of potential energy have I usedduring this day?” They could make a chart. (214-3, 324-3)

Paper and Pencil

• Students could define a practical problem with an energy change, thenpropose a solution. (214-15, 324-3)

• Students could practise writing the different representations forthermochemical equations for both endothermic and exothermicreactions. (324-2)

Presentation

• Students could explain the difference in the energies of reactantsand products with reference to a given thermochemical equation.(324-2)


MGH Chemistry, pp. 641, 645, 648


Outcomes

THERMOCHEMISTRY



Enthalpy Change (continued)

• calculate and compare theenergy involved in changes ofstate and that in chemicalreactions (324-3) (Cont’d)

– define molar enthalpychange, H∆ , as thedifference between thepotential energy ofproducts and the potentialenergy of reactants forphase changes andchemical changes

It is very important for students to understand that during chemical andphase changes there is no change in temperature although temperaturechanges may be observed in the surroundings.

For example, a candle burns and releases energy equal to the change inpotential energy between the reactants (candle wax and oxygen) and theproducts (CO

2 and H

2O). This energy is observed as it warms up the

surrounding air and unburned candle wax (melted wax). This concept ismore easily understood in phase changes because, for example, everyoneis familiar with pure water freezing at 0 oC, and at 0 oC only (at 1 atm).

One way to introduce phase changes and this concept is to have students doa cooling curve of the pure substance, paradichlorobenzene, which freezes at53 oC. Place approximately 10 g of paradichlorobenzene and a thermometerin a small test tube and heat it in a water bath to 100 oC. Ask students toremove the test tube and record temperature every 20 seconds, while stirringcontinuously until the temperature reaches 40.0 oC. The temperature dropsquickly to 53 oC but then remains constant for about 15 minutes beforedropping quickly to 40.0 oC.

Teachers might begin by having students pose questions about energychanges in a system. Students should discuss heating, cooling and phasechanges in terms of forces between particles, particle movement, heatcontent and changes in potential energy. Teachers could identify theseareas if they are not mentioned by students. Changes to particlemovement in systems in terms of change in temperature could beintroduced. Changes in potential energy in matter could be discussed.

– define exothermic andendothermic with respect toheat exchange between thesystem and itssurroundings

– identify and explain thatenergy changes are observedduring phase changes andchemical changes whereforces of attractions betweenparticles are formed orbroken yet no change inthe temperature of thesystem occurs



THERMOCHEMISTRY

Enthalpy Change (continued)

Journal

• Students could analyze the following: “As a living person, my energyexchange position is exothermic.” Discuss this statement. (324-3,324-5, 214-3)

• Fire in a fireplace is started by lighting crumpled paper under logswith a match. Students could explain the energy transfers usingthe terms potential energy, kinetic energy, kindling temperature,system surroundings, endothermic, and exothermic. (324-3, 213-6,213-7)

Paper and Pencil

• Liquid water turns to ice. Students could be asked if thisendothermic or exothermic. They should explain their answer.(324-3)

Presentation

• Students could explain the energy changes in situations such as:

(i) when a frozen pond melts, the surrounding airtemperature drops

(ii) a layer of ice may form on a propane cylinder while it isbeing used

(iii) why vineyards in Ontario spray their crops with water if afrost warning is issued. (324-3)



Outcomes

THERMOCHEMISTRY



Thermochemistry and Potential Energy

• illustrate changes in energy ofvarious chemical reactions,using potential energydiagrams (324-5)

– draw and interpret potentialenergy diagrams based onexperimental data forchemical changes

– label enthalpy diagramsgiven either the H∆ for aprocess, or athermochemical equation

– identify exothermic andendothermic processesfrom the sign of H∆ , fromthermochemical equationsor from labelled enthalpydiagrams

– write thermochemicalequations including thequantity of energyexchanged given either thevalue of H∆ or a labelledenthalpy diagram

– explain that catalysts alterthe reaction mechanismwithout affecting H∆

Teachers could ask students for everyday examples involving endothermicand exothermic changes. Examples might include heating and freezing of ice,hot and cold packs, evaporation and condensation of water, and productionand decomposition of ammonia.

Students should use a properly labelled enthalpy diagram to describe energychanges in a multi-step process. This should help students understand thatan overall enthalpy change (of a multi-step process) is independent of howmany steps, or the magnitude of individual steps. This is the fundamentalbasis of Hess’s Law.



THERMOCHEMISTRY

Thermochemistry and Potential Energy

Paper and Pencil

• Teachers could ask students which would cause a more severe burn:100 g of water at 100°C or 100 g of steam at 100°C? They shouldgive reasons to support their decision. (118-8)

• Students could draw and label a potential energy (enthalpy) diagramfor each of the following:

– exothermic– endothermic– catalyst added to the exothermic/endothermic reaction

(324-5, 214-3)

• Students could begin by looking at various potential energy diagramsshowing heat content of products and of reactants. Students shoulddraw and interpret the potential energy graphs and be able todetermine whether the reaction is exothermic or endothermic. Theyshould identify the reactants and products, and determine theamount of energy involved. They should also draw and correctly labelthe axis and write an interpretation. (324-5, 214-3)


MGH Chemistry, p. 642


Outcomes

THERMOCHEMISTRY



• compare the molar enthalpiesof several combustion reactionsinvolving organic compounds(324-7)

• write and balancethermochemical equationsincluding the combustionreaction of alkanes (324-1)

– write thermochemicalequations to represent theprocess given enthalpynotation for combH∆ , fH∆ ,

fusH∆ , vapH∆ , lnsoH∆

– calculate the heat gained orlost from a system using theformula q n H= ∆ whenH∆ is the molar heat of a

phase change or chemicalreaction

Thermochemical Equations

Students should balance complete hydrocarbon combustion reactions usingup to ten carbon atoms. Equations, including molar enthalpies of possiblecombustion reactions, could be identified. As students become familiar withenergy and thermochemistry throughout this unit, students could add totheir project information. The complete combustion of hydrocarbonsproduces carbon dioxide, CO

2, and water, H

2O. Showing students the

molar enthalpies will help them realize the importance of energy.

CH4(g) + 2O

2(g) → CO2(g) + 2H

2O

(g) + energy (heat)

CH4(g)

+ 2O2(g)

→ CO2(g)

+ 2H2O

(g) + 803kJ

2C3H

6(g) + 9O

2(g) → 6CO

2(g) + 6H

2O

(g) + 4119kJ

54.06 6.03 18.0918.02

kJmolg

mol

mass gq n H x H x kJM

= ∆ = ∆ = =

Since chemical changes and phase changes involve no change in temperaturefor the system, the formula q mc T= ∆ cannot be used to calculate energy

lost or gained because T∆ = 0. Because values of H∆ are in units ofkJ/mol, the formula used to calculate energy lost or gained is q n H= ∆ .(n = number of moles of substance undergoing the chemical or phasechange). It is important that students practise using this formula here. e.g.,Calculate the energy required to melt 54.06 g of ice.

fusH∆ = H2O

(s) = 6.03kJ/mol

Solution:

H∆ values can be determined experimentally using calorimetry similar tothe method and calculations used previously to determine specific heat. Theenergy lost or gained by the chemical reaction or phase change (q

system) is

now calculated using the formula systemq n H= ∆ . This energy normallywarms or cools some substance in the calorimeter, usually water or thecalorimeter and its contents. See CORE LAB #6.

– calculate the H∆ for asystem, given the mass of areactant (or vice versa)



THERMOCHEMISTRY

Thermochemical Equations

Paper and Pencil

• Students could write a balanced chemical equation for thecombustion reaction of each of these alkanes: methane, ethane,propane, butane, and octane. (324-1)

• Students could look up the molar enthalpies of the combustion ofbutane and octane. They should determine what they have incommon. (324-7)

• Students could determine the amount of energy given off fromthe combustion of 26.0 g of methane. (324-1)



MGH Chemistry, pp. 644, 648



Outcomes

THERMOCHEMISTRY



• write and balancethermochemical equationsincluding the combustionreaction of alkanes (324-1)(Cont’d)

– perform calculations whichapply the First Law ofThermodynamics indetermining the heat of areaction (or phase change)given experimental data forchanges to thesurroundings

system surroundingsq q= −

– perform calculations basedon data gathered fromcalorimetry experiments(bomb calorimetry and/orsimple/insulated cupcalorimetry) demonstratingenergy changes in chemicaland phase changes

• perform an experimentidentifying and controllingmajor variables (212-3)

• evaluate instruments forcollecting data (212-8)

• apply and assess methods ofpredicting heats of reaction(214-6)

Students should practise with problem-solving using q mc T= ∆ ,q n H= ∆ , and system surroundingsq q= − . These calculations may beproblematic for some students. Teachers could provide many examplesto help overcome this.

For example, 20.0g of NaOH (s) is dropped into 200 mL of H2O at

22.1 oC. The final temperature of the solution is 43.9 oC. CalculateH∆ for the dissolving of NaOH. Note: It is important for students to

understand that both the NaOH (s) and the 200.0 mL of H2O are initially

at 22.1 oC. Also, the solution (NaOH and H2O) is the substance warmed

by the energy released in the reaction. Therefore, for q mc T= ∆ ,

m = 220.0 g not 200.0 g. Since water makes up the vast majority of thefinal solution, the specific heat capacity of liquid water (4.184 J/g oC) can beused in the calculation.

Another possible example, 0.852 g of glucose, C6H

12O

6,

( combH∆ = -2808 kJ/mol) is burned in a bomb calorimeter. The

temperature of the calorimeter rises from 22.06 oC to 24.29 oC.Calculate the heat capacity (C), of the calorimeter.

Students should recognize that calculation of qsurroundings

may involvemore than one process. For example; a liquid heats up and evaporates,or a liquid and a container undergo a heat change (see question 6a ofCore Lab #5).

The Laboratory outcomes 212-3, 212-8, 214-6 and, in part, 324-1 areaddressed by completing The Heat of Combustion of a Candle, CORELAB #5.

Thermochemical Equations (continued)



THERMOCHEMISTRY

Thermochemical Equations (continued)

Performance

• With a partner, students could design a lab to calculate the molarheat of solution of NH

4Cl and of NaOH. They should include

safety issues that should be addressed. After their plan isapproved, they should carry out their procedure and collectevidence (data) and report their findings. (212-3, 215-6, 212-8,324-2, 324-3)

• Students could select and use appropriate equipment to make aninexpensive hand warmer. (Hint: Use these substances: powderediron; H

2O; NaCl; and vermiculite.) They should design their

experiment and consider safety precautions. If approval isobtained, they could do this experiment. A teacher or student couldtest the results. (212-3, 215-6, 212-8, 324-2)

Paper and Pencil

• Students could calculate the mass of iron at 210.2 oC that is placed inan insulated cup containing 155 mL of water at 25.8 oC. After ashort period of time it is observed that no water remains just as theiron has cooled to 100.0 oC. (specific heat capacity of iron is0.444 J/g . oC). (324-1)


Core Lab #5: “The Heat ofCombustion of a Candle”,pp. 671-672


Outcomes

THERMOCHEMISTRY



• compile, display and interpretevidence and information in agraphical format (214-3)

– draw and interpretheating/cooling curves forphase changes

Heating and Cooling Curves

Students should be able to construct a heating or cooling curve for aparticular substance given the appropriate data (melting point and boilingpoint). In addition, they must be able to calculate the total energy involvedin the process. For example; How much energy is needed to raise thetemperature of 50.0 g of H

2O from -20.0 oC to 140.0 oC?

– calculate the total heat for amulti-step process thatincludes a temperature andphase change

Re-emphasize here that changes of temperature always involve a change inE

K whereas changes of state always involve a change in E

P . Changes in

E

P are

often more difficult for students to understand than changes in EK. One

way to demonstrate this change in EP

is to make a saturated solution ofsodium thiosulfate (10 mL); let it cool to room temperature to form asupersaturated solution; then add a crystal of sodium thiosulfate. Theresulting rapid crystallization of the excess sodium thiosulfate releases a largeamount of energy (an obvious conversion of potential energy to kineticenergy). Ask students to feel the test tube before and after. Another way toemphasize this concept is to generate discussion around issues such as thefollowing:

1. Why does steam at 100oC give you a much more severe burn thanwater at 100oC ?

2. If you are lost in the woods in winter and you are thirsty, why should youdrink water from a stream rather than eating snow or ice to help preventhypothermia ?

In solving these problems, the energy units involving temperaturechange (mc T∆ ) are generally in joules whereas the units for phasechange ( n H∆ ) are generally in kilojoules. Students often forget to convertto one common unit before adding to get the total energy.

– use heating and coolingcurves to represent andexplain changes inpotential energy andkinetic energy of a system



THERMOCHEMISTRY

Heating and Cooling Curves

Paper and Pencil

• Linda wants to know how much heat energy is released when25 kg of steam at 100 °C is cooled to 25 kg of ice at –15 °C.Students could calculate the total heat energy released. (214-3)

• Students could describe, in terms of kinetic and potential energy, apiece of aluminum cooling from 900 °C to room temperature.Students could then identify the values required to calculate theheat released when this occurs. (214-3)




Outcomes

THERMOCHEMISTRY



• analyze the knowledge andskills acquired in their studyof thermochemistry toidentify areas of further study(117-9)

– identify and describe sourcesof energy includingpresent sources andpossible new ones

– compare physical,chemical, and nuclearchanges in terms of thespecies and the magnitudeof energy involved

Students should explore practical situations involving heat and energytransfer, such as a fire in a fireplace, solar collectors, eating food to fuel yourbody, or photosynthesis.

Science Decisions Involving Thermochemistry

One point to emphasize is the difference between the specific heats ofdifferent phases of the same substance. Also, explain in terms of forces ofattraction as to why vapH∆ is always larger than fusH∆ .

Students should compare physical, chemical, and nuclear changes in terms ofthe species and the magnitude of energy changes involved. This works wellwith estimation.

It is enough to indicate that phase change usually involves 10’s of kJ/mol,chemical changes involve 100’s or 1000’s of kJ/mol, and nuclear changesinvolve millions and billions of kJ/mol.



THERMOCHEMISTRY

Science Decisions Involving Thermochemistry

Presentation

• Students could present their project using multimedia,audiovisual, or other suitable format. They should present theirscenario to the class. The following questions might be helpfulwith their thinking:

– What are the characteristics of a good chemical fuel?

– What makes your fuel a good choice?

– What is the most common method of producing your fuel?

– What are some advantages of your fuel? Some disadvantages?

– Outline the process of your fuel’s development.

– What energy efficiency does your fuel have according to industry?

– What impact will the fuel have on the local environment?(117-6, 118-2, 118-8, 118-10, 324-7, 117-9)

Paper and Pencil

• Students could select one of the following and collect and organizeinformation about it. They should describe the science needed tocommercially develop the energy source. Possible topics: coal,petroleum, natural gas, sun biomass, synthetic fuels, nuclearhydrogen, seed oil, methanol, geothermal (heat pumps), oil shale.(213-6, 213-7, 117-9)

MGH Chemistry, pp. 692, 694,695, 699, 701



Outcomes

THERMOCHEMISTRY



Science Decisions Involving Thermochemistry (continued)

• define, calculate and comparefuel values (ACC-8)

• propose and analyze solutionsto solving energy problemsusing the concepts of fuelvalue (214-15)

• analyze examples wheretechnologies were developedbased on thermochemistry,using the idea of the choice offuel in a device (116-4)

• identify perspectives thatinfluence a decision involvingfuels used in devices, and, thechoices made in the Caloriecontent and serving size of thefoods we eat (215-4)

• analyze why scientific andtechnological activities takeplace when making choices formaintaining a healthy lifestyle(117-6)

• analyze the risks and benefits tosociety and the environment inrelation to fossil fuel use andhealthy lifestyle choices(118-2)

• analyze the knowledgeacquired in their study ofthermochemistry to identifyareas of further study,specifically chemicalengineering and nutrition(117-9)

The CORE STSE component of this unit incorporates a broad range ofChemistry 3202 outcomes. More specifically, it targets (in whole or in part)ACC-8, 214-15, 116-4, 215-4, 117-6, 118-2, and 117-9. The STSEcomponent, What Fuels You?, can be found in Appendix A.



THERMOCHEMISTRY

Science Decisions Involving Thermochemistry (continued)

Paper and Pencil

• Students could prepare a newspaper article about an energy sourceand its potential for energy production. They should includeinformation in their article about their energy source, such as itsuseable lifetime as a commercial source, impact on theenvironment, appeal to an individual consumer, and/or appeal toa community of consumers. (114-5, 213-6, 213-7, 215-4)

• Students could consider climatic, economic, and supply factors intheir search for an energy source for the future. They should includethese in the research project that they began at the beginning of thisunit. (114-5, 213-6, 213-7, 215-4)

Core STSE #3: “What Fuels You?”,Appendix A


Outcomes

THERMOCHEMISTRY



Hess’s Law

• calculate the changes in energyof various chemical reactionsusing Hess’s Law (324-4)

– identify, with reference toHess’s law, that the internalenergy and enthalpy of asystem are state functionsand that changes in eachare independent of thereaction path chosen

– use the method of additionof chemical equations andcorresponding enthalpychanges to compute theenthalpy change of theoverall process

• calculate the changes inenergy of various chemicalreactions using heats offormation (324-4)

– use a standard molarenthalpies of formation tableto calculate heat of reactionfor a chemical change

• interpret information on heatsof formation from diagrams,flow charts, tables, andgraphs (214-3)

– recognize a relationship

between the ofH∆ of a

compound and its stability

Teachers should point out that Hess’s law is a method of determining H∆for reactions that are too difficult, too expensive, too slow, or too dangerousto measure H∆ experimentally (e.g., using calorimetry).

An analogy is useful here to illustrate that Hess’s Law is based on theconcept that the energy involved in a chemical reaction is independent of thepath taken to change reactants into products. An example might be tosuggest the whole class meet at a particular spot later in the day. Eachperson may take a different route but the outcome is the same.

Student practice is required here to master the use of Hess’ Law. Problemsused should not include more than 3 equations to combine. The CORELAB could be done at this point to confirm the validity of Hess’s Law.

Teachers should emphasize that the use of ofH∆ values to find ∆ o

rxnH of

reaction is yet another method of determining enthalpies values. The

process of summation of ofH∆ values is an extension of Hess’s Law without

having to combine the equations. Students must practice using the

equation: ( ) (products) (reactants)∆ = ∆ − ∆∑ ∑o of fH rxn n H n H and

tables of ofH∆ values.

There is a relationship between the size of ofH∆ of a compound and its

stability. The more negative the value of ofH∆ (more exothermic) the more

stable the compound formed. Trends also exist in homologous series oforganic compounds which may be represented in tables or graphs.

For example; alkanes

4 2 6

3 8 4 10

( ) 75 ( ) 85

( ) 104 ( ) 126

o okJ kJmol molf f

o okJ kJmol molf f

H CH H C H

H C H H C H

∆ = − ∆ = −

∆ = − ∆ = −

Sample question: Given values of ofH∆ for CH

4, C

3H

8, C

4H

10, predict a

possible value for C2H

6.



THERMOCHEMISTRY

Hess’s Law

Performance

• Students could demonstrate correct use of chemical disposal whiledoing a Hess’s Law lab. (324-4, 214-6, 116-4)

Paper and Pencil

• Given the following reactions and H∆ values:

2NH3(g)

+ 3N2O

(g) → 4N

2(g) + 3H

2O

(l) ∆H = –1012kJ

2N2O

(g) → 2N

2(g) + O

2(g) ∆H =–164kJ

Students could calculate H∆ , using Hess’s Law for

4NH3(g)

+ 3O2(g)

→ 2N2(g)

+ 6H2O

(l) (324-4)

• From their knowledge of standard states and from an enthalpy offormation chart, students could list the standard enthalpy offormation of each of the following substances: (324-4)

a) Cl2(g)

c) C3H

6(g)e) H

2O

(g)

b) H2O

(l)d) Na

(s)f ) P

4(g)

• Students could calculate H∆ for the following reaction:

2C3H

6(g) +9O

2(g) → 6CO2(g)

+ 6H2O

(l) (324-4)

• Students could calculate the heat of formation, ∆ fH , of NO2.

N2(g)

+ 2O2(g)

→ 2NO2(g)

H∆ =+16.2kJ (324-4)

• Hydrazine, N2H

4(g), is used as a fuel in liquid-fuelled rockets. It

can react with O2(g)

or N2O

4(g) both producing N

2(g) and H

2O

(g).

Students could write balanced chemical equations for the tworeactions. They should calculate H∆ for each reaction, usinginformation from an enthalpy table and compare the values. Studentscould then determine which is the more efficient rocket fuel. (324-4)






Outcomes

THERMOCHEMISTRY



Determining Enthalpy Change

• determine experimentally thechanges in energy of variouschemical reactions (324-6)

• apply Hess’s Law and/or Heatsof Formation methods ofpredicting heats of reactions toyour experimentally determinedlab values (214-6)

• select and use apparatus andmaterials safely (213-8)

• propose alternative solutionsto a given practical problem,identify the potentialstrengths and weaknesses ofeach, and select one as thebasis for a plan (214-15)

• evaluate and select appropriateinstruments for collectingevidence and appropriateprocesses for problem solving,inquiring, and decisionmaking (212-8)

The Laboratory outcomes 324-6, 214-6, 213-8, 214-15, 212-8, 324-4 and,in part, 324-6 are addressed by completing Hess’s Law and the Enthalpy ofCombustion of Magnesium, CORE LAB #6.



THERMOCHEMISTRY

Determining Enthalpy Change

Core Lab #6: “Hess’s Law and theEnthalpy of Combustion ofMagnesium”, pp. 682-683


Outcomes

THERMOCHEMISTRY



The use of bond energies is another method of finding H∆ for reactions.However, H∆ values calculated using bond energies will be slightly differentfrom values calculated using heats of formation or determinedexperimentally. The reason for this is that the bond energies in tables areaverage bond energies for each particular bond type. For example, an exactvalue for the bond energy of the H-H bond can be determined because themolecule H

2 contains a single H-H bond only. However, the exact bond

energy for the C-H bond cannot be determined exactly because there is nosuch molecule C-H. The carbon in the C-H bond always has other atomsbonded to it which affects the C-H bond energy. Teachers should note theaverage bond energy values on the mini-table on page 688 of the textbookdo not always match the average bond energy values on the more detailedtable given in the appendix of the textbook.

Take for example, CH4 or CH

2Cl

2, or CH

3F. The bond energy for C-H is

therefore an average of the C-H bond energies in several differentenvironments. Therefore, H∆ values calculated using these average bondenergies will be slightly different.

The formula used to calculate H∆ from bond energies is different from the

ones used for calculating H∆ using ofH∆ values, although they look

similar. In this case, it is: Reactants - Products.

H∆ = ∑ (Bond energies of all reactant bonds broken) - ∑ (Bond energies

of all product bonds formed)

For example; Calculate H∆ for the formation of water from its elementsusing bond energies and the equation below:

2H2(g)

+ O2(g)

→ 2H2O

(g)

Bond Energy

• calculate the changes in energyof various chemical reactionsusing bond energies (324-4)

– define bond energy– use average bond energies to

estimate the H∆ for adesired reaction

[ ] [ ][ ] [ ]

(reactants) (products)

2( ) 1( ) 4( )

2(435 ) 1(498 ) 4(464 )498

(bond breakage) (bond formation)

− = −

∆ =

∆ = −

∆ = + −

∆ = + −∆ = −

−∑ ∑

∑ ∑rxn

reaction

reaction H H O O H O

kJ kJ kJmol mol molreaction

reaction

HH BE BE

H BE BE BE

HH kJ

H H

At this point students could calculate H∆ for the same reaction usingofH∆ values for comparison.

[ ] [ ]2 ( ) 2( ) 2( )

(products) (reactants)

2 ( ) 2 ( ) 1 ( )

2( 241.8 ) 2(0) 1(0)483.6

∆ = ∆ − ∆

∆ = ∆ − ∆ + ∆ ∆ = − − +∆ = −

∑ ∑o orxn f f

o o orxn f g f g f g

kJmolrxn

rxn

H n H n H

H H H O H H H O

HH kJ



THERMOCHEMISTRY

Bond Energy

Paper and Pencil

• Students could, using bond energies, calculate the enthapy of reactionfor:

(i) 2 C4H

10(l) + 13 O

2(g) → 8 CO

2 + 10 H

2O

(g)

(ii) 2 C3H

6(g) + Cl

2(g) → C

3H

6Cl

2(g) (324-4)


CHEMISTRY 3202 CURRICULUM: GUIDE120

THERMOCHEMISTRY

unit 3 thermochemistry specific curriculum outcomes

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