heats of reaction and solution 012-10744 r1.04. the snapshot button is used to capture the screen....

Heats of Reaction and Solution 012-10744 r1.04

The Snapshot button is used to capture the screen.

The Journal is where snapshots are stored and viewed.

The Share button is used to export or print your journal to turn in your work.

IntroductionJournals and Snapshots

Note: You may want to take asnapshot of the first page ofthis lab as a cover page for your Journal.

Each page of this lab that contains the symbol

should be inserted into your journal. After completing a lab page with the snapshot symbol, tap (in the upper right hand corner) to insert the page into your journal.

Heats of Reaction and Solution

Lab Challenges

• Determine the heat (energy) released or absorbed when sodium hydroxide dissolves in water and when ammonium chloride dissolves in water.

• Determine the heat (energy) released or absorbed when magnesium metal reacts with hydrochloric acid.

Chemists make an incredible range of useful products!

Chemists also provide information on the energy associated with physical and chemical changes. This information is used to evaluate new fuels and efficiently make new products!


Background

• Physical changes and chemical reactions both may be accompanied by changes in energy, often in the form of heat or enthalpy.

• Chemical and physical processes that absorb heat from their surroundings are endothermic.

• Chemical and physical processes that release heat into their surroundings are exothermic.

Exothermic

Heat is released and warms the surrounding air.

Endothermic Heat is absorbed and melts the ice cream.


Self-Check1. What happens during an exothermic reaction?

a) Heat is taken from the surroundings.

b) Gas turns into solid.

c) Heat levels remain the same.

d) Heat is released into the surroundings.


...Background

• Phase changes, including the dissolving of a solute into a solvent, are types of physical changes that involve an exchange of heat.

• The heat absorbed or released when a solute dissolves is called the heat of solution (or enthalpy of solution). Heat results from the breaking of attractions

between ions in the crystal lattice

Solute dissolving in water Form attractions

and the forming of attractions between the freed ions and the solvent molecules.

Break attractions


...Background• Heat that is released or absorbed during a chemical reaction is called the heat of

reaction (or enthalpy of reaction).

• The heat of reaction results from the breaking of bonds in the reactants and the forming of bonds in the products.

CH4 (g) + 2 O2 (g) → CO2 (g) + 2 H2O (l)

Reactants- bonds are broken (energy is absorbed)

Products- bonds are formed (energy is released)


...BackgroundEnthalpy diagrams illustrate the internal energy of the reactants compared to the products.

When the final products have more energy than the original reactants, the reaction is endothermic.

ENTHALPY DIAGRAMEndothermic Reaction

Products

Reaction path

Ener

gy

Reactants

When the final products have less energy than the starting reactants, the reaction is exothermic.

ENTHALPY DIAGRAMExothermic Reaction

Reaction path

Products

Ener

gy Reactants


2. In an endothermic reaction the reactants have _____________________________.

a) more energy than the products

b) less energy than the products

c) the same energy as the products

d) more or less energy depending on the chemicals reacted

Self-CheckHeats of Reaction and Solution

Ener

gy

Reaction Path

Self-Check3. In an exothermic reaction, the products have

_________ energy (enthalpy), therefore the surroundings must have ______ this energy.

a) higher, absorbed

b) higher, emitted

c) lower, emitted

d) lower, absorbed

e) zero, infinite


...Background• The heat of reaction q and heat of solution q can both be found by multiplying

the mass m of the water/solution, the specific heat c of water, and change in temperature ΔT together.

q = m x c x ΔT

• The temperature change of the surroundings is measured and thus the heat calculated is the heat change experienced by the surroundings. The enthalpy of reaction ∆H or heat of solution ∆H, which caused the measured change, is the opposite sign of the heat calculated.

ΔH = -q


• Finally, the energy for a physical change or chemical reaction can be described on a per mole basis by dividing the heat of solution or heat of reaction by the number of moles of substance present.

• The resulting term is called the molar heat of solution or molar heat of reaction.

...Background

Molar heat of solution/reaction =

moles of substance

ΔH


Safety

• Follow all common laboratory safety procedures.

• Sodium hydroxide and hydrochloric acid are corrosive irritants. Avoid contact with the skin and eyes.

• Wash hands after handling chemicals, equipment, and glassware.

• Be sure that all acids and bases are neutralized before being disposed of down the drain.

Neutralize!

BE SAFEAlways wash hands to remove residue

before leaving


• Temperature sensor

• Beaker, 250-mL

• Polystyrene cup, 236-mL (8 oz.)

• Centigram balance

• Graduated cylinder, 50-mL

• Spatula

• Weighing paper

Materials and EquipmentCollect all of these materials before beginning the lab.

OR


250-mL

Materials and Equipment

• Wash bottle and waste container

• Stir rod

• Sand paper or steel wool, 1 piece

• Sodium hydroxide, 1 g

• Ammonium chloride, 1 g

• Magnesium metal ribbon, 0.10 g

• 1.0 M Hydrochloric acid solution 25 mL

• Distilled (deionized) water, 50 mL

Collect all of these materials before beginning the lab.

Mg

Na - OH


waste

Distilled Water

Sequencing Challenge The steps to the left are part of the procedure for this lab activity. They are not in the right order. Determine the correct sequence of the steps then take a snapshot of this page.

A. Stop recording temperature data after the temperature has stabilized (or reversed).

B. Record the exact mass of each reactant. Pour the liquid into the calorimeter and start recording temperature data.

C. Calculate the heat (enthalpy) of reaction or solution.

D. After the temperature data has stabilized, add the solid to the solution and mix thoroughly.


Setup: NaOH1. Connect a temperature sensor to the data

collection system.

2. Measure ~1.0 g of sodium hydroxide (NaOH) pellets. Record the exact mass of NaOH in the data table below.*

*To Enter Data into a Table: 1. Tap to open the tool

palette.2. Tap then tap a cell in

the data table to highlight it in yellow.

3. Tap to open the Keyboard screen.


Setup: NaOHQ1: What does it mean to "tare" a balance?






Setup: NaOH3. Measure ~25 mL of water using a graduated cylinder.

Enter the exact volume in the table below.*

4. Place a clean, dry calorimeter on the balance and tare the balance. Pour ~25 mL of water into the calorimeter. Enter the mass of the water in the data table below.*


Q2: Compare the volume of the water with its mass. Is this what you would expect? Explain.


1. Place the calorimeter into a 250-mL beaker and place the temperature sensor in the water.

2. Tap to start collecting data and allow the temperature to stabilize.

Collect Data: NaOH

Continue to the next page.


3. Tap then to re-scale the axes and then add the solid NaOH to the water.

4. Stir the NaOH constantly until it has completely dissolved. This may take several minutes.

5. Continue collecting data until the temperature has remained constant or started to decrease for at least 30 seconds.

6. Tap to stop recording data.


7. Find the initial temperature Ti, the final temperature Tf, and change in temperature ∆T.*

Note: These values will be entered on the next page.

* To Find the Difference Between Two Data Points:

1. Tap to open the tools. 2. Tap and then tap two

points on the data run. 3. Adjust using both buttons

and then tap .4. Tap to display the first

point, last point, and the difference between them.


8. Enter the initial temperature Ti, the final temperature Tf, and change in temperature ∆T for dissolving NaOH in water.*






Setup: NH4Cl

1. Dispose of the NaOH solution according to your teacher's instructions.

2. Thoroughly clean and dry the calorimeter, temperature sensor, and stir rod.

3. On the next several pages you will be repeating the experiment, this time dissolving ammonium chloride (NH4Cl) in water.

Q3: Why must the calorimeter, temperature sensor, and stir rod be dried before using them in the next part of the experiment?






Setup: NH4Cl4. Measure ~1.0 g of NH4Cl using a blance and ~25 mL of

water using a graduated cylinder. Enter the exact values in the table below.*

5. Place a clean, dry calorimeter on the balance and tare the balance. Pour the ~25 mL of water into the calorimeter. Enter the mass of the water below.*




Collect Data: NH4Cl



3. Tap then to re-scale the axes and then add the solid NH4Cl to the water.

4. Stir the NH4Cl constantly until it has completely dissolved. This may take several minutes.

5. Continue collecting data until the temperature has remained constant or started to increase for at least 30 seconds.





* To Find :the Difference Between Two Data Points





Note: Tap then tap next to run 1 to hide the first run of data.


8. Enter the initial temperature Ti, the final temperature Tf, and change in temperature ∆T for dissolving NH4Cl in water.*





Setup: HCl + Mg1. Dispose of the NH4Cl solution according to your

teacher's instructions.

2. Thoroughly clean and dry the calorimeter, temperature sensor, and stir rod.

3. Cut a piece of magnesium (Mg) ribbon that is7-8 cm long (~0.1g).

4. Use sandpaper/steel wool to remove any magnesium oxide that has formed on the Mg ribbon.

5. Cut the cleaned ribbon into ~1 cm pieces.

Q4: Why is the magnesium ribbon cut into pieces?






Setup: HCl + Mg6. Determine the mass of all the pieces together and record

it in the data table below.*

7. Measure ~25 mL of 1.0 M hydrochloric acid (HCl), record its volume, and determine its mass by taring a clean, dry calorimeter and then adding the HCl. Enter the values in the data table below.*




Collect Data: HCl + Mg



3. Tap then to re-scale the axes and then add the Mg pieces to the HCl.

4. Stir until the magnesium has completely reacted. This may take several minutes.

5. Continue collecting data until the temperature has remained constant or started to decrease for at least 30 seconds.



Collect Data: HCl + Mg Q5: What evidence did you observe that a chemical reaction occurred

between HCl and Mg?




* To Find :the Difference Between Two Data Points





Note: Tap then tap next to run 1 and run 2 to hide these data runs.


8. Enter the initial temperature Ti, the final temperature Tf, and change in temperature ∆T for reacting HCl with Mg.*





Data Analysis 1. Calculate the heat absorbed

by the solution in each process. Enter your answers in the data table. Convert joules to kilojoules in the final column.

q = m × c × ΔT q = heat lost or gained by

the solutionm = mass of the liquid c = the specific heat of the

solution (4.18 J/g°C)ΔT= change in temperature


Data Analysis 2. Determine the number of

moles of each solid used. mass moles = molar mass

3. Determine ∆H for each process.

ΔH = -q

4. Determine the molar enthalpy (∆H/mol) for each substance.

Enter all values in the data table on the left.


percent error = × 100 (accepted value - experimental value)

Analysis1. Determine the percent error for each process using the accepted values in the

table below. Enter the percent error in the table below.

accepted value


Analysis2. Suggest possible improvements to the experimental procedure that could

improve the accuracy of the results.


Analysis3. Identify each process as exothermic or endothermic:

a. dissolving NaOH

b. dissolving NH4Cl

c. reacting Mg + HCl


Analysis4. Write the chemical equation that illustrates the heat changes that occurred for

each physical and chemical process you performed in this experiment. Use your experimentally determined values for molar heat of reaction/solution in the equations. Be sure to include state symbols for all reactants and products.


Synthesis1. Describe what happens at the

molecular level to sodium hydroxide when it is dissolved in water.

Solute dissolving in water


Synthesis2. Heat was either released or absorbed in all three processes, but only one of

the processes was a chemical reaction. Explain how this is possible.


Synthesis3. Describe what would have happened if the physical and chemical processes were

performed at 50°C instead of room temperature?


Multiple Choice1. Heat of reaction refers to ______________.

a) the heat released by a chemical reaction.

b) the heat absorbed by a chemical reaction.

c) the temperature of the solution after a chemical reaction occurs.

d) both A and B.


2. Using the chemical equation below, determine the energy released by burning 2 moles of propane (C3H8).

a) 2219.2 kJ

b) 4438.4 kJ

c) 1109.6 kJ

d) 11096 kJ

Multiple Choice

C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(l) + 2219.2 kJ


Multiple ChoiceRead the information below to answer the next three questions.

Two grams of salt A were added to 50 mL of water and the initial and final temperatures were recorded below. In a separate experiment, two grams of salt B were added to 50 mL of water and the initial and final temperatures were recorded below.

SALT A

Initial Temp = 20°C

Final Temp = 35°C

SALT B

Initial Temp = 20°C

Final Temp = 10°C


Multiple Choice3. Using the information on the previous page,

what can be said about flow of energy in the Salt A and Salt B experiments?

a) For salt A: energy flows from the water to the salt; For salt B: energy flows from the water to the salt.

b) For salt A: energy flows from the salt to the water; For salt B: energy flows from the salt to the water.

c) For salt A: energy flows from the salt to the water; For salt B: energy flows from the water to the salt.

d) For salt A: energy flows from the water to the salt; For salt B: energy flows from the salt to the water.

SALT AInitial Temp = 20°CFinal Temp = 35°C

SALT BInitial Temp = 20°CFinal Temp = 10°C


4. What type of process occurred when salt A dissolved in water?

a) enthalpy

b) heat

c) endothermic

d) exothermic

Multiple ChoiceHeats of Reaction and Solution





Multiple Choice5. Which equation illustrates the process that

occurred when salt B dissolved in water?

a) salt B(s) → salt B(aq) + heat

b) salt B(aq) → salt B(aq) + heat

c) salt B(s) + heat → salt B(aq)

d) salt B(aq) + heat → salt B(aq)


You have completed the lab.Congratulations!

Please remember to follow your teacher's instructions for cleaning-up and submitting your lab.


ReferencesHeats of Reaction and Solution

All images were taken from PASCO documentation, public domain clip art, or Wikimedia Foundation Commons.

1.HOT SPRING. Public domain compliments of creator, Matt Fishbach2.ATOMIC SYMBOL. http://www.openstockphotography.org/image-licensing/energy/Nuclear_energy_icon.jpg3.ACTIVATION ENERGY DIAGRAM. http://en.wikipedia.org/wiki/Image:Activation_energy.svg4.GAS PUMP http://freeclipartnow.com/transportation/traffic-signs/us-road-signs/recreation/gas.jpg.html5.SQUEAKY CLEAN http://freeclipartnow.com/household/chores/cleaners/disfectant.jpg.html6.PAINT http://freeclipartnow.com/recreation/art/paint-can-n-brush.jpg.html7.BATTERY http://freeclipartnow.com/science/energy/batteries/battery.jpg.html8.ELECTRIC SOCKET http://freeclipartnow.com/electronics-technology/computers/icons-themes/flat-

theme/applications/energy.png.html9.BURNING MATCH http://freeclipartnow.com/construction/tools/lit-match.jpg.html10.ICE CREAM http://freeclipartnow.com/food/desserts-snacks/ice-cream/ice-cream-cone-3-scoop.jpg.html11.CORROSIVE WARNING http://commons.wikimedia.org/wiki/Image:DIN_4844-2_Warnung_vor_Aetzenden_Stoffen_D-

W004.svg12.BE SAFE http://freeclipartnow.com/signs-symbols/warnings/safety-hands.jpg.html13.BEAKER http://freeclipartnow.com/science/flasks-tubes/beaker.jpg.html14.VINEGAR (DISTILLED WATER) http://freeclipartnow.com/household/chores/cleaners/vinegar.jpg.html15.NAOH http://commons.wikimedia.org/wiki/File:NaOH.gif16.STOVE http://commons.wikimedia.org/wiki/File:MSR_WindPro_portable_stove.jpg

heats of reaction and solution 012-10744 r1.04. the snapshot button is used to capture the screen....

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