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Name ____KEY________________________ Period ____

CRHS Academic Chemistry

Unit 15 Thermochemistry

HOMEWORK

Due Date Assignment On-Time (100) Late (70)

_______ 15.1

_______ 15.2

_______ 15.3

_______ 15.4

_____ Warm Ups

_______ Extra Credit

Page 2 of 12 Unit 15 Homework

Warm-Up

#1 #6

#2 #7

#3 #8

#4 #9

#5 #10

Unit 15 Homework Page 3 of 12

HW 15.1 MATTER AND ENERGY

1. (TRUE / FALSE) Heat energy cannot be transferred but can be lost.

2. If you place an ice cube in a bowl of hot soup, how will heat flow?

The ( ICE CUBE / HOT SOUP ) will transfer ( COLD / HEAT ) energy to the ( ICE CUBE / HOT SOUP).

3. ___Potential_______________ energy is stored in the bonds of molecules.

4. ____Kinetic _______________ energy is related to thermal energy.

5. What are the 3 ways heat can be transferred?

Conduction

Convection

Radiation

6. Conduction, convection, or radiation?

a. The heat you feel from a fireplace Conv/Rad

b. Boiling water Cond/Conv

c. Heat you feel from a hot stove Conv/Rad

d. Frying a pancake Cond

e. Fast particles colliding with slower particles Conv

f. Air travels this way Conv

g. Transfer through solids Cond

h. Transfer through space Rad

i. Moves as a wave Rad _

j. Moves as a current Conv

k. Sun rays reaching earth Rad

l. Occurs with fluids Conv _

7. In chemistry lab, you pour 100 mL of vinegar into a beaker. You measure the temperature of the sample and find that

it is 20° C. You add a tablespoon of calcium chloride to the vinegar and measure the temperature again. You find that it

has risen to 24° C. What type of chemical reaction has taken place?

( EXOTHERMIC / ENDOTHERMIC ) How do you know? Temp of surrounding increases

Page 4 of 12 Unit 15 Homework

8. As water freezes in a freezer, heat is removed from water molecules, slowing their movement enough to allow them

to freeze.

( EXOTHERMIC / ENDOTHERMIC ) How do you know? Heat removed from system

9. You sit down on a steel bench in the middle of summer. The sun has been beating down on the bench for hours.

Heat in the bench transfers to your skin, causing first degree burns and significant mental anguish. From the point of

view of the bench, is this…

( EXOTHERMIC / ENDOTHERMIC ) How do you know? Heat flows from bench and heats up the surroundings_____

Unit 15 Homework Page 5 of 12

HW 15.2 HEAT CALCULATIONS

1. What is specific heat capacity? Heat (Energy) needed to raise 1 g of a pure substance 1oC at constant pressure

2. On a sunny day, why does the concrete deck around an outdoor swimming pool become hot, while the water stays

cool?

Water has higher Cp, which means it takes more energy from sun to raise its temp . If the amount of sun energy

per time is the same then the concrete will get hotter in 1h than will water.

3. Using calories, calculate how much heat 32.0 g of water absorbs when it is heated from 25.0 C to 80.0 C. Look at

the chart in your notes for the specific heat capacity of water.

q=m x Cp x delta T

Cp = 1.00 cal/goC and delta T = 55.0 oC

q = m ∗ Cp ∗ deltaT = 32.0g x 1.00cal

goC 𝑥 55𝑜𝐶 = 1760 𝑐𝑎𝑙

4. When 435 J of heat is added to 3.4 g of olive oil at 21 C, the temperature increases to 85 C. What is the specific

heat of olive oil?

q=m x Cp x delta T

𝑞

𝑚 𝑥 𝑑𝑒𝑙𝑡𝑎 𝑇= 𝐶𝑝 =

435 𝐽

3.4 𝑔 𝑥 64.0𝑜𝐶=

1.999𝐽

𝑔𝑜𝐶= 2.0

𝐽

𝑔𝑜𝐶

5. How much heat is required to raise the temperature of 250.0 g of mercury 52 C? Look at the chart in your notes for

the specific heat of mercury.

q = m ∗ Cp ∗ deltaT = 250.0g x 0.033cal

goC 𝑥 52𝑜𝐶 = 429 𝑐𝑎𝑙

6. How much heat in kilojoules must be added to 178 g of water to increase the temperature of the water by 5.0 C?

q = m ∗ Cp ∗ deltaT = 178g x 4.18J

goC 𝑥 5.0𝑜𝐶 = 3720 𝐽

3720 J x1 kJ

1000 J= 3.72 kJ or 3.7 kJ

1760 cal

2.0𝐽

𝑔𝑜𝐶

429 cal

3.7 kJ

Page 6 of 12 Unit 15 Homework

7. When calcium chloride (CaCl2) dissolves in water, heat is released. This is called the heat of solution. When 10 grams of CaCl2 are dissolved in 505 grams of water at 25 °C in an insulated container, the temperature of the water elevates to 47.1 °C. What is the heat of solution for this mass of CaCl2? Answer -> 11,100 cal or 46,700 J

8. A calorimeter has 230g of an unknown fluid at 22.0 °C. A sample of metal transfers 1109 Joules of heat into the fluid and the temperature rises to 28.3 °C. What is the specific heat capacity of the fluid? Answer -> Cp = 0.77 J/goC

9. A 52.3 gram sample of aluminum is combusted in a bomb calorimeter. The aluminum goes from 24.5 °C to a maximum of 325.4 °C. The bomb contains 312 grams of water at 24.5 °C before the reaction begins. What was the total temperature change of the water? What was the final temperature of the water? The specific heat of aluminum is 0.89 J/g-°C.

Answer -> dT = 75.3 oC

Unit 15 Homework Page 7 of 12

HW 15.3 WHAT IS THERMOCHEMISTRY?

1. What is the definition of thermochemistry - ____Study of transfers of energy as heat that accompany chemical

reactions and changes in physical state ___________________________________________________________

2. What is the “System” in a chemical reaction? ___________The chemicals involved in the reaction._____________

______________________________________________________________________________________________

3. What is the “Surroundings” in a chemical reaction? _______ Everything but the chemicals involved in the reaction

______________________________________________________________________________________________.

4. Explain the source of heat in an exothermic chemical reaction. _____Energy contained in the chemical bonds of the

reactants is greater than the energy contained in the bonds of the products so the excess energy is given off as heat.

5. Classify these processes as exothermic or endothermic. (Think about whether the “object” is gaining or releasing

heat.)

A. Burning rubbing alcohol ( exothermic / endothermic )

B. Cooking an egg ( exothermic / endothermic )

C. Combustion of gasoline ( exothermic / endothermic )

6. Indicate whether Enthalpy of Reaction (ΔH) is positive (+) or negative (–) for the following:

A. the reactants have > enthalpy than the products ΔH = +___

B. the products have > enthalpy than the reactants ΔH = -___

C. the surroundings lose heat as a reaction occurs ΔH = +___

D. the temperature increases as a reaction occurs ΔH = -___

7. Label the graphs as endothermic or exothermic.

_______Exothermic__________ ______Endothermic__

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Determine the ΔH for the following reactions.

3. N2 (g) + O2 (g) + 43.3 kJ → 2 NO (g) ΔH =

4. 2 C2H6 (g) + O2 (g) → 4 CO2 (g) + 6 H2O (l) + 683.5 kJ ΔH =

5. C3H8 (g) → C3H8 (l) + 41.8 kJ ΔH =

Rewrite the following equations by expressing the energy change as a term in the equation:

a) H2O (g) → H2O (l) ΔH = -10.76 kJ

b) 4 Al + 3 O2 → 2 Al2O3 ΔH = -803.8 kJ

c) 2 H2SO4 → 2 SO2 + 2 H2O + O2 ΔH = 130.6 kJ

HW 15.3 Additional Problems (Slip into HW book between page 8 & 9)

2 Na(s) + Cl2 (g) - > 2 NaCl(s) H = -450 kJ

1a. Calculate the heat change (in kJ) for the conversion of 25.0 g of chlorine gas to solid sodium chloride at

constant pressure.

𝟐𝟓. 𝟎 𝒙𝟏 𝒎𝒐𝒍𝒆 𝑪𝒍𝟐

𝟕𝟎. 𝟗𝟎 𝒈 𝑪𝒍𝟐 𝒙

−𝟒𝟓𝟎 𝒌𝑱

𝟏 𝒎𝒐𝒍 𝑪𝒍𝟐= − 𝟏𝟓𝟗 𝒌𝑱 𝒓𝒆𝒍𝒆𝒂𝒔𝒆𝒅

Unit 15 Homework Page 9 of 12

1b. Calculate the heat change (in kJ) for the conversion of 25.0 g of solid sodium to solid sodium chloride at

constant pressure.

𝟐𝟓. 𝟎 𝒙𝟏 𝒎𝒐𝒍𝒆 𝑵𝒂

𝟐𝟐. 𝟗𝟗 𝒈 𝑵𝒂 𝒙

−𝟒𝟓𝟎 𝒌𝑱

𝟐 𝒎𝒐𝒍 𝑵𝒂= − 𝟐𝟒𝟓 𝒌𝑱 𝒓𝒆𝒍𝒆𝒂𝒔𝒆𝒅

3 Mg (s) + 2 AlCl3 (aq) 3 MgCl2 (aq) + 2 Al (s) H = -851 kJ

2. Calculate the heat change (in kJ) for the reaction of 50.2 g of Mg with excess AlCl3 at constant pressure.

𝟓𝟎. 𝟐 𝒈 𝐌𝐠 𝒙𝟏 𝒎𝒐𝒍𝒆 𝐌𝐠

𝟐𝟒. 𝟑𝟎𝟓 𝒈 𝐌𝐠 𝒙

−𝟖𝟓𝟏𝒌𝑱

𝟑 𝒎𝒐𝒍 𝐌𝐠= −𝟓𝟖𝟔 𝒌𝑱 𝒓𝒆𝒍𝒆𝒂𝒔𝒆𝒅

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HW 15.4 DRIVING FORCES FOF CHEMICAL REACTIONS

1. In nature, reactions tend to proceed in a direction that leads to ___lowest energy state.

2. Explain why some endothermic reactions do occur _randomness of products is much greater than the reactants and

this overcomes the fact that the enthalpy of reaction is positive...

3. What is Entropy? A measure of the randomness

4. In nature there is tendency to proceed in direction that INCREASES / DECREASES the randomness of a system.

5. Chemical reactions tend to go in direction of DECREASING/INCREASING order.

6. Predict whether the value of S for each of the following reactions will greater than, less than, or equal to zero.

C3CO2(s) + 5O2(g) 3 CO2(g) + 4 H2O(g) __>___

3O2 (g) + P2 (g) 2 PO3 (g) __<___

NaCl (s) Na+1(aq) + Cl-1(aq) __>___

2Ca (s) + Cl2 (g) -> 2 CaCl2 (s) __<___

7. Decide if G can ever be negative and therefore the reaction is spontaneous and fill in 3rd column with conclusions.

H S G

- value (exothermic) + value (more random) Always negative and spontaneous

- value (exothermic) - value (less random) Negative at lower temp

+ value (endothermic) + value (more random) Negative at high temp

+ value (endothermic -value (less ransom) Always positive and never sponstaneous

Predict whether the reactions will be spontaneous at 298K by calculating Go from the given enthalpies and entropies.

8. NH4Cl (s) NH3 (g) + HCl (g) Ho = -+176 kJ/mol and So= +0.2857 kJ/(mol*K)

Go = 90.9 kcal/mol (not spontaneous)

9. C (s) + O2 (g) CO2 (g) + 393 kJ So= 0.00300 kJ/(mol*K)

Go = - 392 kcal/mol (spontaneous)

Unit 15 Homework Page 11 of 12

Extra Credit - THERMOCHEMISTRY Across 3. in an endothermic reaction heat is ________ 6. 1 calorie = 4.184_____

8. heat flows from a ________ object to a cooler object

9. accurate and precise measurement of heat change in a reaction 11. type of energy stored in chemical or nuclear bonds 12. heat flows out of system - system warms - negative change in enthalpy 15. type of energy – a type of potential energy stored in bonds between atoms in a substance 17. heat of a system at constant pressure 19. type of energy – total of all energy of position and movement of the particles in a system

21. type of energy - due to motion Down 1. substance on right side of reaction 2. a _______________ equation must be balanced 4. capacity for doing work or supplying heat 5. heat capacity is an ___________ property of matter

7. change in enthalpy of a reaction is the same with one step or many steps (answer is HESS’S LAW)

10. heat enters the system - system cools - positive change in enthalpy 13. water has a ________ specific heat capacity (c) 14. water can hold a ________ amount of heat 16. substance on left side of reaction 18. energy that transfers from one object to another because of temperature difference 20. In an exothermic reaction heat is _________

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