the nature of energy - 208.93.184.5

The Nature of Energy

Chapter 6

6 | 2Copyright © Cengage Learning. All rights reserved.


1. Look at Figure 6.1 in your text. Ball A has stopped moving. However, energy must be conserved. So what happened to the energy of Ball A?



2. What if energy was not conserved? How would this affect our lives?



3. The text uses distance traveled and change in elevation to discuss the idea of a state function. Explain which of these is a state function and which is not.



4. A friend of yours reads that the process of water freezing is exothermic. This friend tells you that this can’t be true because exothermic implies “hot,” and ice is cold. Is the process of water freezing exothermic? If so, explain it so your friend can understand it. If not, explain why not.



5. Label the following processes as exothermic or endothermic and explain.

a. Your hand gets cold when you touch ice.

b. The ice gets warmer when you touch it.

c. Water boils in a kettle when heated on a stove.

d. Water vapor condenses on a cold pipe.

e. Ice cream melts.



6. The internal energy of a system is said to be the sum of the kinetic and potential energies of all the particles in the system. The text discusses potential energy and kinetic energy in terms of a ball on a hill, in Section 6.1. Explain potential energy and kinetic energy for a chemical reaction.



7. You strike an unlit match and it burns.

Explain the energy transfers of this scenario using the terms exothermic, endothermic, system, surrounding, potential energy, and kinetic energy in your discussion. Also include an energy-level diagram.



8. For each of the following, define a system and its surroundings, and give the direction of energy transfer.

a. Propane is burning in a Bunsen burner in a laboratory.

b. Water droplets, sitting on your skin after swimming, evaporate.

c. Two chemicals mixing in a beaker give off heat.



9. Hydrogen gas and oxygen gas react violently to form water.

a. Which is lower in energy: a mixture of hydrogen gases, or water? Explain.

b. Sketch an energy-level diagram (like Figure 6.2 or Figure 6.3) for this reaction.



10. Which of the following performs more work?

A gas expanding against a pressure of 2 atm from 1.0 L to 4.0 L

or

A gas expanding against a pressure of 3 atm from 1.0 L to 3.0 L



11. Determine the sign of E for each of the following with the listed conditions.

a. An endothermic process that performs work.

i. *work* > *heat*

ii. *work* < *heat*

b. Work is done on a gas and the process is exothermic.

i. *work* > *heat*

ii. *work* < *heat*



12. There is a law of conservation of energy (the first law of thermodynamics). Is there a law of conservation of heat? Defend your answer.


Enthalpy and Calorimetry



1. Consider four 100.0-g samples of water, each in a separate beaker at 25.0C. Into each beaker you drop 10.0 g of a different metal that has been heated to 95.0C. Assuming no heat loss to the surroundings, which water sample will have the highest final temperature? Explain your answer.

(See next slide)



1. (cont)

a. The water to which you have added aluminum (c=0.89J/gC).

b. The water to which you have added iron (c=0.45J/gC).

c. The water to which you have added copper (c = 0.20J/gC).

d. The water to which you have added lead (c=0.14J/gC).

e. Since the masses of the metals are the same, the final temperatures would be the same.



2. Explain why aluminum cans make good storage containers for soft drinks.



3. A 100.0-g sample of water at 90C is added to a 100.0-g sample of water at 10C.

a. The final temperature of the water should be:

i. Between 50C and 90C

ii. 50C

iii. Between 10C and 50C

b. Calculate the final temperature of the water.



4. A 100.0-g sample of water at 90C is added to a 500.0-g sample of water at 10C.



ii. 50C





5. You have a Styrofoam cup with 50.0 g of water at 10C. You add a 50.0-g iron ball at 90C to the water.



ii. 50C




Hess’s Law and Standard Enthalpies of Formation



1. How is Hess’s law a restatement of the first law of thermodynamics?



2. In Section 6.3 of your text, two characteristics of enthalpy changes for reactions are listed. What are these characteristics? Explain why these characteristics are true.



3. Look at equation 6.1 in Section 6.4 of your text. Does this mean ∆H°reaction is or is not a state function? Explain your answer.



4. Explain why ∆H°f for an element in its standard state is zero.



5. Using the following data, calculate the standard heat of formation of the compound ICl(g) at 25°C, and show your work.

H° (kJ/mol)

Cl2(g) 2Cl(g) 242.3

I2(g) 2I(g) 151.0

ICl(g) I(g) + Cl(g) 211.3

I2(s) I2(g) 62.8

the nature of energy - 208.93.184.5

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