Chapter 8

1. What are the two principal types of energy?

Kinetic-motion

Potential-stored

2. Classify the following according to the type of energy.

  1. You, standing on the tope of a ski slope with your snowboard on.

Potential

  1. You, snowboarding down the mountain.

Kinetic

  1. You, wearing a parachute and peering out of an airplane.

Potential

  1. You, plummeting to Earth just before your parachute opens.

Kinetic

  1. The pizza and beverage that waits for you at the bottom of the mountain.

Potential

3. For each of the following processes, predict whether it is exothermic or endothermic.

a. CO2(s)  CO2(g), solid to gas, endothermic

b. CO2(g)  CO2(s), gas to solid, exothermic

c. O2(g)  2 O(g), 1 molecule of gas becomes 2 molecules, endothermic

d. 2 O(g)  O2(g), 2 molecules of gas become 1 molecule, exothermic

4. Assume that you place two beakers of water out at room temperature (about 20oC). Beaker A contains 50 grams of water at 80oC. Beaker B contains 50 grams of water at 15oC.

  1. What will happen to the temperature of the water in each beaker?

Beaker A the temperature decreases, Beaker B the temperature increases.

  1. For which beaker will this change be complete sooner?

Beaker B, since it is only a five degree change.

c. For one of the beakers, the change is endothermic. For the other beaker, the change

is exothermic. Which beaker is which?

Beaker A is exothermic, releasing heat to its surroundings, Beaker B is endothermic absorbing heat from its surroundings.

5. For each of the following processes, does entropy increase or decrease?

a. Melting ice, solid to liquid, increase

b. Burning paper, increase

c. Stirring sugar into your coffee, increase

d. Breaking an egg, increase

6. Describe how a combustion engine makes a car move?

Here's what happens as the engine goes through its cycle:

  1. The piston starts at the top, the intake valve opens, and the piston moves down to let the engine take in a cylinder-full of air and gasoline. This is the intake stroke. Only the tiniest drop of gasoline needs to be mixed into the air for this to work. (Part 1 of the figure)
  2. Then the piston moves back up to compress this fuel/air mixture. Compression makes the explosion more powerful. (Part 2 of the figure)
  3. When the piston reaches the top of its stroke, the spark plug emits a spark to ignite the gasoline. The gasoline charge in the cylinder explodes, driving the piston down. (Part 3 of the figure)
  4. Once the piston hits the bottom of its stroke, the exhaust valve opens and the exhaust leaves the cylinder to go out the tailpipe. (Part 4 of the figure)

Now the engine is ready for the next cycle, so it intakes another charge of air and gas.

7. The energy diagram for the reaction A  B is shown here under two different conditions, upper and lower traces.

  1. Which reaction is faster, the upper or the lower pathway?

Lower pathway.

b. For the upper pathway, which of the following must be true?

  • Releases energy, True
  • Exothermic, True
  • Increasing entropy, True
  • Relatively fast, No way to predict
  • Spontaneous, True

c. What is the difference between the upper and lower traces if what is shown is the same reaction? The lower trace has a catalyst present to lower the energy of activation, the barrier the reactants must over-some to become products.

8. Why is dissolving ionic solids an endothermic process? Where does the energy go?

Ionic bonds are strong so it requires energy to break them. The energy goes from its surroundings to the dissolved ionic solids.

9. Green plants use the photosynthesis reaction to make sugar from carbon dioxide and water:

6CO2(g) + 6H2O(g)  C6H12O6(s) + 6O2(g)

  1. Would you predict that this is an exothermic or endothermic reaction?

Endothermic because it requires energy from the sun.

  1. Would you predict that this reaction has a positive (increased disorder) or negative entropy change?

Gas (CO2 and H2O) to solid (C6H12O6) decreases entropy.

  1. What is the role of sunlight in this reaction?

Provides energy.