7.E: Energy Changes (Exercises)

 

7.1: Energy and Its Units

Concept Review Exercises

1. What is the relationship between energy and heat?

2. What units are used to express energy and heat?

Answers

1. Heat is the exchange of energy from one part of the universe to another. Heat and energy have the same units.

2. Joules and calories are the units of energy and heat.

Exercises

1. Define energy.

2. What is heat?

3. What is the relationship between a calorie and a joule? Which unit is larger?

4. What is the relationship between a calorie and a kilocalorie? Which unit is larger?

5. Express 1,265 cal in kilocalories and in joules.

6. Express 9,043.3 J in calories and in kilocalories.

7. One kilocalorie equals how many kilojoules?

8. One kilojoule equals how many kilocalories?

9. Many nutrition experts say that an average person needs 2,000 Cal per day from his or her diet. How many joules is this?

10. Baby formula typically has 20.0 Cal per ounce. How many ounces of formula should a baby drink per day if the RDI is 850 Cal?

Answers

1. Energy is the ability to do work.

2. Heat is a form of energy (thermal) that can be transferred from one object to another.

3. 1 cal = 4.184 J; the calorie is larger.

4. 1 kilocalorie(1 Cal) = 1000 cal; the kcal is larger.

5. 1.265 kcal; 5,293 J

6. 2161.4 cal; 2.1614 kcal

7. 1 kcal = 4.184 kJ

8. 1 kJ = 0.239 kcal

9. 8.4 × 10⁶ J

10. 42.5 oz

7.2: Heat and Temperature

Concept Review Exercise

1. Describe the relationship between heat transfer and the temperature change of an object.

2. Describe what happens when two objects that have different temperatures come into contact with one another.

Answer

1. Heat is equal to the product of the mass, the change in temperature, and a proportionality constant called the specific heat.

2. The temperature of the hot object decreases and the temperature of the cold object increases as heat is transferred from the hot object to the cold object. The change in temperature of each depends on the identity and properties of each substance.

Exercises

1. The melting point of mercury is −38.84^oC. Convert this value to degrees Fahrenheit and Kelvin.

2. A pot of water is set on a hot burner of a stove. What is the direction of heat flow?

3. Some uncooked macaroni is added to a pot of boiling water. What is the direction of heat flow?

Answers

1. -37.91⁰F and 234.31 K

2. Heat flows into the pot of water.

3. Heat flows to the macaroni.

7.3: Phase Changes

Concept Review Exercises

1. Explain what happens when heat flows into or out of a substance at its melting point or boiling point.
2. How does the amount of heat required for a phase change relate to the mass of the substance?
3. What is the direction of heat transfer in boiling water?
4. What is the direction of heat transfer in freezing water?
5. What is the direction of heat transfer in sweating?

Answers

1. The energy goes into changing the phase, not the temperature.

2. The amount of heat is a constant per gram of substance.

3. Boiling. Heat is being added to the water to get it from the liquid state to the gas state.

4. Freezing. Heat is exiting the system in order to go from liquid to solid. Another way to look at it is to consider the opposite process of melting. Energy is consumed (endothermic) to melt ice (solid to liquid) so the opposite process (liquid to solid) must be exothermic.

5. Sweating. Heat is consumed to evaporate the moisture on your skin which lowers your temperature.

Exercises

1. How much energy is needed to melt 43.8 g of Au at its melting point of 1,064°C?

2. How much energy is given off when 563.8 g of NaCl solidifies at its freezing point of 801°C?

3. What mass of ice can be melted by 558 cal of energy?

4. How much ethanol (C₂H₅OH) in grams can freeze at its freezing point if 1,225 cal of heat are removed?

5. What is the heat of vaporization of a substance if 10,776 cal are required to vaporize 5.05 g? Express your final answer in joules per gram.

6. If 1,650 cal of heat are required to vaporize a sample that has a heat of vaporization of 137 cal/g, what is the mass of the sample?

7. What is the heat of fusion of water in calories per mole?

8. What is the heat of vaporization of benzene (C₆H₆) in calories per mole?

9. What is the heat of vaporization of gold in calories per mole?

10. What is the heat of fusion of iron in calories per mole?

Answers

1. 670 cal

2. 69,630 cal

3. 6.98 g

4. 27.10 g

5. 8,930 J/g

6. 12.0 g

7. 1,440 cal/mol

8. 7,350 cal/mol

9. 80,600 cal/mol

10. 3,530 cal/mol

 

7.4: Bond Energies and Chemical Reactions

Concept Review Exercises

1. What is the connection between energy and chemical bonds?
2. Why does energy change during the course of a chemical reaction?
3. Two different reactions are performed in two identical test tubes. In reaction A, the test tube becomes very warm as the reaction occurs. In reaction B, the test tube becomes cold. Which reaction is endothermic and which is exothermic? Explain.
4. Classify "burning paper" as endothermic or exothermic processes.

Answers

1. Chemical bonds have a certain energy that is dependent on the elements in the bond and the number of bonds between the atoms.

2. Energy changes because bonds rearrange to make new bonds with different energies.

3. Reaction A is exothermic because heat is leaving the system making the test tube feel hot. Reaction B is endothermic because heat is being absorbed by the system making the test tube feel cold.
4. "Burning paper" is exothermic because burning (also known as combustion) releases heat

Exercises

1. Using the data in Table 7.4.1, calculate the energy of one C–H bond (as opposed to 1 mol of C–H bonds). Recall that 1 mol = 6.022 x 10²³ C–H bonds

2. Using the data in Table 7.4.1, calculate the energy of one C=C bond (as opposed to 1 mol of C=C bonds). Recall that 1 mol = 6.022 x 10²³ C=C bonds

3. Is a bond-breaking process exothermic or endothermic?

4. Is a bond-making process exothermic or endothermic?

5. Is each chemical reaction exothermic or endothermic?

   a. 2SnCl₂(s) + 33 kcal → Sn(s) + SnCl₄(s).         b. CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(ℓ) + 213 kcal

6. Is each chemical reaction exothermic or endothermic?

   a. C₂H₄(g) + H₂(g) → C₂H₆(g) + 137 kJ              b.  C(s, graphite) + 1.9 kJ → C(s, diamond)

Answers

1. 1.661 × 10⁻¹⁹ cal

2. 2.408 x 10^-19 cal

3. endothermic

4. exothermic

5. a. endothermic  b. exothermic

6. a. exothermic.  b. endothermic

7.5: The Energy of Biochemical Reactions

Concept Review Exercise

1. What is the energy content per gram of proteins, carbohydrates, and fats?

Answer

1. proteins and carbohydrates: 4 kcal/g; fats: 9 kcal/g

Exercises

1. An 8 oz serving of whole milk has 8.0 g of fat, 8.0 g of protein, and 13 g of carbohydrates. Approximately how many kilocalories does it contain?

2. A serving of potato chips has 160 kcal. If the chips have 15 g of carbohydrates

    and 2.0 g of protein, about how many grams of fat are in a serving of potato chips?

3. The average body temperature of a person is 37°C, while the average surrounding temperature is 22°C. Is overall human metabolism exothermic or endothermic?

4. Cold-blooded animals absorb heat from the environment for part of the energy they need to survive. Is this an exothermic or an endothermic process?

5. If the reaction ATP → ADP gives off 7.5 kcal/mol, then the reverse process, ADP → ATP requires 7.5 kcal/mol to proceed. How many moles of ADP can be converted to ATP using the energy from 1 serving of potato chips (see Exercise 2)?

6. If the oxidation of glucose yields 670 kcal of energy per mole of glucose oxidized, how many servings of potato chips (see Exercise 2) are needed to provide the same amount of energy?

Answers

1. 156 kcal

2. 10.2 g

3. exothermic

4. endothermic

5. 21.3 mol

6. 4.2 servings

Additional Exercises

1. Sulfur dioxide (SO₂) is a pollutant gas that is one cause of acid rain. It is oxidized in the atmosphere to sulfur trioxide (SO₃), which then combines with water to make sulfuric acid (H₂SO₄).

   1. Write the balanced reaction for the oxidation of SO₂ to make SO₃. (The other reactant is diatomic oxygen.)
   2. When 1 mol of SO₂ reacts to make SO₃, 23.6 kcal of energy are given off. If 100 lb (1 lb = 454 g) of SO₂ were converted to SO₃, what would be the total energy change?

2. Ammonia (NH₃) is made by the direct combination of H₂ and N₂ gases according to this reaction:

   N₂(g) + 3H₂(g) → 2NH₃(g) + 22.0 kcal

   1. Is this reaction endothermic or exothermic?
   2. What is the overall energy change if 1,500 g of N₂ are reacted to make ammonia?

3. When steam condenses, heat energy is _____ (absorbed or released?). 

4. When water freezes, heat energy is _______  (absorbed or released?)

5. The change in energy is +65.3 kJ for each mole of calcium hydroxide [Ca(OH)₂] according to the following reaction:

   Ca(OH)₂(s) → CaO(s) + H₂O(g)

   How many grams of Ca(OH)₂ could be reacted if 575 kJ of energy were available?

6. The thermite reaction gives off so much energy that the elemental iron formed as a product is typically produced in the liquid state:

   2Al(s) + Fe₂O₃(s) → Al₂O₃(s) + 2Fe(ℓ) + 204 kcal

   How much heat will be given off if 250 g of Fe are to be produced?

7. A normal adult male requires 2,500 kcal per day to maintain his metabolism.

   1. Nutritionists recommend that no more than 30% of the calories in a person’s diet come from fat. At 9 kcal/g, what is the maximum mass of fat an adult male should consume daily?
   2. At 4 kcal/g each, how many grams of protein and carbohydrates should an adult male consume daily?

8. A normal adult male requires 2,500 kcal per day to maintain his metabolism.

   1. At 9 kcal/g, what mass of fat would provide that many kilocalories if the diet was composed of nothing but fats?
   2. At 4 kcal/g each, what mass of protein and/or carbohydrates is needed to provide that many kilocalories?

9. Some biology textbooks represent the conversion of adenosine triphosphate (ATP) to adenosine diphosphate (ADP) and phosphate ions as follows:

   ATP → ADP + phosphate + energy

   What is wrong with this reaction?

Answers

1.  

   1. 2SO₂ + O₂ → 2SO₃
   2. 16,700 kcal

2.   exothermic; 1200 kcal

3.   released

4.   released

 

5.   652 g

 

6.   457 kcal

7.

1. 83.3 g
2. 438 g

8.   278 g; 625 g

9. A reactant is missing: H₂O is missing.