7.6 End-of-Chapter Material

To ensure that you understand the material in this chapter, you should review the meanings of the following bold terms in the following summary and ask yourself how they relate to the topics in the chapter.

Energy is the ability to do work. The transfer of energy from one place to another is heat. Heat and energy are measured in units of joules, calories, or kilocalories (equal to 1,000 calories). The amount of heat gained or lost when the temperature of an object changes can be related to its mass and a constant called the specific heat of the substance.

The transfer of energy can also cause a substance to change from one phase to another. During the transition, called a phase change, heat is either added or lost. Despite the fact that heat is going into or coming out of a substance during a phase change, the temperature of the substance does not change until the phase change is complete; that is, phase changes are isothermal. Analogous to specific heat, a constant called the heat of fusion of a substance describes how much heat must be transferred for a substance to melt or solidify (that is, to change between solid and liquid phases), while the heat of vaporization describes the amount of heat transferred in a boiling or condensation process (that is, to change between liquid and gas phases).

Every chemical change is accompanied by an energy change. This is because the interaction between atoms bonding to each other has a certain bond energy, the energy required to break the bond (called lattice energy for ionic compounds), and the bond energies of the reactants will not be the same as the bond energies of the products. Reactions that give off energy are called exothermic, while reactions that absorb energy are called endothermic. Energy-level diagrams can be used to illustrate the energy changes that accompany chemical reactions.

Even complex biochemical reactions have to follow the rules of simple chemistry, including rules involving energy change. Reactions of carbohydrates and proteins provide our bodies with about 4 kcal of energy per gram, while fats provide about 9 kcal per gram.

Additional Exercises

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?
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?
When 1 g of steam condenses, 540 cal of energy is released. How many grams of ice can be melted with 540 cal?
When 1 g of water freezes, 79.9 cal of energy is released. How many grams of water can be boiled with 79.9 cal?
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?
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?
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?
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?
The volume of the world’s oceans is approximately 1.34 × 10²⁴ cm³.
1. How much energy would be needed to increase the temperature of the world’s oceans by 1°C? Assume that the heat capacity of the oceans is the same as pure water.
2. If Earth receives 6.0 × 10²² J of energy per day from the sun, how many days would it take to warm the oceans by 1°C, assuming all the energy went into warming the water?
Does a substance that has a small specific heat require a small or large amount of energy to change temperature? Explain.

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Chapter 7: Energy and Chemical Processes

Chapter Summary

Additional Exercises

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