Chapter 15

A fruit-and-oatmeal bar contains 142 nutritional Calories. Convert this energy to calories.

An exothermic reaction releases 86.5 kJ. How many kilocalories of energy are released?

Challenge Define a new energy unit, named after yourself, with a magnitude of one-tenth of a calorie. What conversion factors relate this new unit to joules? To Calories?

If the temperature of 34.4 \({g}\) of ethanol increases from \(25.0^{\circ} {C}\) to \(78.8^{\circ} {C}\) how much heat has been absorbed by the ethanol? Refer to Table 15.2.

A 155-g sample of an unknown substance was heated from 25.0°C to 40.0°C. In the process, the substance absorbed 5696 J of energy. What is the specific heat of the substance? Identify the substance among those listed in Table 15.2.

Challenge A 4.50-g nugget of pure gold absorbed 276 J of heat. The initial temperature was 25.0°C. What was the final temperature?

Explain how energy changes from one form to another in an exothermic reaction. In an endothermic reaction.

Distinguish between kinetic and potential energy in the following examples: two separated magnets; an avalanche of snow; books on library shelves; a mountain stream; a stock-car race; separation of charge in a battery

Explain how the light and heat of a burning candle are related to chemical potential energy.

Calculate the amount of heat absorbed when 5.50 g of aluminum is heated from 25.0ºC to 95.0ºC. The specific heat of aluminum is 0.897 J/(g?ºC)

A 90.0-g sample of an unknown metal absorbed 25.6 J of heat as its temperature increased 1.18°C. What is the specific heat of the metal?

How much heat is absorbed by a \(2.00 \times 10^{3}-{g}\) granite boulder \(({c}_{\text { granite }}=0.803 {J} /({g} \cdot^{\circ} {C}))\) as its temperature changes from 10.0ºC to 29.0ºC?

Challenge If 335 g of water at 65.5°C loses 9750 J of heat, what is the final temperature of the water?

Describe how you would calculate the amount of heat absorbed or released by a substance when its temperature changes.

Explain why ?H for an exothermic reaction always has a negative value.

Explain why a measured volume of water is an essential part of a calorimeter.

Explain why you need to know the specific heat of a substance in order to calculate how much heat is gained or lost by the substance as a result of a temperature change.

Describe what the system means in thermodynamics, and explain how the system is related to the surroundings and the universe.

Calculate the specific heat in J/(g?ºC) of an unknown substance if a 2.50-g sample releases 12.0 cal as its temperature changes from 25.0ºC to 20.0ºC.

Design an Experiment Describe a procedure you could follow to determine the specific heat of a 45-g piece of metal.

Write a complete thermochemical equation for the combustion of ethanol \(({C}_{2} {H}_{5} {OH}) . \Delta H_{{comb}}=-1367 {kJ} / {mol}\)

Determine Which of the following processes are exothermic? Endothermic? a. \(C_{2} H_{5} O H(1) \rightarrow C_{2} H_{5} O H(g)\) d. \({NH}_{3}({g}) \rightarrow {NH}_{3}({l})\) b. \(B r_{2}(1) \rightarrow B r_{2}(s)\) e. \({NaCl}({s}) \rightarrow {NaCl}({l})\) c. \({C}_{5} {H}_{12}({g})+8 {O}_{2}({g}) \rightarrow 5 {CO}_{2}({g})+6 {H}_{2} {O}({l})\)

Explain how you could calculate the heat released in freezing 0.250 mol water

Calculate How much heat is released by the combustion of 206 g of hydrogen gas \(? \Delta H_{\text { comb }}=-286 {k} / / {mol}\)

Apply The molar heat of vaporization of ammonia is 23.3 kJ/mol. What is the molar heat of condensation of ammonia?

Use Equations a and b to determine ?H for the following reaction. \(2 {CO}({g})+2 {NO}({g}) \rightarrow 2 {CO}_{2}({g})+{N}_{2}({g}) \Delta H=?\) a. \(2 {CO}({g})+{O}_{2}({g}) \rightarrow 2 {CO}_{2}(\\}) \Delta H=-566.0 {kJ}\) b. \(N_{2}(g)+O_{2}(g) \rightarrow 2 N O(g) \Delta H=-180.6 {kJ}\)

Challenge ?H for the following reaction is -1789 kJ. Use this and Equation a to determine ?H for Equation b. \(4 {Al}({s})+3 {MnO}_{2}({s}) \rightarrow 2 {Al}_{2} {O}_{3}({s})+3 {Mn}({s}) \quad \Delta H=-1789 {kJ}\) a. \(4 \mathrm{Al}(\mathrm{s})+3 \mathrm{O}_{2}(\mathrm{g}) \rightarrow 2 \mathrm{Al}_{2} \mathrm{O}_{3}(\mathrm{s}) \Delta H=-3352 \mathrm{kJ}\) b. \(\mathrm{Mn}(\mathrm{s})+\mathrm{O}_{2}(\mathrm{g}) \rightarrow \mathrm{MnO}_{2}(\mathrm{s}) \Delta H=?\)

Show how the sum of enthalpy of formation equations produces each of the following reactions. You do not need to look up and include ?H values a. \(2 \mathrm{NO}(\mathrm{g})+\mathrm{O}_{2}(\mathrm{g}) \rightarrow 2 \mathrm{NO}_{2}(\mathrm{g})\) b. \(\mathrm{SO}_{3}(\mathrm{g})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightarrow \mathrm{H}_{2} \mathrm{SO}_{4}(\mathrm{aq})\)

Use standard enthalpies of formation from Table \(R-11\) on page 975 to calculate \(\Delta H_{\mathrm{rxn}}^{\circ}\) for the following reaction. \(4 \mathrm{NH}_{3}(\mathrm{g})+7 \mathrm{O}_{2}(\mathrm{g}) \rightarrow 4 \mathrm{NO}_{2}(\mathrm{g})+6 \mathrm{H}_{2} \mathrm{O}(\mathrm{l})\)

Determine \(\Delta H_{\text { comb }}^{\circ}\) for butanoic acid, \(\mathrm{C}_{3} \mathrm{H}_{7} \mathrm{COOH}(1)+5 \mathrm{O}_{2}(\mathrm{g}) \rightarrow 4 \mathrm{CO}_{2}(\mathrm{g})+4 \mathrm{H}_{2} \mathrm{O}(\mathrm{l})\) Use data in Table \(\mathrm{R}\) -11 on page 975 and the following equation. \(4 \mathrm{C}(\mathrm{s})+4 \mathrm{H}_{2}(\mathrm{g})+\mathrm{O}_{2}(\mathrm{g}) \rightarrow \mathrm{C}_{3} \mathrm{H}_{7} \mathrm{COOH}(1) \Delta H=-534 \mathrm{kJ}\)

Challenge Two enthalpy of formation equations, a and b, combine to form the equation for the reaction of nitrogen oxide and oxygen. The product of the reaction is nitrogen dioxide: \(\mathrm{NO}(\mathrm{g})+\frac{1}{2} \mathrm{O}_{2}(\mathrm{g}) \rightarrow \mathrm{NO}_{2}(\mathrm{g}) \Delta H_{\mathrm{rxn}}^{\circ}=-58.1 \mathrm{kJ}\) a. \(\frac{1}{2} \mathrm{N}_{2}(\mathrm{g})+\frac{1}{2} \mathrm{O}_{2}(\mathrm{g}) \rightarrow \mathrm{NO}(\mathrm{g}) \Delta H_{\mathrm{f}}^{\circ}=91.3 \mathrm{kJ}\) b. \(\frac{1}{2} \mathrm{N}_{2}(\mathrm{g})+\mathrm{O}_{2}(\mathrm{g}) \rightarrow \mathrm{NO}_{2}(\mathrm{g}) \Delta H_{\mathrm{f}}^{\circ}=?\) What is \(\Delta H_{\mathrm{f}}^{\circ}\) for Equation \(\mathrm{b}\) ?

Explain what is meant by Hess’s law and how it is used to determine \(\Delta H_{{ran}}^{0}\)

Explain in words the formula that can be used to determine \(\Delta H_{{rm}}^{\circ}\) when using Hess’s law.

Describe how the elements in their standard states are defined on the scale of standard enthalpies of formations

Calculate Use Hess's law to determine \(\Delta H\) for the reaction \({NO ({g})+{O}({g}) \rightarrow\) \({NO}_{2}({g}) \Delta H=?\) given the following reactions. Show your work. \({O}_{2}({g}) \rightarrow 20({g}) \Delta H=+495 {k}\) \(20_{3}({g}) \rightarrow 30_{2}({g}) \Delta H=-427 {kJ}\) \({NO}({g})+{O}_{3}({g}) \rightarrow {NO}_{2}({g})+\mathrm{O}_{2}({g}) \Delta H=-199 {kJ}\)

Predict the sign of \(\Delta S_{\text { system for each of the following changes. }}\) a. \(C I F(g)+F_{2}(g) \rightarrow C | F_{3}(g)\) c. \(C H_{3} O H(1) \rightarrow C H_{3} O H(a q)\) b. \(N H_{3}(g) \rightarrow N H_{3}(a q)\) d. \(C_{10} H_{8}(1) \rightarrow C_{10} H_{8}(s)\)

Challenge Comment on the sign of \(\Delta S_{\text { system for the following reaction. }}\) \({Fe}({s})+{Zn}^{2+}({aq}) \rightarrow {Fe}^{2+}({aq})+{Zn}({s})\)

Determine whether each of the following reactions is spontaneous. a. \(\Delta H_{\text { system }}=-75.9 \mathrm{k} J, T=273 \mathrm{K}, \Delta S_{\text { system }}=138 \mathrm{J} / \mathrm{K}\) b. \(\Delta H_{\text { system }}=-27.6 \mathrm{k} \mathrm{J}, T=535 \mathrm{K}, \Delta S_{\mathrm{system}}=-55.2 \mathrm{J} / \mathrm{K}\) c. \(\Delta H_{\text { system }}=365 \mathrm{kJ}, T=388 \mathrm{K}, \Delta S_{\text { system }}=-55.2 \mathrm{J} / \mathrm{K}\) d. \(\Delta H_{\text { system }}=452 \mathrm{kJ}, T=165 \mathrm{K}, \Delta S_{\text { system }}=55.7 \mathrm{J} / \mathrm{K}\)

Compare and contrast spontaneous and nonspontaneous reactions.

Describe how a system’s entropy changes if the system becomes more disordered during a process.

Decide Does the entropy of a system increase or decrease when you dissolve a cube of sugar in a cup of tea? Define the system, and explain your answer.

Determine whether the system \(\Delta H_{\mathrm{system}}=-20.5 \mathrm{k}, T=298 \mathrm{K}, \mathrm{and}\) \(\Delta S_{\text { system }}=-35.0 \mathrm{J} / \mathrm{K}\) is spontaneous or nonspontaneous.

Compare and contrast temperature and heat

How does the chemical potential energy of a system change during an endothermic reaction?

Describe a situation that illustrates potential energy changing to kinetic energy.

Cars How is the energy in gasoline converted and released when it burns in an automobile engine?

Nutrition How does the nutritional Calorie compare with the calorie? What is the relationship between the Calorie and a kilocalorie?

Ethanol has a specific heat of 2.44 J/(g·°C). What does this mean

Explain how the amount of energy required to raise the temperature of an object is determined.

Nutrition A food item contains 124 nutritional Calories. How many calories does the food item contain?