Chapter 15

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

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?

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 \(25.0^{\circ} \mathrm{C}\) to \(95.0^{\circ} \mathrm{C}\) . The specific heat of aluminum is 0.897 \(\mathrm{J} / \mathrm{g} \cdot^{\circ} \mathrm{C}\) .

A 90.0 -g sample of an unknown metal absorbed 25.6 \(\mathrm{J}\) of heat as its temperature increased \(1.18^{\circ} \mathrm{C}\) . What is the specific heat of the metal?

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

Challenge If 335 g of water at \(65.5^{\circ} \mathrm{C}\) loses 9750 \(\mathrm{J}\) of heat, what is the final temperature of the water?

Explain why \(\Delta H\) for an exothermic reaction always has a negative value.

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 \(\mathrm{J} /\left(\mathrm{q} \cdot^{\circ} \mathrm{C}\right)\) of an unknown substance if a \(2.50-\mathrm{g}\) sample releases 12.0 cal as its temperature changes from \(25.0^{\circ} \mathrm{C}\) to \(20.0^{\circ} \mathrm{C}\)

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 $$\left(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\right) . \Delta \mathrm{H}_{\mathrm{comb}}=-1367 \mathrm{k} / \mathrm{mol}$$

Determine Which of the following processes are exothermic? Endothermic? \begin{equation} \begin{array}{ll}{\text { a. } C_{2} \mathrm{H}_{5} \mathrm{OH}(\mathrm{l}) \rightarrow \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}(\mathrm{g})} & {\text { d. } \mathrm{NH}_{3}(\mathrm{g}) \rightarrow \mathrm{NH}_{3}(\mathrm{l})} \\\ {\text { b. } \mathrm{Br}_{2}(\mathrm{l}) \rightarrow \mathrm{Br}_{2}(\mathrm{s})} & {\text { e. } \mathrm{NaCl}(\mathrm{s}) \rightarrow \mathrm{NaCl}(\mathrm{l})} \\ {\text { c. } \mathrm{C}_{5} \mathrm{H}_{12}(\mathrm{g})+8 \mathrm{O}_{2}(\mathrm{g}) \rightarrow 5 \mathrm{CO}_{2}(\mathrm{g})+6 \mathrm{H}_{2} \mathrm{O}(\mathrm{l})}\end{array} \end{equation}

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

Apply The molar heat of vaporization of ammonia is 23.3 \(\mathrm{kl} / \mathrm{mol} .\) What is the molar heat of condensation of ammonia?

Use Equations a and b to determine \(\Delta H\) for the following reaction. \(2 \mathrm{CO}(\mathrm{g})+2 \mathrm{NO}(\mathrm{g}) \rightarrow 2 \mathrm{CO}_{2}(\mathrm{g})+\mathrm{N}_{2}(\mathrm{g}) \Delta H=?\) $$\begin{array}{l}{\text { a. } 2 \mathrm{CO}(\mathrm{g})+\mathrm{O}_{2}(\mathrm{g}) \rightarrow 2 \mathrm{CO}_{2}(\mathrm{g}) \Delta H=-566.0 \mathrm{kJ}} \\ {\text { b. } \mathrm{N}_{2}(\mathrm{g})+\mathrm{O}_{2}(\mathrm{g}) \rightarrow 2 \mathrm{NO}(\mathrm{g}) \Delta H=-180.6 \mathrm{kJ}}\end{array}$$

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

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

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: \(\mathrm{NO}(\mathrm{g})+\frac{1}{2} \mathrm{O}_{2}(\mathrm{g}) \rightarrow \mathrm{NO}_{2}(\mathrm{g}) \Delta H_{\mathrm{mn}}^{\circ}=-58.1 \mathrm{kJ}\) $$\begin{array}{l}{\text { a. } \frac{1}{2} \mathrm{N}_{2}(\mathrm{g})+\frac{1}{2} \mathrm{O}_{2}(\mathrm{g}) \rightarrow \mathrm{NO}(\mathrm{g}) \Delta H_{\mathrm{f}}^{\circ}=91.3 \mathrm{kJ}} \\\ {\text { b. } \frac{1}{2} \mathrm{N}_{2}(\mathrm{g})+\mathrm{O}_{2}(\mathrm{g}) \rightarrow \mathrm{NO}_{2}(\mathrm{g}) \Delta H_{\mathrm{f}}^{\circ}=?} \\ {\text { What is } \Delta H_{\mathrm{f}}^{\circ} \text { for Equation } \mathrm{b} ?}\end{array}$$

Explain what is meant by Hess's law and how it is used to determine \(\Delta H_{\mathrm{mn}}^{\circ}\)

\(\begin{array}{l}{\text { Interpret Scientific lillustrations Use the data below to draw a diagram of }} \\ {\text { standard heats of formation similar to Figure } 15.15 \text { on page } 538 \text { and use your }} \\ {\text { diagram to determine the heat of vaporization of water at } 298 \mathrm{K} \text { . }}\end{array}\) \(\begin{array}{l}{\text { Liquid water: } \Delta H_{\mathrm{f}}^{\circ}=-285.8 \mathrm{kJ} / \mathrm{mol}} \\ {\text { Gaseous water: } \Delta H_{\mathrm{f}}^{\circ}=-241.8 \mathrm{kJ} / \mathrm{mol}}\end{array}\)

Predict the sign of \(\Delta S_{\text { system }}\) for each of the following changes. \(\begin{array}{ll}{\text { a. } \mathrm{ClF}(\mathrm{g})+\mathrm{F}_{2}(\mathrm{g}) \rightarrow \mathrm{ClF}_{3}(\mathrm{g})} & {\text { c. } \mathrm{CH}_{3} \mathrm{OH}(1) \rightarrow \mathrm{CH}_{3} \mathrm{OH}(\mathrm{aq})} \\ {\text { b. } \mathrm{NH}_{3}(\mathrm{g}) \rightarrow \mathrm{NH}_{3}(\mathrm{aq})} & {\text { d. } \mathrm{C}_{10} \mathrm{H}_{8}(\mathrm{l}) \rightarrow \mathrm{C}_{10} \mathrm{H}_{8}(\mathrm{s})}\end{array}\)

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

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

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 disolve a cube of sugar in a cup of tea? Define the system, and explain your answer.

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

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

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

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

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

What quantity has the units \(\mathrm{J} /\left(\mathrm{g} \cdot^{\circ} \mathrm{C}\right) ?\)

Ethanol has a specific heat of 2.44 \(\mathrm{J} /\left(\mathrm{g} \cdot^{\circ} \mathrm{C}\right) .\) What does this mean?

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

How many joules are absorbed in a process that absorbs 0.5720 kcal?

Transportation Ethanol is being used as an additive to gasoline. The combustion of 1 mol of ethanol releases 1367 \(\mathrm{kJ}\) of energy. How many Calories are released?

To vaporize 2.00 \(\mathrm{g}\) of ammonia, 656 calories are required. How many kilojoules are required to vaporize the same mass of ammonia?

The combustion of one mole of ethanol releases 326.7 Calories of energy. How many kilojoules are released?

Metallurgy \(\mathrm{A} 25.0\) -g bolt made of an alloy absorbed 250 \(\mathrm{J}\) of heat as its temperature changed from \(25.0^{\circ} \mathrm{C}\) to \(78.0^{\circ} \mathrm{C} .\) What is the specific heat of the alloy?

Why is a foam cup used in a student calorimeter rather than a typical glass beaker?

Give two examples of chemical systems and define the universe in terms of those examples.

The enthalpy change for a reaction, \(\Delta H\) , is negative. What does this indicate about the chemical potential energy of the system before and after the reaction?

What is the sign of \(\Delta H\) for an exothermic reaction? An endothermic reaction?

How many joules of heat are lost by 3580 \(\mathrm{kg}\) of granite as it cools from \(41.2^{\circ} \mathrm{C}\) to \(-12.9^{\circ} \mathrm{C}\) ? The specific heat of granite is 0.803 \(\mathrm{J} /\left(\mathrm{g} \cdot^{\circ} \mathrm{C}\right)\)

Swimming Pool A swimming pool measuring 20.0 \(\mathrm{m} \times 12.5 \mathrm{m}\) is filled with water to a depth of 3.75 \(\mathrm{m} .\) If the initial temperature is \(18.4^{\circ} \mathrm{C}\) , how much heat must be added to the water to raise its temperature to \(29.0^{\circ} \mathrm{C} ?\) Assume that the density of water is 1.000 \(\mathrm{g} / \mathrm{mL}\) .

Food Preparation When 10.2 \(\mathrm{g}\) of canola oil at \(25.0^{\circ} \mathrm{C}\) is placed in a wok, 3.34 \(\mathrm{kJ}\) of heat is required to heat it to a temperature of \(196.4^{\circ} \mathrm{C}\) . What is the specific heat of the canola oil?