Chapter 7

What is energy? What is work? List some examples of each.

What is kinetic energy? What is potential energy? List some examples of each.

State the law of conservation of energy. How does it relate to energy exchanges between a thermodynamic system and its surroundings?

What is the SI unit of energy? List some other common units of energy.

State the first law of thermodynamics. What are its implications?

A friend claims to have constructed a machine that creates electricity but requires no energy input. Explain why you should be suspicious of your friend's claim.

What is a state function? List some examples of state functions.

What is internal energy? Is internal energy a state function?

If energy flows out of a chemical system and into the surroundings, what is the sign of \(\Delta E_{\text {system }} ?\)

If the internal energy of the products of a reaction is higher than the internal energy of the reactants, what is the sign of \(\Delta E\) for the reaction? In which direction does energy flow?

What is heat? Explain the difference between heat and temperature.

What is heat capacity? Explain the difference between heat capacity and specific heat capacity.

Explain how the high specific heat capacity of water can affect the weather in coastal regions.

If two objects, A and B, of different temperature come into direct contact, what is the relationship between the heat lost by one object and the heat gained by the other? What is the relationship between the temperature changes of the two objects? (Assume that the two objects do not lose any heat to anything else.)

What is pressure-volume work? How is it calculated?

What is calorimetry? Explain the difference between a coffeecup calorimeter and a bomb calorimeter. What is each designed to measure?

Explain the difference between an exothermic and an endother- mic reaction. Give the sign of \(\Delta H\) for each type of reaction.

From a molecular viewpoint, where does the energy emitted in an exothermic chemical reaction come from? Why does the reaction mixture undergo an increase in temperature even though energy is emitted?

From a molecular viewpoint, where does the energy absorbed in an endothermic chemical reaction go? Why does the reaction mixture undergo a decrease in temperature even though energy is absorbed?

Is the change in enthalpy for a reaction an extensive property? Explain the relationship between \(\Delta H\) for a reaction and the amounts of reactants and products that undergo reaction.

What is Hess's law? Why is it useful?

What is a standard state? What is the standard enthalpy change for a reaction?

What is the standard enthalpy of formation for a compound? For a pure element in its standard state?

Convert between energy units. MISSED THIS? a. \(534 \mathrm{kWh}\) to \(\mathrm{J}\) b. \(215 \mathrm{~kJ}\) to \(\mathrm{Cal}\) c. 567 Cal to \(J\) d. \(2.85 \times 10^{3} \mathrm{~J}\) to cal

Convert between energy units. a. 231 cal to kJ b. \(132 \times 10^{4} \mathrm{~kJ}\) to \(\mathrm{kcal}\) c. \(4.99 \times 10^{3} \mathrm{~kJ}\) to \(\mathrm{kWh}\) d. \(2.88 \times 10^{4} \mathrm{~J}\) to \(\mathrm{Cal}\)

Which statement is true of the internal energy of a system and its surroundings during an energy exchange with a negative \(\Delta E_{\text {sys }} ?\) a. The internal energy of the system increases and the internal energy of the surroundings decreases. b. The internal energy of both the system and the surroundings increases. c. The internal energy of both the system and the surroundings decreases. d. The internal energy of the system decreases and the internal energy of the surroundings increases.

During an energy exchange, a chemical system absorbs energy from its surroundings. What is the sign of \(\Delta E_{\text {sys }}\) for this process? Explain.

Identify each energy exchange as primarily heat or work and determine whether the sign of \(\Delta E\) is positive or negative for the system. MISSED THIS? a. Sweat evaporates from skin, cooling the skin. (The evaporating sweat is the system.) b. A balloon expands against an external pressure. (The contents of the balloon is the system.) c. An aqueous chemical reaction mixture is warmed with an external flame. (The reaction mixture is the system.)

Identify each energy exchange as primarily heat or work and determine whether the sign of \(\Delta E\) is positive or negative for the system. a. A rolling billiard ball collides with another billiard ball. The first billiard ball (defined as the system) stops rolling after the collision. b. A book falls to the floor. (The book is the system.) c. A father pushes his daughter on a swing. (The daughter and the swing are the system.)

A system releases \(622 \mathrm{~kJ}\) of heat and does \(105 \mathrm{~kJ}\) of work on the surroundings. What is the change in internal energy of the system?

A system absorbs \(196 \mathrm{~kJ}\) of heat, and the surroundings do \(117 \mathrm{~kJ}\) of work on the system. What is the change in internal energy of the system?

The gas in a piston (defined as the system) warms and absorbs \(655 \mathrm{~J}\) of heat. The expansion performs \(344 \mathrm{~J}\) of work on the surroundings. What is the change in internal energy for the system?

The air in an inflated balloon (defined as the system) warms over a toaster and absorbs \(115 \mathrm{~J}\) of heat. As it expands, it does \(77 \mathrm{~kJ}\) of work. What is the change in internal energy for the system?

A kilogram of aluminum metal and a kilogram of water are each warmed to \(75^{\circ} \mathrm{C}\) and placed in two identical insulated containers. One hour later, the two containers are opened, and the temperature of each substance is measured. The aluminum has cooled to \(35^{\circ} \mathrm{C},\) while the water has cooled only to \(66{ }^{\circ} \mathrm{C}\). Explain this difference.

How much heat is required to warm \(1.50 \mathrm{~L}\) of water from \(25.0^{\circ} \mathrm{C}\) to \(100.0^{\circ} \mathrm{C}\) ? (Assume a density of \(1.0 \mathrm{~g} / \mathrm{mL}\) for the water.)

When 1 mol of a fuel burns at constant pressure, it produces \(3452 \mathrm{~kJ}\) of heat and does \(11 \mathrm{~kJ}\) of work. What are \(\Delta E\) and \(\Delta H\) for the combustion of the fuel?

The change in internal energy for the combustion of \(1.0 \mathrm{~mol}\) of octane at a pressure of \(1.0 \mathrm{~atm}\) is \(5084.3 \mathrm{~kJ}\). If the change in enthalpy is 5074.1 kJ, how much work is done during the combustion?

Determine whether each process is exothermic or endothermic and indicate the sign of \(\Delta H\). a. natural gas burning on a stove b. isopropyl alcohol evaporating from skin c. water condensing from steam

Determine whether each process is exothermic or endothermic and indicate the sign of \(\Delta H\). a. dry ice evaporating b. a sparkler burning c. the reaction that occurs in a chemical cold pack used to ice athletic injuries

What mass of natural gas ( \(\mathrm{CH}_{4}\) ) must burn to emit \(267 \mathrm{~kJ}\) of heat? $$\begin{array}{r}\mathrm{CH}_{4}(g)+2 \mathrm{O}_{2}(g) \longrightarrow \mathrm{CO}_{2}(g)+2 \mathrm{H}_{2} \mathrm{O}(g) \\\\\Delta H_{\mathrm{rxn}}^{\circ}=-802.3 \mathrm{~kJ}\end{array}$$

Charcoal is primarily carbon. Determine the mass of \(\mathrm{CO}_{2}\) produced by burning enough carbon (in the form of charcoal) to produce \(5.00 \times 10^{2} \mathrm{~kJ}\) of heat. $$\mathrm{C}(s)+\mathrm{O}_{2}(g) \longrightarrow \mathrm{CO}_{2}(g) \quad \Delta H_{\mathrm{rxn}}^{\circ}=-393.5\mathrm{~kJ}$$

In order to obtain the largest possible amount of heat from a chemical reaction in which there is a large increase in the number of moles of gas, should you carry out the reaction under conditions of constant volume or constant pressure? Explain.

When \(0.514 \mathrm{~g}\) of biphenyl \(\left(\mathrm{C}_{12} \mathrm{H}_{10}\right)\) undergoes combustion in a bomb calorimeter, the temperature rises from \(25.8^{\circ} \mathrm{C}\) to \(29.4^{\circ} \mathrm{C}\). Find \(\Delta E_{\mathrm{rxn}}\) for the combustion of biphenyl in \(\mathrm{kJ} / \mathrm{mol}\) biphenyl. The heat capacity of the bomb calorimeter, determined in a separate experiment, is \(5.86 \mathrm{~kJ} /{ }^{\circ} \mathrm{C}\).

Instant cold packs used to ice athletic injuries on the field contain ammonium nitrate and water separated by a thin plastic divider. When the divider is broken, the ammonium nitrate dissolves according to the endothermic reaction: $$\mathrm{NH}_{4} \mathrm{NO}_{3}(s) \longrightarrow \mathrm{NH}_{4}^{+}(a q)+\mathrm{NO}_{3}^{-}(a q)$$ In order to measure the enthalpy change for this reaction, \(1.25 \mathrm{~g}\) of \(\mathrm{NH}_{4} \mathrm{NO}_{3}\) is dissolved in enough water to make \(25.0 \mathrm{~mL}\) of solution. The initial temperature is \(25.8^{\circ} \mathrm{C}\) and the final temperature (after the solid dissolves) is \(21.9^{\circ} \mathrm{C}\). Calculate the change in enthalpy for the reaction in kJ. (Use \(1.0 \mathrm{~g} / \mathrm{mL}\) as the density of the solution and \(4.18 \mathrm{~J} / \mathrm{g} \cdot{ }^{\circ} \mathrm{C}\) as the specific heat capacity.)

For each generic reaction, determine the value of \(\Delta H_{2}\) in terms of \(\Delta H_{1} .\) a. \(\mathrm{A}+\mathrm{B} \longrightarrow 2 \mathrm{C} \quad \Delta H_{1}\) \(2 \mathrm{C} \longrightarrow \mathrm{A}+\mathrm{B} \quad \Delta H_{2}=?\) b. \(A+1 / 2 B \longrightarrow C\) \(\Delta H_{1}\) \(2 \mathrm{~A}+\mathrm{B} \longrightarrow 2 \mathrm{C} \quad \Delta H_{2}=?\) \(\begin{array}{ll}\text { c. } A \longrightarrow B+2 C & \Delta H_{1}\end{array}\) \(1 / 2 \mathrm{~B}+\mathrm{C} \longrightarrow 1 / 2 \mathrm{~A} \quad \Delta H_{2}=?\)

Consider the generic reaction: $$\mathrm{A}+2 \mathrm{~B} \longrightarrow \mathrm{C}+3 \mathrm{D} \quad \Delta H=155 \mathrm{~kJ}$$ Determine the value of \(\Delta H\) for each related reaction. a. \(3 \mathrm{~A}+6 \mathrm{~B} \longrightarrow 3 \mathrm{C}+9 \mathrm{D}\) b. \(C+3 D \longrightarrow A+2 B\) c. \(1 / 2 \mathrm{C}+{ }^{3} /{ }_{2} \mathrm{D} \longrightarrow{ }^{1} /{ }_{2} \mathrm{~A}+\mathrm{B}\)

Calculate \(\Delta H_{\mathrm{rxn}}\) for the reaction: $$ \mathrm{Fe}_{2} \mathrm{O}_{3}(s)+3 \mathrm{CO}(g) \longrightarrow 2 \mathrm{Fe}(s)+3 \mathrm{CO}_{2}(g) $$ Use the following reactions and given \(\Delta H^{\prime}\) s: \(2 \mathrm{Fe}(s)+3 / 2 \mathrm{O}_{2}(g) \longrightarrow \mathrm{Fe}_{2} \mathrm{O}_{3}(s) \quad \Delta H=-824.2 \mathrm{~kJ}\) $$\mathrm{CO}(g)+{ }^{1} /{ }_{2} \mathrm{O}_{2}(g) \longrightarrow \mathrm{CO}_{2}(g) \quad \Delta H=-282.7\mathrm{~kJ}$$

Calculate \(\Delta H_{\mathrm{rxn}}\) for the reaction: $$ \mathrm{CaO}(s)+\mathrm{CO}_{2}(g) \longrightarrow \mathrm{CaCO}_{3}(s) $$ Use the following reactions and given \(\Delta H\) 's: \(\mathrm{Ca}(s)+\mathrm{CO}_{2}(g)+{ }^{1} /{ }_{2} \mathrm{O}_{2}(g) \longrightarrow \mathrm{CaCO}_{3}(s) \quad \Delta H=-812.8 \mathrm{~kJ}\) $$2 \mathrm{Ca}(s)+\mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{CaO}(s) \quad \Delta H=-1269.8 \mathrm{~kJ}$$

Calculate \(\Delta H_{\mathrm{rxn}}\) for the reaction: $$5 \mathrm{C}(s)+6 \mathrm{H}_{2}(g) \longrightarrow \mathrm{C}_{5} \mathrm{H}_{12}(l)$$Use the following reactions and given \(\Delta H^{\prime}\) s:$$ \begin{array}{l} \mathrm{C}_{5} \mathrm{H}_{12}(l)+8 \mathrm{O}_{2}(g) \longrightarrow 5 \mathrm{CO}_{2}(g)+6 \mathrm{H}_{2} \mathrm{O}(g) \\\\\qquad \begin{array}{ll}\Delta H=-3244.8 \mathrm{~kJ} \\\\\mathrm{C}(s)+\mathrm{O}_{2}(g) \longrightarrow \mathrm{CO}_{2}(g) & \Delta H=-393.5 \mathrm{~kJ} \\ 2 \mathrm{H}_{2}(g)+\mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{H}_{2} \mathrm{O}(g) & \Delta H=-483.5\mathrm{~kJ}\end{array}\end{array}$$

Use standard enthalpies of formation to calculate \(\Delta H_{\mathrm{rxn}}^{\circ}\) for each reaction. a. \(\mathrm{C}_{2} \mathrm{H}_{4}(g)+\mathrm{H}_{2}(g) \longrightarrow \mathrm{C}_{2} \mathrm{H}_{6}(g)\) b. \(\mathrm{CO}(g)+\mathrm{H}_{2} \mathrm{O}(g) \longrightarrow \mathrm{H}_{2}(g)+\mathrm{CO}_{2}(g)\) c. \(3 \mathrm{NO}_{2}(g)+\mathrm{H}_{2} \mathrm{O}(l) \longrightarrow 2 \mathrm{HNO}_{3}(a q)+\mathrm{NO}(g)\) d. \(\mathrm{Cr}_{2} \mathrm{O}_{3}(s)+3 \mathrm{CO}(g) \longrightarrow 2 \mathrm{Cr}(s)+3 \mathrm{CO}_{2}(g)\)