Chapter 15

Alloys When a 58.8 -g piece of hot alloy is placed in 125 g of cold water in a calorimeter, the temperature of the alloy decreases by \(106.1^{\circ} \mathrm{C}\) , while the temperature of the water increases by \(10.5^{\circ} \mathrm{C}\) . What is the specific heat of the alloy?

Write the sign of \(\Delta H_{\text { system for each of the following }}\) changes in physical state. a. \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}(\mathrm{s}) \rightarrow \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}(1)\) b. \(\mathrm{H}_{2} \mathrm{O}(\mathrm{g}) \rightarrow \mathrm{H}_{2} \mathrm{O}(\mathrm{l})\) c. \(\mathrm{CH}_{3} \mathrm{OH}(1) \rightarrow \mathrm{CH}_{3} \mathrm{OH}(\mathrm{g})\) d. \(\mathrm{NH}_{3}(\mathrm{l}) \rightarrow \mathrm{NH}_{3}(\mathrm{s})\)

The molar enthalpy of fusion of methanol is 3.22 \(\mathrm{k} / \mathrm{mol}\) What does this mean?

Explain how perspiration can help cool your body.

Grilling what mass of propane \(\left(\mathrm{C}_{3} \mathrm{H}_{8}\right)\) must be burned in a barbecue grill to release 4560 \(\mathrm{kJ}\) of heat? The \(\Delta H_{\mathrm{comb}}\) of propane is \(-2219 \mathrm{kJ} / \mathrm{mol}\)

Heating with Coal How much heat is liberated when 5.00 kg of coal is burned if the coal is 96.2\(\%\) carbon by mass and the other materials in the coal do not react? \(\Delta H_{\mathrm{comb}}\) of carbon is \(-394 \mathrm{kJ} / \mathrm{mol}\)

A sample of ammonia \(\left(\Delta H_{\text { solid }}=-5.66 \mathrm{k}\right] / \mathrm{mol} )\) liber- ates 5.66 \(\mathrm{kJ}\) of heat as it solidifies at its melting point. What is the mass of the sample?

For a given compound, what does the standard enthalpy of formation describe?

How does \(\Delta H\) for a thermochemical equation change when the amounts of all substances are tripled and the equation is reversed?

Use Hess' law and the following thermochemical equations to produce the thermochemical equation for the reaction \(\mathrm{C}(\mathrm{s},\) diamond) \(\rightarrow \mathrm{C}(\mathrm{s},\) graphite).What is \(\Delta H\) for the reaction? $$ \begin{array}{ll}{\text { a. } \mathrm{C}(\mathrm{s}, \text { graphite })+\mathrm{O}_{2}(\mathrm{g}) \rightarrow \mathrm{CO}_{2}(\mathrm{g})} & {\Delta H=-394 \mathrm{kJ}} \\ {\text { b. } \mathrm{C}(\mathrm{s}, \text { diamond })+\mathrm{O}_{2}(\mathrm{g}) \rightarrow \mathrm{CO}_{2}(\mathrm{g})} & {\Delta H=-396 \mathrm{kJ}}\end{array} $$

Use Hess' law and the changes in enthalpy for the following two generic reactions to calculate \(\Delta H\) for the reaction \(2 \mathrm{A}+\mathrm{B}_{2} \mathrm{C}_{3} \rightarrow 2 \mathrm{B}+\mathrm{A}_{2} \mathrm{C}_{3}\) \(2 \mathrm{A}+\frac{3}{2} \mathrm{C}_{2} \rightarrow \mathrm{A}_{2} \mathrm{C}_{3} \quad \Delta \mathrm{H}=-1874 \mathrm{kJ}\) \(2 \mathrm{B}+\frac{3}{2} \mathrm{C}_{2} \rightarrow \mathrm{B}_{2} \mathrm{C}_{3} \quad \Delta H=-285 \mathrm{kJ}\)

Under what conditions is an endothermic chemical reaction in which the entropy of the system increases likely to be spontaneous?

Predict how the entropy of the system changes for the reaction \(\mathrm{CaCO}_{3}(\mathrm{s}) \rightarrow \mathrm{CaO}(\mathrm{s})+\mathrm{CO}_{2}(\mathrm{g}) .\) Explain.

Explain how an exothermic reaction changes the entro- py of the surroundings. Does the enthalpy change for such a reaction increase or decrease \(\Delta G_{\text { system }} ?\) Explain.

Calculate \(\Delta G_{\text { system }}\) for each process, and state whether the process is spontaneous or nonspontaneous. a. \(\Delta H_{\text { system }}=145 \mathrm{kJ}, T=293 \mathrm{K}, \Delta S_{\mathrm{system}}=195 \mathrm{J} / \mathrm{K}\) b. \(\Delta H_{\text { system }}=-232 \mathrm{kJ}, T=273 \mathrm{K}, \Delta S_{\mathrm{system}}=138 \mathrm{J} / \mathrm{K}\) c. \(\Delta H_{\mathrm{system}}=-15.9 \mathrm{kJ}, T=373 \mathrm{K}, \Delta S_{\mathrm{system}}=-268 \mathrm{J} / \mathrm{K}\)

Calculate the temperature at which \(\Delta G_{\text { system }}=0\) if \(\Delta H_{\text { system }}=4.88 \mathrm{kJ}\) and \(\Delta S_{\text { system }}=55.2 \mathrm{J} / \mathrm{K}\)

For the change \(\mathrm{H}_{2} \mathrm{O}(1) \rightarrow \mathrm{H}_{2} \mathrm{O}(\mathrm{g}), \Delta G_{\mathrm{sssstem}}^{\circ}\) is 8.557 \(\mathrm{kJ}\) and \(\Delta H_{\text { system is } 44.01 \mathrm{kJ} . \text { What is } \Delta S_{\text { system }}^{\circ}}\) for the change?

Is the following reaction to convert copper(II) sulfide to copper(II) sulfate spontaneous under standard conditions $$ \operatorname{Cus}(\mathrm{s})+2 \mathrm{O}_{2}(\mathrm{g}) \rightarrow \mathrm{CuSO}_{4}(\mathrm{s}) . \Delta \mathrm{H}_{\mathrm{rxn}}^{\circ}=-718.3 $$ $$ \mathrm{k} \mathrm{J}, \text { and } \Delta S_{\mathrm{rxn}}^{\circ}=-368 \mathrm{J} / \mathrm{K} $$. Explain.

Calculate the temperature at which \(\Delta G_{\text { system }}=-34.7 \mathrm{kJ}\) if \(\Delta H_{\text { system }}=-28.8 \mathrm{kJ}\) and \(\Delta S_{\text { system }}=22.2 \mathrm{J} / \mathrm{K}\) .

Bicycling Describe the energy conversions that occur when a bicyclist coasts down a long grade, then struggles to ascend a steep grade.

Hiking Imagine that on a cold day you are planning to take a thermos of hot soup with you on a hike. Explain why you might fill the thermos with hot water first before filling it with the hot soup.

Predict which of the compounds, methane gas \(\left(\mathrm{CH}_{4}\) or methanal vapor \(\left(\mathrm{CH}_{2} \mathrm{O}\right),\) has the greater molar \right. Write and compare the balanced chemical equations for the two combustion reactions.)

Why is it necessary to perform repeated experiments in order to support a hypothesis? (Chapter 1\()\)

Phosphorus has the atomic number 15 and an atomic mass of 31 amu. How many protons, neutrons, and electrons are in a neutral phosphorus atom? (Chapter 4)

What element has the electron configuration \([\) Ar \(] 4 s^{1} 3 \mathrm{d}^{5} ?(\)Chapter 5\()\)

Determine the molar mass for the foloowing compounds. (Chapter 10\()\) \begin{equation}\begin{array}{l}{\text { a. } \operatorname{Co}\left(\mathrm{NO}_{3}\right)_{2} \cdot 6 \mathrm{H}_{2} \mathrm{O}} \\ {\text { b. } \mathrm{Fe}(\mathrm{OH})_{3}}\end{array}\end{equation}

What is the molarity of a solution made by dissolving 25.0 g of sodium thiocyanate (NaSCN) in enough water to make 500 mL of solution? (Chapter 14)

List three colligative properties of solutions. (Chapter 14)

Alternate Fuels Use library and Internet sources to explain how hydrogen might be produced, transported, and used as a fuel for automobiles. Summarize the benefits and drawbacks of using hydrogen as an alternative fuel for internal combustion engines.

Wind Power Research the use of wind as a source of electrical power. Explain the possible benefits, disadvantages, and limitations of its use.