Chapter 5

Complete combustion of 1 mol of acetone $\left({\mathrm{C}}_3{\mathrm{H}}_6\mathrm{O}\right)$ liberates $1790\mathrm{kJ}:$ $$\begin{array}{rl}{\mathrm{C}}_3{\mathrm{H}}_6\mathrm{O}(l)+4{\mathrm{O}}_2(g)⟶3{\mathrm{CO}}_2(g)+3{\mathrm{H}}_2\mathrm{O}(l)& \\ \mathrm{\Delta }{H}^{\circ }=& -1790\mathrm{kJ}\end{array}$$ Using this information together with the standard enthalpies of formation of ${\mathrm{O}}_2(g),{\mathrm{CO}}_2(g),$ and ${\mathrm{H}}_2\mathrm{O}(l)$ from Appendix C, calculate the standard enthalpy of formation of acetone.

Calcium carbide $\left({\mathrm{CaC}}_2\right)$ reacts with water to form acetylene $\left({\mathrm{C}}_2{\mathrm{H}}_2\right)$ and $\mathrm{Ca}(\mathrm{OH}{)}_2$. From the following enthalpy of reaction data and data in Appendix C, calculate $\mathrm{\Delta }{H}_f^{\circ }$ for ${\mathrm{CaC}}_2(s);$ $$\begin{array}{rl}{\mathrm{CaC}}_2(s)+2{\mathrm{H}}_2\mathrm{O}(l)⟶\mathrm{Ca}(\mathrm{OH}{)}_2(s)+{\mathrm{C}}_2{\mathrm{H}}_2(g)& \\ \mathrm{\Delta }{H}^{\circ }=& -127.2\mathrm{kJ}\end{array}$$

Ethanol $\left({\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}\right)$ is blended with gasoline as an automobile fuel. (a) Write a balanced equation for the combustion of liquid ethanol in air. (b) Calculate the standard enthalpy change for the reaction, assuming ${\mathrm{H}}_2\mathrm{O}(g)$ as a product. (c) Calculate the heat produced per liter of ethanol by combustion of ethanol under constant pressure. Ethanol has a density of $0.789\mathrm{g}/\mathrm{mL}$. (d) Calculate the mass of ${\mathrm{CO}}_2$ produced per kJ of heat emitted.

Without doing any calculations, predict the sign of $\mathrm{\Delta }H$ for each of the following reactions: (a) $\mathrm{NaCl}(s)⟶{\mathrm{Na}}^{+}(g)+{\mathrm{Cl}}^{-}(\mathrm{g})$ (b) $2\mathrm{H}(g)⟶{\mathrm{H}}_2(g)$ (c) $\mathrm{Na}(g)⟶{\mathrm{Na}}^{+}(g)+{\mathrm{e}}^{-}$ (d) ${\mathrm{I}}_2(s)⟶{\mathrm{I}}_2(l)$

Without doing any calculations, predict the sign of $\mathrm{\Delta }H$ for each of the following reactions: (a) $2{\mathrm{NO}}_2(g)⟶{\mathrm{N}}_2{\mathrm{O}}_4(g)$ (b) $2\mathrm{F}(g)⟶{\mathrm{F}}_2(g)$ (c) ${\mathrm{Mg}}^{2+}(g)+2{\mathrm{Cl}}^{-}(g)⟶{\mathrm{MgCl}}_2(s)$ (d) $\mathrm{HBr}(g)⟶\mathrm{H}(g)+\mathrm{Br}(g)$

Use bond enthalpies in Table 5.4 to estimate $\mathrm{\Delta }H$ for each of the following reactions: (a) $\mathrm{H}-\mathrm{H}(g)+\mathrm{Br}-\mathrm{Br}(g)⟶2\mathrm{H}-\mathrm{Br}(g)$ (b)

(a) Use enthalpies of formation given in Appendix C to calculate $\mathrm{\Delta }H$ for the reaction ${\mathrm{Br}}_2(g)⟶2\mathrm{Br}(g)$, and use this value to estimate the bond enthalpy $D(\mathrm{Br}-\mathrm{Br})$. (b) How large is the difference between the value calculated in part (a) and the value given in Table 5.4 ?

(a) What is meant by the term fuel value? (b) Which is a greater source of energy as food, $5\mathrm{g}$ of fat or $9\mathrm{g}$ of carbohydrate? (c) The metabolism of glucose produces ${\mathrm{CO}}_2(g)$ and ${\mathrm{H}}_2\mathrm{O}(l)$. How does the human body expel these reaction products?

(a) Which releases the most energy when metabolized, $1\mathrm{g}$ of carbohydrates or $1\mathrm{g}$ of fat? (b) A particular chip snack food is composed of $12\mathrm{\%}$ protein, $14\mathrm{\%}$ fat, and the rest carbohydrate. What percentage of the calorie content of this food is fat? (c) How many grams of protein provide the same fuel value as $25\mathrm{g}$ of fat?

(a) A serving of a particular ready-to-serve brown \& wild rice meal contains $4.5\mathrm{g}$ fat, $42\mathrm{g}$ carbohydrate, and $4.0\mathrm{g}$ protein. Estimate the number of calories in a serving. (b) According to its nutrition label, the same meal also contains $140\mathrm{mg}$ of potassium ions. Do you think the potassium contributes to the caloric content of the food?