17.13.8: Chapter 9

1.

The temperature of 1 gram of burning wood is approximately the same for both a match and a bonfire. This is an intensive property and depends on the material (wood). However, the overall amount of produced heat depends on the amount of material; this is an extensive property. The amount of wood in a bonfire is much greater than that in a match; the total amount of produced heat is also much greater, which is why we can sit around a bonfire to stay warm, but a match would not provide enough heat to keep us from getting cold.

3.

Heat capacity refers to the heat required to raise the temperature of the mass of the substance 1 degree; specific heat refers to the heat required to raise the temperature of 1 gram of the substance 1 degree. Thus, heat capacity is an extensive property, and specific heat is an intensive one.

5.

(a) 47.6 J/°C; 11.38 cal °C⁻¹; (b) 407 J/°C; 97.3 cal °C⁻¹

7.

1310 J; 313 cal

9.

7.15 °C

11.

(a) 0.390 J/g °C; (b) Copper is a likely candidate.

13.

We assume that the density of water is 1.0 g/cm³(1 g/mL) and that it takes as much energy to keep the water at 85 °F as to heat it from 72 °F to 85 °F. We also assume that only the water is going to be heated. Energy required = 7.47 kWh

15.

lesser; more heat would be lost to the coffee cup and the environment and so ΔT for the water would be lesser and the calculated q would be lesser

17.

greater, since taking the calorimeter’s heat capacity into account will compensate for the thermal energy transferred to the solution from the calorimeter; this approach includes the calorimeter itself, along with the solution, as “surroundings”: q_rxn = −(q_solution + q_calorimeter); since both q_solution and q_calorimeter are negative, including the latter term (q_rxn) will yield a greater value for the heat of the dissolution

19.

The temperature of the coffee will drop 1 degree.

21.

5.7 $\times$ 10² kJ

23.

38.5 °C

25.

−2.2 kJ; The heat produced shows that the reaction is exothermic.

27.

1.4 kJ

29.

22.6. Since the mass and the heat capacity of the solution is approximately equal to that of the water, the two-fold increase in the amount of water leads to a two-fold decrease of the temperature change.

31.

11.7 kJ

33.

30%

35.

0.24 g

37.

1.4 $\times$ 10² Calories

39.

The enthalpy change of the indicated reaction is for exactly 1 mol HCL and 1 mol NaOH; the heat in the example is produced by 0.0500 mol HCl and 0.0500 mol NaOH.

41.

25 kJ mol⁻¹

43.

81 kJ mol⁻¹

45.

5204.4 kJ

47.

1.83 $\times$ 10⁻² mol

49.

–802 kJ mol⁻¹

51.

15.5 kJ/ºC

53.

7.43 g

55.

Yes.

57.

459.6 kJ

59.

−494 kJ/mol

61.

44.01 kJ/mol

63.

−394 kJ

65.

265 kJ

67.

90.3 kJ/mol

69.

(a) −1615.0 kJ mol⁻¹; (b) −484.3 kJ mol⁻¹; (c) 164.2 kJ; (d) −232.1 kJ

71.

−54.04 kJ mol⁻¹

73.

−2660 kJ mol⁻¹

75.

–66.4 kJ

77.

−122.8 kJ

79.

3.7 kg

81.

On the assumption that the best rocket fuel is the one that gives off the most heat, B₂H₆ is the prime candidate.

83.

−88.2 kJ

85.

(a) ${\text{C}}_3{\text{H}}_8(g)+5{\text{O}}_2(g)⟶3{\text{CO}}_2(g)+4{\text{H}}_2\text{O}\left(l\right);$ (b) 330 L air; (c) −104.5 kJ mol⁻¹; (d) 75.4 °C

88.

(a) −114 kJ;
(b) 30 kJ;
(c) −1055 kJ

91.

The specific average bond distance is the distance with the lowest energy. At distances less than the bond distance, the positive charges on the two nuclei repel each other, and the overall energy increases.

92.

The greater bond energy is in the figure on the left. It is the more stable form.

94.

$\begin{array}{}\\ \text{HCl}\left(g\right)⟶\frac{1}{2}{\text{H}}_2(g)+\frac{1}{2}{\text{Cl}}_2(g)\text{Δ}{H}_1^{}{°}_{}^{}=\text{−}\text{Δ}{H}_{\text{f}}^{}{\left[\text{HCl}\left(g\right)\right]}_{°}^{}\\ \frac{1}{2}{\text{H}}_2(g)⟶\text{H}\left(g\right)\text{Δ}{H}_2^{}{°}_{}^{}=\text{Δ}{H}_{\text{f}}^{}{\left[\text{H}\left(g\right)\right]}_{°}^{}\\ \underset{¯}{\frac{1}{2}{\text{Cl}}_2(g)⟶\text{Cl}\left(g\right)\text{Δ}{H}_3^{}{°}_{}^{}=\text{Δ}{H}_{\text{f}}^{}{\left[\text{Cl}\left(g\right)\right]}_{°}^{}}\\ \text{HCl}\left(g\right)⟶\text{H}\left(g\right)+\text{Cl}\left(g\right)\text{Δ}{H}_{298}^{}{°}_{}^{}=\text{Δ}{H}_1^{}{°}_{}^{}+\text{Δ}{H}_2^{}{°}_{}^{}+\text{Δ}{H}_3^{}{°}_{}^{}\end{array}$
$\begin{array}{ll}{D}_{\text{HCl}}=\text{Δ}{H}_{298}^{}{°}_{}^{}& =\text{Δ}{H}_{\text{f}}^{}{[\text{HCl}(g)]}_{°}^{}+\text{Δ}{H}_{\text{f}}^{}{[\text{H}(g)]}_{°}^{}+\text{Δ}{H}_{\text{f}}^{}{[\text{Cl}(g)]}_{°}^{}\\ \\ \hfill & =-(\mathrm{-92.307}\text{kJ})+217.97\text{kJ}+121.3\text{kJ}\\ & =431.6\text{kJ}\end{array}$

96.

The S–F bond in SF₄ is stronger.

98.

The C–C single bonds are longest.

100.

(a) When two electrons are removed from the valence shell, the Ca radius loses the outermost energy level and reverts to the lower n = 3 level, which is much smaller in radius. (b) The +2 charge on calcium pulls the oxygen much closer compared with K, thereby increasing the lattice energy relative to a less charged ion. (c) Removal of the 4s electron in Ca requires more energy than removal of the 4s electron in K because of the stronger attraction of the nucleus and the extra energy required to break the pairing of the electrons. The second ionization energy for K requires that an electron be removed from a lower energy level, where the attraction is much stronger from the nucleus for the electron. In addition, energy is required to unpair two electrons in a full orbital. For Ca, the second ionization potential requires removing only a lone electron in the exposed outer energy level. (d) In Al, the removed electron is relatively unprotected and unpaired in a p orbital. The higher energy for Mg mainly reflects the unpairing of the 2s electron.

102.

(d)

104.

4008 kJ/mol; both ions in MgO have twice the charge of the ions in LiF; the bond length is very similar and both have the same structure; a quadrupling of the energy is expected based on the equation for lattice energy

106.

(a) Na₂O; Na⁺ has a smaller radius than K⁺; (b) BaS; Ba has a larger charge than K; (c) BaS; Ba and S have larger charges; (d) BaS; S has a larger charge

108.

(e)