Deck 2: The First Law of Thermodynamics

Question 1

Calculate the expansion work done on the system when exactly 1 mol of solid ammonium chloride, NH₄Cl, decomposes completely to yield gaseous ammonia, NH₃ and hydrogen chloride, HCl at a temperature of 1250 K. Treat the expansion as irreversible and the gases formed as perfect.

A) -15.4 kJ
B) -4.96 kJ
C) -16.6 kJ
D) -20.8 kJ

Answer 1

D

Question 2

Calculate the heat transferred to the system when 1.00 mol of a perfect gas expands reversibly at a constant temperature of 25°C so that its volume doubles.

A) -144 J
B) -746 J
C) -1.72 kJ mol^-1
D) 2.48 kJ

Answer 2

C

Question 3

The constant pressure molar heat capacity of zinc is 25.40 J K^-1 mol^-1 at 298 K. Calculate the constant pressure specific heat capacity of zinc at this temperature.

A) 1.660 kJ K^-1 kg^-1
B) 388.6 J K^-1 kg^-1
C) 17.09 J K^-1 kg^-1
D) 33.71 J K^-1 kg^-1

Answer 3

B

Question 4

The molar heat capacity of solid aluminium is 24.4 J K^-1 mol^-1 at 25°C. Calculate the change in internal energy when 1.00 mol of solid aluminium is heated from a temperature of 20°C to 30°C.

A) 244 J
B) 24.4 J
C) 171 J
D) 327 J

Question 5

The constant pressure molar heat capacity of ammonia, NH₃, has been found to vary with temperature according to the relation
C_p.m / J K^-1 mol^-1=a+bT+c/T²
With a = 29.73, b = 25.1  10^-3 K^-1 and c = -1.55  10⁵ K². Calculate the value of the constant pressure molar heat capacity at 25.0 °C.

A) 217.6 J K^-1 mol^-1
B) 29.73 J K^-1 mol^-1
C) 35.47 J K^-1 mol^-1
D) 657.2 J K^-1 mol^-1

Question 6

In the calibration step of a thermochemistry experiment, a current of 117 mA, from a 24.0 V source was allowed to flow through the electrical heater for 247 s and was found to result in an increase in the temperature of the calorimeter and its contents of +1.25 K. Calculate the heat capacity of the calorimeter and its contents.

A) 694 J K^-1
B) 277 J K^-1
C) 555 J K^-1
D) 867 J K^-1

Question 7

The constant pressure molar heat capacity of methane, CH₄, is 35.31 J K^-1 mol^-1 at temperatures close to 298 K. Calculate the enthalpy change when 2.00 mol of methane is heated from a temperature of 278 K to 318 K.

A) 21.2 kJ
B) 353 J
C) 1.41 kJ
D) 2.83 kJ

Question 8

The constant pressure molar heat capacity of argon is 20.79 J K^-1 mol^-1 at 298 K. Predict the value of the constant volume molar heat capacity of argon at this temperature.

A) 29.11 J K^-1 mol^-1
B) 8.31 J K^-1 mol^-1
C) 4.16 J K^-1 mol^-1
D) 12.48 J K^-1 mol^-1

Question 9

The heat capacity of a bomb calorimeter and its contents was measured to be 6.14 kJ K^-1. Calculate the change in temperature when a current of 245 mA from a 12.0 V source was allowed to flow through the electrical heater for a period of 254 s.

A) +0.122 K
B) +4.59 K
C) +18.7 K
D) +2.45 K

Question 10

The constant pressure molar heat capacity of nitrogen gas, N₂, is 29.125 J K^-1 mol^-1 at 298.15 K. Calculate the change in the internal energy when 2.00 mol of nitrogen gas is heated so that its temperature increases by 25.0°C. Assume that the value of the heat capacity does not vary with temperature.

A) 1.04 kJ
B) 1.46 kJ
C) 1.87 kJ
D) 520 J

Question 11

Use the following data to determine the standard enthalpy change for the reaction
K(g) + ½Cl₂(g)  K⁺(g) + ½Cl^-(g) at a temperature of 298.15 K. The values refer to standard enthalpy changes at this temperature.
Ionization enthalpy of potassium
_ionH^o = +418 kJ mol^-1
Enthalpy of formation of atomic chlorine
_fH^o = +121 kJ mol^-1
Electron gain enthalpy of atomic chlorine
_egH^o = -349 kJ mol^-1

A) +190 kJ mol^-1
B) +888 kJ mol^-1
C) -52 kJ mol^-1
D) +646 kJ mol^-1

Question 12

A sample of liquid butan-1-ol, C₄H₇OH was brought to the boil in an open calorimeter. An electric current of 289 mA from a 12.0 V source was then passed through a resistive heater coil which was immersed in the liquid. The current was allowed to flow for a period of 245 s, during which time the temperature remained constant and 1.416 g of butan-1-ol was found to have evaporated. Calculate the molar enthalpy of vaporization of butan-1-ol.

A) 86.8 kJ mol^-1
B) 850 J mol^-1
C) 16.7 kJ mol^-1
D) 43.3 kJ mol^-1

Question 13

The mean bond enthalpy of a P-Cl bond is 331 kJ mol^-1 and of a Cl-Cl bond is 242 kJ mol^- at 298 K. If the mean standard enthalpy of atomization of white phosphorus is 315 kJ mol^-1, estimate the standard enthalpy of formation of gaseous phosphorus trichloride, PCl₃, at this temperature.

A) -315 kJ mol^-1
B) +226 kJ mol^-1
C) +1671 kJ mol^-1
D) -1041 kJ mol^-1

Question 14

The standard enthalpy of formation of methane, CH₄, is -75 kJ mol^-1 at 298.15 K. Calculate the mean bond enthalpy of a H-CH₃ bond, given that, at this temperature, the standard enthalpy of atomization of carbon
C(s, graphite)  C(g)
Is +717 kJ mol^-1 and the standard bond enthalpy of hydrogen, H₂, is 436 kJ mol^-1.

A) +1664 kJ mol^-1
B) +1078 kJ mol^-1
C) +379 kJ mol^-1
D) +416 kJ mol^-1

Question 15

When 7.82 g of benzaldehyde, C₆H₅CHO, was burned in a bomb calorimeter at 298.15 K, the heat released was 259.2 kJ. Calculate the enthalpy of combustion of benzaldehyde at this temperature.

A) 262 kJ mol^-1
B) 3515 kJ mol^-1
C) 257 kJ mol^-1
D) -3520 kJ mol^-1

Question 16

Use the following data to determine the standard enthalpy of reaction at 298 K for the addition of hydrogen chloride, HCl, to ethene, C₂H₄
C₂H₄(g) + HCl(g) ? C₂H₅Cl(g)
$\begin{array}{lc}& \Delta \$ \mathrm{H}^{\circ}(298 \mathrm{~K}) / \mathrm{kJ} \mathrm{mol}^{-1} \\\mathrm{C}_{2} \mathrm{H}_{4} \\& +52.2 \\\mathrm{HCl} \\& -92.3 \\\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl} \\ & -109.8\end{array}$

A) -34.7 kJ mol^-1
B) 149.9 kJ mol^-1
C) -69.7 kJ mol^-1
D) 254.3 kJ mol^-1

Question 17

The standard enthalpy of formation of benzoic acid, C₆H₅COOH, is -385 kJ mol^-1 at 298 K. Calculate the standard enthalpy of combustion of benzoic acid at this temperature, given that the standard enthalpy of formation of liquid water, H₂O is -285.8 kJ mol^-1 and gaseous carbon dioxide, CO₂, is -393.51 kJ mol^-1.

A) -294.3 kJ mol^-1
B) -3997 kJ mol^-1
C) -3227.0 kJ mol^-1
D) 2282.2 kJ mol^-1

Question 18

Hydrazine, N₂H₄, may be produced by the reaction
2 NH₃(g) + H₂O₂(l)  N₂H₄(l) +H₂O(l)
Use the following data for the standard enthalpies of formation at 298 K for the various species to calculate the standard enthalpy of reaction at this temperature.
_fH^o / (kJ mol^-1
NH₃(g) -46
H₂O₂(l) -188
N₂H₄(l) +51
H₂O(l) -286

A) +45 kJ mol^-1
B) -139 kJ mol^-1
C) -1 kJ mol^-1
D) -469 kJ mol^-1

Question 19

The enthalpy of vaporization of tetrachloromethane, CCl₄, is 30.5 kJ mol^-1 at its normal boiling temperature of 350 K. Estimate the enthalpy of vaporization at 298 K, given that the molar constant pressure heat capacities of liquid and gaseous tetrachloromethane are 83.5 and 135 J K^-1 mol^-1.

A) 27.8 kJ mol^-1
B) 33.2 kJ mol^-1
C) 30.5 kJ mol^-1
D) 2.7 kJ mol^-1

Question 20

The standard enthalpy of formation of gaseous diborane, B₂H₆, is +31 kJ mol^-1 at 298 K. Use the following data for the molar heat capacities at constant pressure to determine the standard enthalpy of reaction at 450 K.
C_p,m^o / kJ mol^-1
B(s) 12.0
H₂(g) 28.8
B₂H₆(g) 56.4

A) +39 kJ mol^-1
B) +23 kJ mol^-1
C) +33 kJ mol^-1
D) +29 kJ mol^-1