EXERCISES
6.1 Choose the correct answer. A thermodynamic state function is a
6.1 Choose the correct answer. A thermodynamic state function is a
quantity
(i) used to determine heat changes
(ii) whose value is independent of path
(iii) used to determine pressure volume work
(iv) whose value depends on temperature only.
6.2 For the process to occur under adiabatic conditions, the
correct
condition is:
(i) ÄT = 0
(ii) Äp = 0
(iii) q = 0
(iv) w = 0
6.3 The enthalpies of all elements in their standard states are:
(i) unity
(ii) zero
(iii) < 0
(iv) different for each element
6.4 ÄU0of combustion of methane is – X kJ mol–1. The value of ÄH0 is
(i) = ÄU0
(ii) > ÄU0
(iii) < ÄU0
(iv) = 0
6.5 The enthalpy of combustion of methane, graphite and
dihydrogen
at 298 K are, –890.3 kJ mol–1 –393.5 kJ mol–1, and –285.8 kJ mol–1
respectively. Enthalpy of formation of CH4(g) will be
(i) –74.8 kJ mol–1 (ii) –52.27 kJ mol–1
(iii) +74.8 kJ mol–1 (iv) +52.26 kJ mol–1.
6.6 A reaction, A + B → C + D + q is found to have a positive entropy
change. The reaction will be
(i) possible at high temperature
(ii) possible only at low temperature
(iii) not possible at any temperature
(v) possible at any temperature
6.7 In a process, 701 J of heat is absorbed by a system and 394
J of
work is done by the system. What is the change in internal
energy
for the process?
6.8 The reaction of cyanamide, NH2CN (s), with dioxygen was
carried
out in a bomb calorimeter, and ÄU was found to be –742.7 kJ mol–1
at 298 K. Calculate enthalpy change for the reaction at 298 K.
NH2CN(g) +32O2(g) → N2(g) + CO2(g) + H2O(l)
6.9 Calculate the number of kJ of heat necessary to raise the
temperature
of 60.0 g of aluminium from 35°C to 55°C. Molar heat capacity of
Al
is 24 J mol–1 K–1.
6.10 Calculate the enthalpy change on freezing of 1.0 mol of
water
at10.0°C to ice at –10.0°C. ÄfusH = 6.03 kJ mol–1 at 0°C.
Cp [H2O(l)] = 75.3 J mol–1 K–1
Cp [H2O(s)] = 36.8 J mol–1 K–1
6.11 Enthalpy of combustion of carbon to CO2 is –393.5 kJ mol–1.
Calculate
the heat released upon formation of 35.2 g of CO2 from carbon
and
dioxygen gas.
6.12 Enthalpies of formation of CO(g), CO2(g), N2O(g) and N2O4(g)
are –110,
– 393, 81 and 9.7 kJ mol–1 respectively. Find the value of ÄrH for the
reaction:
N2O4(g) + 3CO(g) → N2O(g) + 3CO2(g)
6.13 Given
N2(g) + 3H2(g) → 2NH3(g) ; ÄrH0 = –92.4 kJ mol–1
What is the standard enthalpy of formation of NH3 gas?
6.14 Calculate the standard enthalpy of formation of CH3OH(l)
from the
following data:
CH3OH (l) +32O2(g) → CO2(g) + 2H2O(l) ; ÄrH0 = –726 kJ mol–1
C(g) + O2(g) → CO2(g) ; ÄcH0 = –393 kJ mol–1
H2(g) +12O2(g) → H2O(l) ; Äf H0 = –286 kJ mol–1.
6.15 Calculate the enthalpy change for the process
CCl4(g) → C(g) + 4 Cl(g)
and calculate bond enthalpy of C – Cl in CCl4(g).
ÄvapH0(CCl4) = 30.5 kJ mol–1.
ÄfH0 (CCl4) = –135.5 kJ mol–1.
ÄaH0 (C) = 715.0 kJ mol–1 , where ÄaH0 is enthalpy of atomisation
ÄaH0 (Cl2) = 242 kJ mol–1
6.16 For an isolated system, ÄU = 0, what will be ÄS ?
6.17 For the reaction at 298 K,
2A + B → C
ÄH = 400 kJ mol–1 and ÄS = 0.2 kJ K–1 mol–1
At what temperature will the reaction become spontaneous
considering ÄH and ÄS to be constant over the
temperature range.
6.18 For the reaction,
2 Cl(g) → Cl2(g), what are the signs of ÄH and ÄS ?
6.19 For the reaction
2 A(g) + B(g) → 2D(g)
ÄU 0 = –10.5 kJ and ÄS0 = –44.1 JK–1.
Calculate ÄG0 for the reaction, and predict whether the reaction
may occur spontaneously.
6.20 The equilibrium constant for a reaction is 10. What will be
the value
of ÄG0 ? R = 8.314 JK–1 mol–1, T = 300 K.
6.21 Comment on the thermodynamic stability of NO(g), given
12
N2(g) +12O2(g) → NO(g) ; ÄrH0 = 90 kJ mol–1
NO(g) +12O2(g) → NO2(g) : ÄrH0= –74 kJ mol–1
6.22 Calculate the entropy change in surroundings when 1.00 mol
of
H2O(l)
is formed under standard conditions. Äf H0 = –286 kJ mol–1.