NIOS Chemical Thermodynamics Intext Questions With Answer
National Institute of Open Schooling (NIOS)
Sr. Secondary, Module-4
Lession-9: NIOS Chemical Thermodynamics Intext Questions With Answer
INTEXT QUESTIONS 8.1
1. Which of the following is false ?
(a) The reaction H2(g) + Cl2(g) → 2HCl (g) + 185 kJ is endothermic.
(b) Enthalpy change is a state function.
(c) Standard state condition for a gaseous system is 1 bar pressure at a specified temperature
Answer: Option (a) is false because it is an exothermic process due to its release of energy of amount 185KJ.2. For the reaction at 298 K,
1/2 N2(g) + 3/2 H2(g) → NH3(g) ; Δ H = - 46 kJ
(a) What is the value of Δng?
(b) Calculate the value of ΔU at 298 K?
Answer: (a) From the relation between change in enthalpy and change in internal energy state in gaseous state we get the following equation-ΔH = ΔU + ΔngRT
The temperature and pressure is kept constant. For liquids and solids, the difference between ΔU and ΔH is negligible and thus this relationship is only applicable to gaseous state.
From the equation, no. of moles in the product = 1 mole
No. of moles in the reactant = 1/2 + 3/2 = 2moles
∴ Δng = 1 − 2 = −1
(b) From the equation, ΔH = ΔU + ΔngRT
−46000Jmol−1 = ΔU +(−1)(8.314JKmol−1)(298K)
or, ΔU = −46000Jmol−1 + 2247.6Jmol−1 = −43.5kJmol−1
3. Which of the following will increase the internal energy of a system?
(a) Heat given to the system
(b) Work done by the system
Answer: The internal energy equation is ΔU = q + wWhere,
ΔU is the change in internal energy
q is the heat absorbed and
w is the work done on the system.
From this equation, internal energy (U) will increase when the heat is being supplied to the system.
INTEXT QUESTIONS 8.2
1. Which of the following is true?
(a) Enthalpy of formation is the heat evolved or absorbed when one gram of a substance is formed from its elements in their most stable states.
(b) When one mole of H+ (aq) and 1 mole of OH— (aq) react, 57.1 kJ of energy is absorbed.
(c) In the thermochemical equation,
C(graphite) + O2(g) → CO2(g) ; ΔfHo = –394 kJ mol–1
ΔfHo is known as enthalpy of formation of CO2(g)
Answer: Statement (c) is true since CO2(g) is being formed from the reactants which are in their elemental and most stable states and the enthalpy change would be ‘enthalpy of formation’.2. Calculate the enthalpy change for complete combustion of 29.0 g of butane, if
C4H0(g) + 3/2 O2(g) → 4CO2(g) + 5H2O (l) ; ΔcombHo = – 2658 kJ
Answer: The molar mass of butane is (12 × 4) + (1 × 10) = 58g/mol-1329KJ is the enthalpy change for butane combustion when 29.0g butane is taken.
3.Calculate the standard enthalpy of the reaction
2H2S(g) + SO2(g) → 3S (s) + 2H2O(l)
given that
ΔfHo(H2S) = – 20.6 kJ mol–1
ΔfHo(SO2) = – 296.9 kJ mol–1
ΔfHo(H2O) = – 289.9 kJ mol–1
Answer: Given-ΔfHo(H2S) = – 20.6 kJ mol–1
ΔfHo(SO2) = – 296.9 kJ mol–1
ΔfHo(H2O) = – 289.9 kJ mol–1
Thus, using the formula, the enthalpy of reaction is
ΔfHoreaction = 2 ΔfHo(H2O) − 2 ΔfHo(H2S) − ΔfHo(SO2)
ΔfHoreaction = 2(−289.9)−2(−20.6)−(−296.9)
ΔfHoreaction = −241.7KJ
Thus, the enthalpy of the reaction is −241.7KJ in the reaction
2H2S(g) + SO2(g) → 3S(s)+2H2O(l)
INTEXT QUESTIONS 8.3
1. Write True or false
(a) Enthalpy of a reaction is equal to the sum of the enthalpy of formation of products minus the sum of the enthalpy of formation of the reactants.
(b) Enthalpy of formation of any elementary substance is equal to zero.
(c) If a reaction can be carried out in several steps, the enthalpy change.
for the overall reaction is equal to enthalpy change in the last step.
(d) Bond enthalpy and bond dissociation energy are same for polyatomic molecules.
Answer: a. Trueb. True
c. False
d. False
2. Calculate the bond enthalpy of N – H in NH3(g)
Given
1/2 N2(g) + 3/2 H2(g) → NH3(g) ; ΔfHo = – 46 kJ mol-1
1/2 H2(g) → H (g); ΔrHo = 218 kJ mol-1
1/2 N2(g) → N(g); ΔrHo = 973 kJ mol-1
Answer: ΔrHo = ∑ B.E. (reactant bonds) – ∑ B.E. (products bonds)or – 46 (kJ mol-1) = 3 × 218 (kJ mol-1) + 973 (kJ mol-1) – B.E. (NH3(g)Bonds)
B.E. (NH3(g) Bonds) = 1673 kJ mol-1
B.E. (N – H) = 557.7 kJ mol-1
