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Thermodynamics · Question 59

AIPMT 2005MCQMedium📈 Topic in 0% papers+4−1
1

The absolute enthalpy of neutralisation of the reaction : MgO(s) + 2HCl(aq) MgCl2(aq) + H2O(l) will be

  1. -57.33 kJ mol-1
  2. greater than -57.33 kJ mol-1
  3. less than -57.33 kJ mol-1
  4. 57.33 kJ mol-1

Answer

(3) less than -57.33 kJ mol-1

Solution

As per NCERT, the enthalpy of neutralisation of a strong acid by a strong base in dilute solution is -57.33 kJ mol-1, since only H+(aq) + OH-(aq) H2O(l) takes place.

Here the base is solid MgO, so the reaction also includes breaking of the MgO lattice and hydration of Mg2+ ions; the overall enthalpy change is therefore not -57.33 kJ mol-1.

Using standard enthalpies of formation ( kJ mol-1): MgO(s) −601.7, HCl(aq) −167.2, MgCl2(aq) −801.2, H2O(l) −285.8

r H = [-801.2 + (-285.8)] - [-601.7 + 2(-167.2)]

r H = -1087.0 + 936.1

r H -151 kJ mol-1 (per mole of water formed)

∴ The enthalpy of neutralisation is less than -57.33 kJ mol-1 (more heat is evolved).

Answer: (3)

AIPMT 2005

Keep practising — Thermodynamics

2

If the bond energies of H-H, Br-Br, and H-Br are 433, 192 and 364 kJ mol-1 respectively, the H for the reaction H2(g) + Br2(g) 2HBr(g) is

  1. -261 kJ
  2. +103 kJ
  3. +261 kJ
  4. -103 kJ
AIPMT 2004
3

Standard enthalpy and standard entropy changes for the oxidation of ammonia at 298 K are -382.64 kJ mol-1 and -145.6 J K-1 mol-1, respectively. Standard Gibbs energy change for the same reaction at 298 K is

  1. -221.1 kJ mol-1
  2. -339.3 kJ mol-1
  3. -439.3 kJ mol-1
  4. -523.2 kJ mol-1
AIPMT 2004
4

Considering entropy (S) as a thermodynamic parameter, the criterion for the spontaneity of any process is

  1. Ssystem + Ssurroundings > 0
  2. Ssystem - Ssurroundings > 0
  3. Ssystem > 0 only
  4. Ssurroundings > 0 only
AIPMT 2004
5

The work done during the expansion of a gas from a volume of 4 dm3 to 6 dm3 against a constant external pressure of 3 atm is (1 L atm = 101.32 J)

  1. -6 J
  2. -608 J
  3. +304 J
  4. -304 J
AIPMT 2004
6

For the reaction, C3H8(g) + 5O2(g) 3CO2(g) + 4H2O(l) at constant temperature, H - E is

  1. +RT
  2. -3RT
  3. +3RT
  4. -RT
AIPMT 2003
7

The densities of graphite and diamond at 298 K are 2.25 and 3.31 g cm-3, respectively. If the standard free energy difference ( G ) is equal to 1895 J mol-1, the pressure at which graphite will be transformed into diamond at 298 K is

  1. 11.14 108 Pa
  2. 11.14 107 Pa
  3. 11.14 106 Pa
  4. 11.14 105 Pa
AIPMT 2003
8

What is the entropy change (in J K-1 mol-1) when one mole of ice is converted into water at 0 C? (The enthalpy change for the conversion of ice to liquid water is 6.0 kJ mol-1 at 0 C).

  1. 20.13
  2. 2.013
  3. 2.198
  4. 21.98
AIPMT 2003
9

For which one of the following equations is H react equal to H f for the product ?

  1. Xe(g) + 2F2(g) XeF4(g)
  2. 2CO(g) + O2(g) 2CO2(g)
  3. N2(g) + O3(g) N2O3(g)
  4. CH4(g) + 2Cl2(g) CH2Cl2(l) + 2HCl(g)
AIPMT 2003
10

The molar heat capacity of water at constant pressure, C, is 75 J K-1 mol-1. When 1.0 kJ of heat is supplied to 100 g of water which is free to expand, the increase in temperature of water is

  1. 1.2 K
  2. 2.4 K
  3. 4.8 K
  4. 6.6 K
AIPMT 2003
11

Unit of entropy is

  1. J K-1 mol-1
  2. J mol-1
  3. J-1 K-1 mol-1
  4. J K mol-1
AIPMT 2002

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