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Thermodynamics · Question 56

AIPMT 2006MCQMedium📈 Topic in 12% papers+4−1
1

The enthalpy of hydrogenation of cyclohexene is -119.5 kJ mol-1. If resonance energy of benzene is -150.4 kJ mol-1, its enthalpy of hydrogenation would be

  1. -358.5 kJ mol-1
  2. -508.9 kJ mol-1
  3. -208.1 kJ mol-1
  4. -269.9 kJ mol-1

Answer

(3) -208.1 kJ mol-1

Solution

If benzene had three isolated C=C bonds (Kekulé structure), its enthalpy of hydrogenation would be:

Hexpected = 3 (-119.5) = -358.5 kJ mol-1

Resonance makes benzene more stable by 150.4 kJ mol-1, so less heat is released:

Hactual = -358.5 - (-150.4)

= -208.1 kJ mol-1

Answer: (3)

AIPMT 2006

Keep practising — Thermodynamics

2

Which of the following pairs of a chemical reaction is certain to result in a spontaneous reaction ?

  1. Exothermic and increasing disorder
  2. Exothermic and decreasing disorder
  3. Endothermic and increasing disorder
  4. Endothermic and decreasing disorder
AIPMT 2005
3

A reaction occurs spontaneously if

  1. T S < H and both H and S are +ve
  2. T S > H and H is +ve and S is −ve
  3. T S > H and both H and S are +ve
  4. T S = H and both H and S are +ve
AIPMT 2005
4

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
AIPMT 2005
5

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
6

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
7

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
8

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
9

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
10

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
11

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

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