Tag: energetics and thermochemistry

Questions Related to energetics and thermochemistry

The value of $log _{10}$ K for a reaction $A\rightleftharpoons B$ is:

$( Given : \Delta _{r}H^{0} _{298k}=-54.07 kJ mol^{-1},$ $\Delta _{r}S^{0} _{298k}=10JK^{-1}mol^{-1}$ $and\ R=8.314 JK^{-1}mol^{-1};$ 
$2.303\times 8.314\times 298=5705 )$

chemistry energetics and thermochemistry gibbs energy change and equilibrium gibbs free energy entropy and spontaneity

  1. 5

  2. 10

  3. 95

  4. 100

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Correct Option: B
Explanation:

$\Delta G^{0}=\Delta H^{0}-T\Delta S^{0}=-54.07 \times 1000 - 298 \times 10$

$=-54070-2980=-57050$

$\Delta G^{0}=-2.303 RT log _{10}K$

$-57050=-2.303\times 298\times 8.314 log _{10}K=-5705 log _{10}K$


$ log _{10}K=10$

The dissociation energy of $CH _4$ and $C _2H _6$ are respectively 360 and 620 kcal /mole. the bond energy $C-C$ is:

chemistry energetics and thermochemistry bond energies and enthalpy changes bond enthalpies enthalpies for different types of reactions

  1. 260 kcal /mole

  2. 180 kcal /mole

  3. 130 kcal / mole

  4. 80 kcal / mole

Show answer
Correct Option: D
Explanation:
Dissociation of energy of $CH _4$ base
=$\cfrac {\text{dissociation energy of }{CH _4}}{4}=\cfrac {360}{4}=90Kcal/mole$

Dissociation energy of $C _2H _6$

620=1(B.E of C-C bond+ 6 B.E of C-H bond)

620=B.E of C-C bond +6 $\times $90

B.E of C-C bond=620-540=80 kcal / mole

The heat of neutralisation of $HCl$ by $NaOH$ is -55.9 KJ/mole. If the heat of neutralisation of $HCN$ by$ NaOH$ is -12.1 KJ/mole, the energy of dissociation of $HCN$ is:

chemistry energetics and thermochemistry bond energies and enthalpy changes bond enthalpies enthalpies for different types of reactions

  1. -43.8 KJ

  2. 43.8 KJ

  3. 68 KJ

  4. -68 KJ

Show answer
Correct Option: B
Explanation:

Heat of dissociation $=(55.9-12.1)=43.8\ KJ$