Question

The standard reaction Gibbs energy for a chemical reaction at an absolute temperature $T$ is given by
$\Delta_r G^{\circ} = A - Bt$
Where $A$ and $B$ are non-zero constants. Which of the following is TRUE about this reaction ?

• A. Exothermic if $B < 0$
• B. Exothermic if $A > 0$ and $B < 0$
• C. Endothermic if $A < 0$ and $B > 0$
• D. Endothermic if $A > 0$

Answer 1

Answer: (D)

Gibb's free energy $(\Delta G )$ :
$\Delta G =\Delta H - T \Delta S$
where,
$\Delta G =$ Gibb's free energy
$\Delta H =$ enthalpy of the reaction
$\Delta S =$ entropy change
$T =$ temperature
Under standard condition:
$\Delta G ^{0}=\Delta H ^{0}- T \Delta S ^{0}\,\,\ \equiv \quad \Delta G ^{0}= A - BT$
For a spontaneous process, $\Delta G$ must be negative.
$\Delta G ^{0}= A - BT =- ve$
$\Delta H < T \Delta S -\cdots(1)$
$A =\Delta H$ can $- ve$ or $+ ve$ if the condition of eqn (1) is satisfied.
$\Delta H = A =+ ve \rightarrow$ Endothermic