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:
$\Delta G^0=\Delta H^0-T\Delta S^0\equiv \Delta G^0=A-BT$
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.
$\Delta H=A=+ve\to$ Endothermic