Question

The standard reaction Gibbs energy for a chemical reaction at an absolute temperature $T$ is given by:

${\Delta }_r{G}^o=A-BT$

Where $A$ and $B$ are non-zero constants. Which of the following is true about this reaction?

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

Answer 1

The correct option is D Endothermic if $A>0$

Gibb's free energy $\left(\Delta G\right)$:

$\Delta G=\Delta H-T\Delta S$

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.

$\Delta G^0=A-BT=-ve$

$\Delta H<T\Delta S$ -----(1)

$A=\Delta H$ can $-ve$ or $+ve$ if the condition of eqn (1) is satisfied.

$\Delta H=A=+ve\to$ Endothermic