For the reaction; A(l) → 2B(g) = 2.1kcal, = 20cal K ^ - 1 at 300K ....

$∆ H = ∆ U + ∆ ngRT$
$= 2.1 \times {10}^{3} + 2\left(2\right)\left(300\right)$
$= 2100 + 1200$
$= 3300cal$
$∆ G = ∆ H - T ∆ S$
$= 3300 - \left(300\right)\left(20\right)$
$= 3300 - 6000$
$= - 2700cals = - 2.7kcal$

Gibbs free energy can be visualized as the amount of useful energy present in a thermodynamic system that can be utilized to perform some useful work.