What happens during a spontaneous process?

Consider the following reaction:
$A_{(g)} + 3 B_{(g)} \longrightarrow 2 C_{(g)} ; \Delta H^{\ominus} = -24 \ kJ$.
At $25^{\circ} C$,if $\Delta G^{\ominus}$ of the reaction is $-9 \ kJ$,the standard entropy change (in $J \ K^{-1}$) of the same reaction at the same temperature is:

The occurrence of a reaction is impossible if

Consider the graph of $Gibbs$ free energy $G$ vs. Extent of reaction. The number of statement$(s)$ from the following which are true with respect to points $(a)$,$(b)$,and $(c)$ is $.................$
$A$. Reaction is spontaneous at $(a)$ and $(b)$
$B$. Reaction is at equilibrium at point $(b)$ and non-spontaneous at point $(c)$
$C$. Reaction is spontaneous at $(a)$ and non-spontaneous at $(c)$
$D$. Reaction is non-spontaneous at $(a)$ and $(b)$

$\Delta G$ is net energy available to do useful work and is thus a measure of "free energy". Show mathematically that $\Delta G$ is a measure of free energy. Find the unit of $\Delta G$. If a reaction has positive enthalpy change and positive entropy change,under what condition will the reaction be spontaneous?

An ideal gas expands from an initial volume of $5 \text{ dm}^3$ to $15 \text{ dm}^3$ against a constant external pressure of $2 \text{ atm}$. What is the work done by the gas (in $\text{ J}$)?