State the criteria for a spontaneous process in terms of $\Delta G$ at constant pressure and temperature.

The criterion for a spontaneous process is

$A$ reaction,$A + B \rightarrow C + D + q$,is found to have a positive entropy change. The reaction will be:

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 $\text{ J}$)?

For the reaction $2 \ NO_{2(g)} \rightleftharpoons N_2O_{4(g)}$,when $\Delta S = -176.0 \ J \ K^{-1} \ mol^{-1}$ and $\Delta H = -57.8 \ kJ \ mol^{-1}$,the magnitude of $\Delta G$ at $298 \ K$ for the reaction is $...... \ kJ \ mol^{-1}$. (Nearest integer)

When is a reaction spontaneous?