1. For spontaneity, the Gibbs free energy change must be negative:

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

A reaction becomes spontaneous when:

$\Delta G < 0$

The threshold temperature is found by setting:

$\Delta G = 0$

2. Given:

$\Delta H = 400\ \text{kJ mol}^{-1}$ $\Delta S = 0.2\ \text{kJ mol}^{-1}\text{K}^{-1}$

So,

$0 = 400 - T(0.2)$

3. Solve for $T$:

$T(0.2) = 400$

$T = \frac{400}{0.2} = 2000\ \text{K}$

4. Since both $\Delta H$ and $\Delta S$ are positive, the reaction is spontaneous at sufficiently high temperature, i.e. for:

$T > 2000\ \text{K}$

Hence, the reaction becomes spontaneous above $2000\ \text{K}$.