How is a reaction with more products than reactants spontaneous?
Note that whether the forward or reverse reaction is favorable depends entirely on the relative magnitudes of Q and K (the reaction quotient versus the equilibrium constant):

$$\begin{align} Q&amp;&gt;K \rightarrow \text{reactants favored}  \\ K&amp;&gt;Q \rightarrow \text{products favored}  \end{align}$$

Next, consider that when you declare a value for $\Delta G^\circ$ you can translate that into a value for K, using the following equation:

$$\Delta G^\circ = -RT\log K$$

For instance, if $\Delta G^\circ = 0$ then $K=1$.

For the example here, this means that

$$\begin{align} Q&amp;&gt;1 \rightarrow \text{reactants favored}  \\ 1&amp;&gt;Q \rightarrow \text{products favored}  \end{align}$$

Next, consider the standard state of all substances. For solutions this is typically $\pu{c^\circ = 1 M}$. Standard states are important because the reaction quotients and equilibrium constants, when described in terms of concentrations, contain ratios of the concentrations in solution to the standard concentrations, for instance

$$Q= \frac{\prod_{\text{products i}} (c_i/c^\circ)^{\nu_i}}{\prod_{\text{reactants i}} (c_i/c^\circ)^{\nu_i}}$$

If all substances are present at the standard concentration, $Q=1$. In the present example $K=1$ so the equilibrium state consists of all substances being at standard concentrations ($c_i=\pu{1 M}$).

Finally, consider the following way to relate Q, K and $\Delta G$: 

$$\Delta G = RT\log \left( \frac{Q}{K}\right)$$

If $K=1$ as in the present example then

$$\Delta G = RT\log Q $$ 

This just reinforces what was stated earlier for the special example presented here: when $Q&lt;1$ the free energy difference is negative and products will be favored, and vice-versa when $Q&gt;1$. 

Inserting the definition of the reaction quotient

$$\begin{align} \Delta G &amp;= RT\log \left( \frac{\prod_{\text{products i}} (c_i/c^\circ)^{\nu_i}}{\prod_{\text{reactants i}} (c_i/c^\circ)^{\nu_i}}\right) \\ &amp;= RT\log \left( \frac{c_B^2c_C^2}{c_Ac^{\circ\,3}}\right) \end{align}$$ 

If we assume all substances are present at the same concentration c, then

$$  \Delta G= 3RT\log \left( \frac{c}{c^{\circ}}\right) $$

Clearly if $c&lt;\pu{1 M}$ then $\Delta G&lt;0$ and products are favored.