From the balanced chemical equation, the mole ratio of \( NH_3 \) to \( H_2 \) is \( 2:3 \). Therefore, the rate of change of \( H_2 \) is \( 3 \times 10^{-4} \, \text{mol L}^{-1} \text{s}^{-1} \).
Final Answer:
\[
\boxed{3 \times 10^{-4} \, \text{mol L}^{-1} \text{s}^{-1}}
\]