For the reaction \( N_2 + 3H_2 \rightarrow 2NH_3 \), if \( \frac{\Delta [NH_3]}{\Delta t} = 2 \times 10^{-4} \, \text{mol L}^{-1} \text{s}^{-1} \), then the value of \( \frac{\Delta [H_2]}{\Delta t} \) would be:
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In rate law calculations, use the stoichiometric coefficients to determine the relationship between reactants and products.
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:
\[
\boxed{3 \times 10^{-4} \, \text{mol L}^{-1} \text{s}^{-1}}
\]