Pressure increases reactant concentrations, accelerating the forward rate

Steps:

• Forward rate depends on concentrations of N2 and H2 via rate = k[N2][H2]^3.
• Gas concentrations are proportional to partial pressures (PV = nRT).
• Reducing volume at constant T increases total pressure and partial pressures of reactants.
• Higher concentrations raise collision frequency, increasing forward rate.

Why C is correct:

• Collision theory states that increased reactant concentrations enhance effective collisions, directly speeding the forward reaction.

Why the others are wrong:

• A: Inert argon addition at constant volume keeps reactant partial pressures unchanged, so concentrations and rate stay the same.
• B: Lower temperature reduces molecular speeds, decreasing collision energy and forward rate per Arrhenius equation.
• D: Removing NH3 shifts equilibrium right but leaves reactant concentrations unaffected, so forward rate unchanged.

Final answer: C