For the reaction N2(g) + 3 H2(g) ⇌ 2 NH3(g) with Kc = 1.0×10^-3 at a given temperature, what happens if NH3 is added?

• A. Adding NH3 shifts equilibrium toward reactants; Kc remains unchanged.
• B. Adding NH3 shifts equilibrium toward products; Kc increases.
• C. Adding NH3 shifts toward reactants; Kc changes with temperature.
• D. No effect on the equilibrium.

Answer: (A)

This question tests how a system at equilibrium responds to adding a product. According to Le Chatelier’s principle, when you increase the concentration of a product, the system shifts to oppose that change by consuming some of the added product. For this reaction, that means shifting toward the left, toward the reactants N2 and H2, so some NH3 is converted back into N2 and H2. At the same time, the equilibrium constant Kc does not change because the temperature is fixed; Kc depends only on temperature and remains the same even as concentrations adjust.

So adding NH3 makes the mixture shift toward the reactants, while Kc stays the same. The other ideas—shifting toward products, increasing Kc, or having no effect—don’t fit because the shift is determined by the direction that counteracts the change (adding product pushes left), and Kc would only change if temperature changed.