Question

consider the reaction below.
2nh₃(g) ⇌ n₂(g) + 3h₂(g)
which is most likely the effect to the forward reaction if there is an increase in pressure of the system?
○ the reactant surface area increases.
○ the reaction rate decreases.
○ the reaction is not affected at all.
○ the reaction stops completely.

Explanation:

To solve this, we analyze the reaction \( 2NH_{3(g)}
ightleftharpoons N_{2(g)} + 3H_{2(g)} \) and Le Chatelier's principle for pressure changes. Pressure affects reactions with gaseous reactants/products by changing volume (since \( P \propto \frac{1}{V} \) for constant \( n,T \)). The number of moles of gas: reactants have \( 2 \) moles (\( 2NH_3 \)), products have \( 1 + 3 = 4 \) moles (\( N_2 + 3H_2 \)). Increasing pressure shifts equilibrium to the side with fewer moles of gas (left, reverse reaction favored), but the question is about the forward reaction rate. Wait, no—wait, the options: let's re - evaluate. Wait, the reaction is gaseous. But the options: "reactant surface area increases" – pressure doesn't affect surface area (surface area is about solids/liquids, not gases). "Reaction rate decreases" – when pressure increases, gas concentration increases (since \( P = nRT/V \), so \( n/V \) (concentration) increases with \( P \) at constant \( T \)). Wait, no—wait, the forward reaction: the equilibrium shifts left, but the rate of forward reaction? Wait, no, maybe I made a mistake. Wait, the reaction is \( 2NH_3
ightleftharpoons N_2 + 3H_2 \). Moles of gas: reactants \( 2 \), products \( 4 \). Increasing pressure: the system will try to reduce pressure by moving to the side with fewer moles (left, so reverse reaction is favored). But the rate of the forward reaction: when pressure increases, concentration of \( NH_3 \) (reactant) increases. Wait, no—wait, the forward reaction is \( 2NH_3 \to N_2 + 3H_2 \). If pressure increases, the concentration of \( NH_3 \) increases, but the equilibrium shifts left. However, the options: let's check each option:

1. "The reactant surface area increases." – Reactants are gaseous (\( NH_3 \)), surface area is for solids/liquids. So this is wrong.

1. "The reaction rate decreases." – Wait, when pressure increases, for gaseous reactions, if we consider the forward reaction: the concentration of \( NH_3 \) is higher, but the equilibrium shifts left. Wait, no—maybe the question is about the effect on the forward reaction rate. Wait, no, maybe I messed up. Wait, the reaction is in equilibrium. When pressure increases, the system shifts to the left (reverse reaction favored), so the forward reaction rate will decrease (because the equilibrium is moving left, so the rate of forward reaction is less than reverse). Alternatively, maybe the question is not about equilibrium shift but about rate. Wait, pressure affects gas concentration. If pressure increases, concentration of all gases increases. But for the forward reaction, the reactant is \( NH_3 \), product is \( N_2 \) and \( H_2 \). Wait, no—maybe the question has a mistake, but among the options, "The reaction rate decreases" is the only plausible one? Wait, no, maybe I'm wrong. Wait, let's re - examine the options.

1. "The reaction is not affected at all." – No, pressure affects gaseous equilibria, so this is wrong.

1. "The reaction stops completely." – Reactions in equilibrium never stop, they are dynamic. So this is wrong.

Wait, maybe the question is about the effect on the forward reaction (not equilibrium shift). Wait, when pressure increases, for a gaseous reaction, if the forward reaction produces more moles of gas (here, 2 moles react to make 4 moles), increasing pressure will slow down the forward reaction (because the system resists the pressure increase by favoring the reverse reaction, which has fewer moles of gas). So the forward reaction rate decreases. So the correct option is "The reaction rate decreases."

Answer:

The reaction rate decreases.