Question 1

For the reaction: N₂(g) + 3H₂(g) ⇌ 2NH₃(g), if the equilibrium constant is Kc = 0.5 at 400°C, what is the relationship between forward and reverse reaction rates at equilibrium?

Accepted Answer

At chemical equilibrium, the forward and reverse reaction rates are equal regardless of the Kc value; Kc only indicates the position of equilibrium, not the rates.

Question 2

For the equilibrium: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), the equilibrium constant expression Kc is:

Accepted Answer

Kc is the ratio of products raised to their stoichiometric coefficients divided by reactants raised to their stoichiometric coefficients.

Question 3

When the pressure on the equilibrium system: N₂(g) + O₂(g) ⇌ 2NO(g) is increased, how does the equilibrium shift?

Accepted Answer

Since there are 2 moles of gas on both sides of the equation, the equilibrium position is unaffected by pressure changes according to Le Chatelier's principle.

Question 4

For the reaction: H₂(g) + I₂(g) ⇌ 2HI(g), Kc = 50 at 440°C. What is the value of Kc for the reverse reaction: 2HI(g) ⇌ H₂(g) + I₂(g)?

Accepted Answer

The equilibrium constant for the reverse reaction is the reciprocal of the forward reaction: Kc(reverse) = 1/50 = 0.02.

Question 5

In the equilibrium: PCl₅(g) ⇌ PCl₃(g) + Cl₂(g), if temperature is increased, the equilibrium shifts to the right. This indicates that the forward reaction is:

Accepted Answer

According to Le Chatelier's principle, if increasing temperature shifts equilibrium to the right, the forward reaction must be endothermic (absorbs heat).

Question 6

For the equilibrium reaction N₂O₄(g) ⇌ 2NO₂(g), if the equilibrium constant Kc = 4.63 × 10⁻³ at 298 K, what is the value of Kp at the same temperature?

Accepted Answer

Using Kp = Kc(RT)^Δn, where Δn = 2 - 1 = 1, Kp = 4.63 × 10⁻³ × (0.0821 × 298)¹ ≈ 0.113.

Question 7

At equilibrium, a 2 L container contains 0.5 mol H₂, 0.5 mol I₂, and 1.0 mol HI. What is the equilibrium constant Kc for the reaction H₂(g) + I₂(g) ⇌ 2HI(g)?

Accepted Answer

Kc = [HI]²/([H₂][I₂]) = (1.0/2)²/((0.5/2)(0.5/2)) = (0.5)²/(0.25 × 0.25) = 0.25/0.0625 = 16.0.

Question 8

For an equilibrium reaction, if ΔG° = 0 kJ/mol at 298 K, which statement is correct?

Accepted Answer

When ΔG° = 0, using ΔG° = -RT ln K, we get ln K = 0, therefore K = 1, indicating the system is at equilibrium.

Question 9

In the equilibrium 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), if the initial concentrations are [SO₂] = 0.6 M, [O₂] = 0.3 M, and [SO₃] = 0 M, and x mol/L of O₂ reacts, which expression correctly represents the equilibrium concentration of SO₃?

Accepted Answer

From stoichiometry, for every 1 mol of O₂ that reacts, 2 mol of SO₃ forms; if x mol/L of O₂ reacts, then [SO₃] = 2x.

Question 10

For the reaction A(g) ⇌ B(g) + C(g), the equilibrium constant Kp is 2.0 atm at 400 K. If the initial pressure of A is 1 atm and it dissociates by 50%, what is the partial pressure of B at equilibrium?

Accepted Answer

If A dissociates 50%, then 0.5 atm of A remains, 0.5 atm of B forms, and 0.5 atm of C forms; therefore PB = 0.50 atm.

Question 11

Which of the following changes will shift the equilibrium of the reaction CaCO₃(s) ⇌ CaO(s) + CO₂(g) to the right?

Accepted Answer

Decreasing CO₂ pressure removes the product, shifting equilibrium right by Le Chatelier's principle; solid amounts don't affect Kc, and temperature decrease favors exothermic reverse reaction.

Question 12

For the equilibrium PCl₅(g) ⇌ PCl₃(g) + Cl₂(g), if Kc = 1.8 × 10⁻² at 250°C, what is the degree of dissociation (α) if initial concentration of PCl₅ is 0.5 M?

Accepted Answer

Using Kc = α²C/(1-α) where C = 0.5 M, solving 0.018 = 0.5α²/(1-α) gives α ≈ 0.19.

Question 13

In a reversible reaction at constant temperature, if more reactants are added while the system is at equilibrium, which quantity increases immediately?

Accepted Answer

Adding reactants increases their concentration, which immediately increases the forward reaction rate (rate = k[reactants]); Kc, ΔG°, and product concentrations change only after re-equilibration.

Question 14

At 500 K, the equilibrium constant Kc for the reaction N₂(g) + 3H₂(g) ⇌ 2NH₃(g) is 1.7 × 10⁻³. What is Kp for this reaction at the same temperature? (R = 0.0821 L·atm·mol⁻¹·K⁻¹)

Accepted Answer

Using Kp = Kc(RT)^Δn where Δn = 2 - 4 = -2: Kp = 1.7 × 10⁻³ × (0.0821 × 500)⁻² = 1.7 × 10⁻³ × (41.05)⁻² ≈ 2.97 × 10⁻⁵.

Question 15

A sealed vessel contains PCl₅(g) at equilibrium with PCl₃(g) and Cl₂(g). If the volume of the container is decreased isothermally, which statement correctly describes the shift in equilibrium?

Accepted Answer

For PCl₅(g) ⇌ PCl₃(g) + Cl₂(g), Δn = +1; decreasing volume increases pressure, shifting equilibrium toward fewer moles (reactant side, leftward).

Question 16

For the reaction 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), ΔH = −198 kJ/mol. If initial concentrations are [SO₂] = 0.5 M, [O₂] = 0.8 M, and [SO₃] = 0 M, and Kc = 280 at 700 K, will the reaction proceed forward or backward?

Accepted Answer

Q = [SO₃]²/([SO₂]²[O₂]) = 0/((0.5)²(0.8)) = 0, which is much less than Kc = 280, so the reaction proceeds forward to establish equilibrium.

CAPF AC Chemical Equilibrium

Chemical Equilibrium — Practice Questions