CBSE Class 10 Chemistry Chemical Equilibrium MCQs Set 09

Answer: (d) 0.68

Explanation:
1. SO₃ left = 1 - 0.6 = 0.4 mol/L and O₂ formed = 0.3 mol/L.

Teacher's Note:
1. \( K = \dfrac{[SO_2]^2[O_2]}{[SO_3]^2} = \dfrac{(0.6)^2(0.3)}{(0.4)^2} = \dfrac{0.108}{0.16} = 0.675 \approx 0.68 \).
2. Check each option before choosing.

Answer: (b) the rate of formation of ammonia would be very slow

Explanation:
1. Low temperature favours the equilibrium yield of ammonia (the reaction is exothermic) but the rate becomes very slow, so a compromise temperature is used.

Teacher's Note:
1. The optimum temperature balances rate and yield.
2. The catalyst still works but too slowly.

Answer: (c) high temperature

Explanation:
1. The forward reaction absorbs heat, so a rise in temperature favours it. The number of moles of gas does not change, so pressure has no effect.

Teacher's Note:
1. Le Chatelier's principle: heating favours the endothermic direction.
2. Pressure matters only if the number of gas moles changes.

Answer: (c) increases both forward and backward reactions to the same extent

Explanation:
1. A catalyst lowers the activation energy of both directions equally, so equilibrium is reached faster but its position is unchanged.

Teacher's Note:
1. A catalyst does not change the equilibrium constant.
2. It does not change the yield.

Answer: (a) \( K = \dfrac{[NO_2]^2}{[NO]^2[O_2]} \)

Explanation:
1. By the law of mass action, K = [products]^coefficients / [reactants]^coefficients, so \( K = \dfrac{[NO_2]^2}{[NO]^2[O_2]} \).

Teacher's Note:
1. Each concentration is raised to the power of its coefficient.
2. Products go in the numerator.

Answer: (a) the velocities of the opposite reactions become equal

Explanation:
1. At equilibrium the rate of the forward reaction equals the rate of the backward reaction, so the concentrations stay constant (not necessarily equal).

Teacher's Note:
1. Equilibrium is dynamic; both reactions go on.
2. Equal concentrations are not required.

Answer: (d) active mass (molar concentration)

Explanation:
1. The law of mass action states that the rate is proportional to the active masses (molar concentrations) of the reactants.

Teacher's Note:
1. Active mass is the molar concentration in mol/L.
2. It is used to derive the equilibrium constant.

Answer: (b) increase 4 times

Explanation:
1. Rate = k[A][B]. Doubling both gives 2 x 2 = 4 times the rate.

Teacher's Note:
1. The rate is proportional to the product of the concentrations.
2. Doubling only one reactant doubles the rate.

Answer: (d) increases nine times

Explanation:
1. Rate = k[A]²[B], so trebling [A] multiplies the rate by 3² = 9.

Teacher's Note:
1. The power of each concentration equals its coefficient in the balanced equation.
2. Rate increases, not decreases.

Answer: (d) KNO₃(aq) + NaCl(aq) ⇌ KCl(aq) + NaNO₃(aq)

Explanation:
1. In the last case all reactants and products stay in solution and can react back, so equilibrium is established. In the others a precipitate or a gas leaves the system.

Teacher's Note:
1. A reaction that forms a precipitate or an escaping gas goes to completion.
2. Reversible reactions need a closed system.

Answer: (d) NH₄Cl(s) ⇌ NH₃(g) + HCl(g) in a closed vessel

Explanation:
1. Heated in a closed vessel, ammonium chloride dissociates into ammonia and hydrogen chloride which recombine on cooling.

Teacher's Note:
1. Closed systems allow reversibility.
2. The other reactions go to completion.

Answer: (c) It can never proceed to completion

Explanation:
1. In a reversible reaction products also react to give back the reactants, so a state of equilibrium is reached before the reactants are used up.

Teacher's Note:
1. A catalyst can speed up any reaction, so (b) is not special to reversible reactions.
2. Equilibrium mixtures always contain both reactants and products.

Answer: (d) CO₂ escapes continuously

Explanation:
1. CO₂ is removed continuously, so the equilibrium shifts to the right until all the limestone decomposes.

Teacher's Note:
1. Removing a product shifts the equilibrium forward.
2. In a closed vessel the reaction would stop at equilibrium.

Answer: (b) the rates of the forward and backward reactions are equal

Explanation:
1. Equilibrium means the two opposing rates are equal, so there is no net change in concentrations.

Teacher's Note:
1. The amounts need not be equal.
2. The reaction has not stopped; it is dynamic.

Answer: (d) the rate of transformation of A to B equals the rate of transformation of B to A

Explanation:
1. Equilibrium is defined by equal forward and backward rates, whatever the percentage conversion.

Teacher's Note:
1. The position of equilibrium depends on the reaction conditions.
2. Equal rates give constant concentrations.

Answer: (a) closed vessel

Explanation:
1. If hydrogen escapes from an open vessel the reaction goes to completion, so the system must be closed.

Teacher's Note:
1. A closed system has no loss of reactants or products.
2. Steam passed over hot iron in an open tube gives complete reaction.

Answer: (a) backward (decomposition) reaction

Explanation:
1. Heating favours the endothermic direction, which here is the decomposition of ammonia into nitrogen and hydrogen.

Teacher's Note:
1. Le Chatelier's principle governs the shift.
2. Low temperature favours ammonia but is slow.

Answer: (a) high pressure

Explanation:
1. \( 2SO_2 + O_2 \rightleftharpoons 2SO_3 \) decreases the number of gas moles from 3 to 2, so high pressure favours SO₃.

Teacher's Note:
1. Pressure favours the side with fewer gas moles.
2. Low temperature also favours SO₃ because the reaction is exothermic.

Answer: (a) the velocity constants of the forward and backward reactions

Explanation:
1. \( K = \dfrac{k_f}{k_b} \), which equals the ratio of product concentrations to reactant concentrations at equilibrium.

Teacher's Note:
1. At equilibrium forward rate = backward rate, giving this ratio.
2. K changes only with temperature.