Chemical Kinetics and Solutions — Practice

Question 1

For a reaction rate = k[A][B]², the overall order and the units of k are:
(a) 3; L² mol⁻² s⁻¹ (b) 2; L mol⁻¹ s⁻¹ (c) 3; mol L⁻¹ s⁻¹ (d) 1; s⁻¹

Accepted Answer

(a) 3; L² mol⁻² s⁻¹.

Overall order = 1 + 2 = 3. Units of k = (mol L⁻¹)^(1−3) s⁻¹ = (mol L⁻¹)⁻² s⁻¹ = L² mol⁻² s⁻¹.

Question 2

Which statement is correct?
(a) Order is experimental and can be fractional; molecularity is a whole number (b) Both are always equal (c) Molecularity can be zero (d) Order is always a whole number

Accepted Answer

(a) Order is experimental and can be fractional; molecularity is a whole number.

Order is found from the rate law and may be zero, fractional or negative; molecularity counts species in an elementary step and must be a positive integer.

Question 3

A first-order reaction has a rate constant of 0.0693 s⁻¹. Its half-life is:
(a) 10 s (b) 20 s (c) 6.93 s (d) 100 s

Accepted Answer

(a) 10 s.

t₁/₂ = 0.693/k = 0.693/0.0693 = 10 s.

Question 4

For a first-order reaction, the fraction of reactant remaining after 3 half-lives is:
(a) 1/8 (b) 1/3 (c) 1/6 (d) 1/2

Accepted Answer

(a) 1/8.

Each half-life halves the amount, so after 3 it is (1/2)³ = 1/8, independent of the starting concentration.

Question 5

In a first-order reaction, 75% of the reactant is consumed. The number of half-lives elapsed is:
(a) 2 (b) 1 (c) 3 (d) 4

Accepted Answer

(a) 2.

75% consumed means 25% (1/4) remains. Since (1/2)ⁿ = 1/4 gives n = 2, two half-lives have elapsed.

Question 6

A catalyst increases the reaction rate by:
(a) lowering the activation energy (b) increasing ΔH (c) shifting the equilibrium (d) raising the temperature

Accepted Answer

(a) lowering the activation energy.

A catalyst offers an alternative path with lower Eₐ, so more molecules can cross the barrier. It does not change ΔH or the equilibrium position.

Question 7

The rate of a reaction typically ______ for every 10 °C rise in temperature:
(a) doubles (b) halves (c) is unchanged (d) becomes zero

Accepted Answer

(a) doubles.

The temperature coefficient is about 2–3: a 10 °C rise roughly doubles (to triples) the rate because far more molecules exceed the activation energy (Arrhenius/Maxwell distribution).

Question 8

The molality of a solution containing 0.5 mol of solute in 250 g of solvent is:
(a) 2 m (b) 0.5 m (c) 1 m (d) 0.25 m

Accepted Answer

(a) 2 m.

Molality = mol solute / kg solvent = 0.5 / 0.250 = 2 mol/kg.

Question 9

According to Henry's law, the solubility of a gas in a liquid, as temperature increases:
(a) decreases (b) increases (c) stays constant (d) becomes infinite

Accepted Answer

(a) decreases.

Gas solubility falls with rising temperature (warm carbonated drinks go flat), and it is proportional to the partial pressure of the gas above the liquid.

Question 10

A solution of ethanol and water shows a higher vapour pressure than predicted by Raoult's law. This is:
(a) positive deviation (b) negative deviation (c) ideal behaviour (d) no deviation

Accepted Answer

(a) positive deviation.

Ethanol–water interactions are weaker than the pure-component interactions, so molecules escape more easily and the vapour pressure exceeds Raoult's prediction — a positive deviation, giving a minimum-boiling azeotrope.

Question 11

The depression in freezing point of a solution of 0.1 mol of a non-electrolyte in 1 kg of water (Kf = 1.86 K kg mol⁻¹) is:
(a) 0.186 K (b) 1.86 K (c) 0.0186 K (d) 18.6 K

Accepted Answer

(a) 0.186 K.

ΔTf = Kf·m = 1.86 × 0.1 = 0.186 K, so the solution freezes at −0.186 °C.

Question 12

The depression in freezing point of 0.1 m NaCl (i = 2, Kf = 1.86) in water is:
(a) 0.372 K (b) 0.186 K (c) 0.093 K (d) 3.72 K

Accepted Answer

(a) 0.372 K.

ΔTf = i·Kf·m = 2 × 1.86 × 0.1 = 0.372 K — double the non-electrolyte value because NaCl gives two ions per formula unit.

Question 13

The osmotic pressure of a 0.1 M solution at 300 K (R = 0.0821 L atm K⁻¹ mol⁻¹) is:
(a) 2.46 atm (b) 0.246 atm (c) 24.6 atm (d) 1.23 atm

Accepted Answer

(a) 2.46 atm.

π = CRT = 0.1 × 0.0821 × 300 = 2.46 atm.

Question 14

Which of the following is NOT a colligative property?
(a) vapour pressure of the pure solvent (b) osmotic pressure (c) elevation of boiling point (d) depression of freezing point

Accepted Answer

(a) vapour pressure of the pure solvent.

Colligative properties depend on the number of solute particles: relative lowering of vapour pressure, boiling-point elevation, freezing-point depression and osmotic pressure. The vapour pressure of the pure solvent is a property of the solvent alone.

Question 15

Acetic acid dimerises in benzene. Its van't Hoff factor i is:
(a) about 0.5 (b) about 2 (c) exactly 1 (d) about 3

Accepted Answer

(a) about 0.5.

Dimerisation halves the number of particles (two molecules become one), so i ≈ 0.5 (i < 1 for association), giving an abnormally high observed molar mass.

Practice — Chemical Kinetics and Solutions

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