A-Level A-Level Chemistry

Question 1

What does Hess's Law state?

Accepted Answer

The enthalpy change for a reaction is the same regardless of the route taken, provided initial and final states are the same

Answer

Hess's Law (conservation of energy) states that the total enthalpy change for a chemical reaction is independent of the route taken. This allows calculation of enthalpy changes that cannot be measured directly using thermochemical cycles.

Question 2

What is electronegativity and what is the trend across Period 3?

Accepted Answer

A measure of an atom's ability to attract the bonding electron pair; it generally increases across Period 3 (left to right)

Answer

Electronegativity is the ability of an atom in a covalent bond to attract the shared electron pair. Across Period 3, electronegativity increases from Na to Cl as nuclear charge increases while shielding stays similar, pulling electrons more strongly.

Question 3

What is the rate-determining step in a reaction mechanism?

Accepted Answer

The slowest step in the mechanism, which determines the overall rate of the reaction

Answer

The rate-determining step (RDS) is the slowest step in a multi-step reaction mechanism. It acts as the bottleneck. Only the species involved in the RDS appear in the rate equation, making it key to understanding kinetics.

Question 4

What is Le Chatelier's Principle?

Accepted Answer

When a system at equilibrium is subjected to a change, the equilibrium shifts to oppose that change

Answer

Le Chatelier's Principle states that if a system at dynamic equilibrium is disturbed (by changing concentration, pressure, or temperature), the equilibrium shifts in the direction that opposes the change to restore equilibrium.

Question 5

What is the difference between SN1 and SN2 nucleophilic substitution?

Accepted Answer

SN1 is a two-step mechanism via a carbocation intermediate (first order); SN2 is a one-step concerted mechanism (second order) with backside attack

Answer

SN1 proceeds via a carbocation intermediate; rate = k[RX] (first order, favoured by tertiary halogenoalkanes). SN2 involves simultaneous attack and departure in one step; rate = k[RX][Nu] (second order, favoured by primary halogenoalkanes, inversion of configuration).

A-Level Revision Practice Test

A-Level Revision Practice Test

A-Level A-Level Chemistry