A-Level Chemistry Flashcards

What does Hess's Law state?

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

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.

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

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

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.

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

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

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.

What is Le Chatelier's Principle?

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

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.

What is the difference between SN1 and SN2 nucleophilic substitution?

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

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).

What is Avogadro's constant and what does it represent?

Answer: The number of particles in one mole of a substance: 6.022 × 10²³

Avogadro's constant (Nₐ = 6.022 × 10²³ mol⁻¹) is the number of entities (atoms, molecules, ions) in one mole of any substance. It links the macroscopic (moles) to the microscopic (individual particles).