A-Level Chemistry Flashcards

What is a buffer solution and how does it resist pH change?

Answer: A solution that resists changes in pH when small amounts of acid or alkali are added, typically containing a weak acid and its conjugate base

A buffer contains a weak acid (HA) and its conjugate base (A⁻, from a salt). Adding acid: A⁻ + H⁺ → HA (base removes excess H⁺). Adding alkali: HA + OH⁻ → A⁻ + H₂O (acid neutralises OH⁻). pH change is minimal.

What is the Born-Haber cycle used for?

Answer: Calculating lattice enthalpy using Hess's Law, combining several measurable enthalpy changes

A Born-Haber cycle is a Hess's Law cycle used to calculate lattice enthalpy (difficult to measure directly). It incorporates atomisation, ionisation, electron affinity, and formation enthalpies to find the lattice energy.

What is Markovnikov's rule in addition reactions?

Answer: When HX adds across an asymmetric double bond, the hydrogen adds to the carbon already bearing the most hydrogens

Markovnikov's rule states that when HX adds to an unsymmetrical alkene, H adds to the C with more hydrogens (forming the more stable, more substituted carbocation intermediate). This determines the major product.

What is the difference between an endothermic and an exothermic reaction in terms of enthalpy?

Answer: Endothermic: ΔH > 0 (heat absorbed); exothermic: ΔH < 0 (heat released)

In an endothermic reaction, the system absorbs heat from surroundings: ΔH > 0 (positive). Products have higher energy than reactants. In exothermic reactions, heat is released: ΔH < 0 (negative). Products are lower in energy than reactants.

What is the test for a halide ion and the expected observations?

Answer: Add silver nitrate solution (in dilute HNO₃): Cl⁻ gives white ppt (AgCl); Br⁻ gives cream ppt (AgBr); I⁻ gives yellow ppt (AgI)

Halide ions are identified by adding AgNO₃ solution (acidified with dilute HNO₃ to remove interfering ions). AgCl is white and soluble in dilute NH₃; AgBr is cream and soluble only in conc NH₃; AgI is yellow and insoluble in NH₃.

What is a nucleophile?

Answer: An electron-pair donor — a species rich in electrons that attacks electron-deficient (δ+) carbon atoms

A nucleophile is an electron-rich species that donates an electron pair to an electron-deficient atom. Common nucleophiles include OH⁻, CN⁻, NH₃, and halide ions. They attack δ+ carbons in polar covalent bonds.