A-Level A-Level Chemistry 2 — Questions and Answers
Question 1: What is a buffer solution and how does it resist pH change?
- A solution that changes pH easily
- 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 (Correct answer)
- A solution at exactly pH 7
- A solution that neutralises strong acids only
Correct 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.
Question 2: What is the Born-Haber cycle used for?
- Calculating reaction rates
- Calculating lattice enthalpy using Hess's Law, combining several measurable enthalpy changes (Correct answer)
- Predicting the products of organic reactions
- Measuring bond enthalpies directly
Correct 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.
Question 3: What is Markovnikov's rule in addition reactions?
- The less electronegative carbon gets the hydrogen
- When HX adds across an asymmetric double bond, the hydrogen adds to the carbon already bearing the most hydrogens (Correct answer)
- The halide always adds to the terminal carbon
- Addition always forms a major and minor product in equal amounts
Correct 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.
Question 4: What is the difference between an endothermic and an exothermic reaction in terms of enthalpy?
- Endothermic: ΔH > 0 (heat absorbed); exothermic: ΔH < 0 (heat released) (Correct answer)
- Endothermic: ΔH < 0; exothermic: ΔH > 0
- Both have ΔH = 0
- Exothermic reactions absorb heat
Correct 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.
Question 5: What is the test for a halide ion and the expected observations?
- Add dilute nitric acid; precipitate forms with iodide only
- Add silver nitrate solution (in dilute HNO₃): Cl⁻ gives white ppt (AgCl); Br⁻ gives cream ppt (AgBr); I⁻ gives yellow ppt (AgI) (Correct answer)
- Add universal indicator and measure pH
- Add sodium hydroxide; colour changes indicate halide present
Correct 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.
Question 6: What is a nucleophile?
- An electron-pair acceptor with a positive charge
- An electron-pair donor — a species rich in electrons that attacks electron-deficient (δ+) carbon atoms (Correct answer)
- A type of catalyst in organic reactions
- A species that only reacts with alkenes
Correct 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.
A-Level A-Level Chemistry 2
What is a buffer solution and how does it resist pH change?