A-Level A-Level Chemistry — Questions and Answers
Question 1: What does Hess's Law state?
- The enthalpy of a reaction depends on the conditions
- The enthalpy change for a reaction is the same regardless of the route taken, provided initial and final states are the same (Correct answer)
- Energy cannot be created or destroyed
- The rate of reaction depends on temperature
Correct 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.
Question 2: What is electronegativity and what is the trend across Period 3?
- The energy needed to remove an electron; it decreases across Period 3
- A measure of an atom's ability to attract the bonding electron pair; it generally increases across Period 3 (left to right) (Correct answer)
- The number of electrons in the outer shell
- The size of an atom
Correct 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.
Question 3: What is the rate-determining step in a reaction mechanism?
- The fastest step
- The slowest step in the mechanism, which determines the overall rate of the reaction (Correct answer)
- The step that produces the most energy
- The final step of the mechanism
Correct 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.
Question 4: What is Le Chatelier's Principle?
- A principle about the rate of reactions
- When a system at equilibrium is subjected to a change, the equilibrium shifts to oppose that change (Correct answer)
- The principle that reactions always go to completion
- The principle that exothermic reactions are always faster
Correct 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.
Question 5: What is the difference between SN1 and SN2 nucleophilic substitution?
- They produce different types of products
- SN1 is a two-step mechanism via a carbocation intermediate (first order); SN2 is a one-step concerted mechanism (second order) with backside attack (Correct answer)
- SN2 only occurs with primary halogenoalkanes
- SN1 is faster than SN2 always
Correct 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).
Question 6: What is Avogadro's constant and what does it represent?
- The speed of light
- The number of particles in one mole of a substance: 6.022 × 10²³ (Correct answer)
- The charge on an electron
- The molar gas volume at STP
Correct 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).
A-Level A-Level Chemistry
What does Hess's Law state?