A-Level Chemistry Fundamentals 2 — Questions and Answers
Question 1: Which statement about the reaction between sodium and water is correct?
- Sodium floats, melts into a ball, and produces hydrogen gas and sodium hydroxide (Correct answer)
- Sodium sinks and reacts slowly to produce sodium oxide
- Sodium reacts violently producing sodium chloride and oxygen
- Sodium dissolves without any gas being produced
Correct answer: Sodium floats, melts into a ball, and produces hydrogen gas and sodium hydroxide
Sodium is less dense than water so it floats. The exothermic reaction (2Na + 2H₂O → 2NaOH + H₂) melts the sodium into a ball and produces hydrogen gas, which can ignite with larger pieces.
Question 2: What type of reaction mechanism does the hydrolysis of a primary halogenoalkane with aqueous NaOH follow?
- SN2 (nucleophilic substitution, bimolecular) (Correct answer)
- SN1 (nucleophilic substitution, unimolecular)
- Electrophilic addition
- Free radical substitution
Correct answer: SN2 (nucleophilic substitution, bimolecular)
Primary halogenoalkanes undergo SN2 mechanisms because the relatively unhindered carbon allows the nucleophile (OH⁻) to attack simultaneously as the halide leaves, in a one-step concerted process. The rate depends on concentrations of both reactants.
Question 3: Le Chatelier's principle predicts that increasing the pressure of a gaseous equilibrium will favour which side?
- The side with fewer moles of gas (Correct answer)
- The side with more moles of gas
- The exothermic direction
- Neither side — pressure has no effect on equilibrium
Correct answer: The side with fewer moles of gas
According to Le Chatelier's principle, increasing pressure shifts the equilibrium to reduce pressure, which means favouring the side with fewer moles of gas molecules. This only applies to reactions where the number of gas moles differs between sides.
Question 4: What is the enthalpy change for the reaction if 50 cm³ of 1.0 mol dm⁻³ HCl reacts with 50 cm³ of 1.0 mol dm⁻³ NaOH, raising the temperature by 6.8°C? (Assume c = 4.18 J g⁻¹ K⁻¹, density = 1 g cm⁻³)
- -56.8 kJ mol⁻¹ (Correct answer)
- -2.84 kJ mol⁻¹
- +56.8 kJ mol⁻¹
- -28.4 kJ mol⁻¹
Correct answer: -56.8 kJ mol⁻¹
q = mcΔT = 100 × 4.18 × 6.8 = 2842.4 J. Moles of acid = 0.050 × 1.0 = 0.050 mol. ΔH = -2842.4 / 0.050 = -56,848 J mol⁻¹ ≈ -56.8 kJ mol⁻¹. Negative because the reaction is exothermic (temperature rises).
Question 5: In mass spectrometry, what does the peak with the highest m/z value in the spectrum of a compound typically represent?
- The molecular ion (M⁺) (Correct answer)
- The base peak
- The most stable fragment
- A doubly charged ion
Correct answer: The molecular ion (M⁺)
The peak at the highest m/z value (ignoring isotope peaks like M+1) represents the molecular ion M⁺, formed when the molecule loses one electron. This gives the relative molecular mass of the compound.
Question 6: Which of the following is the correct order of increasing boiling points?
- CH₄ < NH₃ < H₂O (Correct answer)
- NH₃ < CH₄ < H₂O
- H₂O < NH₃ < CH₄
- CH₄ < H₂O < NH₃
Correct answer: CH₄ < NH₃ < H₂O
CH₄ has only weak London dispersion forces (bp -164°C). NH₃ has hydrogen bonding (bp -33°C). H₂O has stronger hydrogen bonding due to two lone pairs and two O-H bonds creating extensive H-bond networks (bp 100°C).
A-Level Chemistry Fundamentals 2
Which statement about the reaction between sodium and water is correct?