A-Level H2 Chemistry 5 — Questions and Answers
Question 1: Predict the product of the reaction between propan-1-ol and concentrated H₂SO₄ at 170°C.
- Propan-2-ol
- Propene (Correct answer)
- Propanoic acid
- Propyl sulfate
Correct answer: Propene
At 170°C, concentrated H₂SO₄ dehydrates the alcohol by elimination of water: CH₃CH₂CH₂OH → CH₃CH=CH₂ + H₂O (propene). At 140°C, it would form an ether.
Question 2: What is the difference between SN1 and SN2 mechanisms?
- SN1 is faster; SN2 is slower
- SN1 is a one-step mechanism; SN2 involves a carbocation intermediate
- SN1 has a carbocation intermediate (two steps); SN2 is one-step with backside attack (Correct answer)
- They are the same mechanism
Correct answer: SN1 has a carbocation intermediate (two steps); SN2 is one-step with backside attack
SN1: two-step — carbocation intermediate formed, then nucleophile attacks. SN2: one-step — concerted backside attack by nucleophile, inversion of configuration. SN1 favoured by tertiary; SN2 by primary halides.
Question 3: What is the molecular formula of aspirin (acetylsalicylic acid)?
- C₆H₅COOH
- C₉H₈O₄ (Correct answer)
- C₇H₆O₃
- C₈H₉NO₂
Correct answer: C₉H₈O₄
Aspirin (acetylsalicylic acid) has the molecular formula C₉H₈O₄. It is formed by the esterification of salicylic acid with ethanoic anhydride.
Question 4: What is shown by a Boltzmann distribution curve in kinetics?
- The variation in reaction rate with temperature
- The distribution of energies among molecules in a gas at a given temperature (Correct answer)
- The activation energy profile of a reaction
- The concentration changes over time
Correct answer: The distribution of energies among molecules in a gas at a given temperature
The Boltzmann distribution shows the spread of molecular kinetic energies in a gas sample at a given temperature. A catalyst or higher temperature shifts the distribution, increasing the proportion of molecules with energy ≥ Ea.
Question 5: What is meant by 'degree of unsaturation' (or index of hydrogen deficiency)?
- The number of lone pairs in a molecule
- The number of rings and multiple bonds in a molecule compared to the saturated equivalent (Correct answer)
- The ratio of carbon to hydrogen
- The number of functional groups
Correct answer: The number of rings and multiple bonds in a molecule compared to the saturated equivalent
Degree of unsaturation = (2C + 2 + N - H - X)/2 for a molecule. Each ring or double bond contributes 1; each triple bond contributes 2. It helps determine molecular structure.
Question 6: Describe the trend in electronegativity across Period 3 (Na to Cl).
- Decreases from Na to Cl
- Increases from Na to Cl (Correct answer)
- Remains constant
- First increases then decreases
Correct answer: Increases from Na to Cl
Electronegativity increases across Period 3 from Na to Cl because nuclear charge increases while atomic radius decreases and shielding remains roughly constant, increasing the pull on bonding electrons.
A-Level H2 Chemistry 5
Predict the product of the reaction between propan-1-ol and concentrated H₂SO₄ at 170°C.