CAIE Chemistry (9701)

2020 May/June
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Questions & Answers

Q1c(i).
Short Answer 1 mark

for the activation energy of reaction A.

Model Answer:
Activation energy of reaction A = the energy difference between the reactants and the peak of the energy profile for reaction A.
Explanation: Activation energy is the minimum energy needed for reaction to occur.
Q1c(ii).
Short Answer 1 mark

The enthalpy change for reaction A is -14 kJ mol-1. Predict a value for the enthalpy change for reaction B.

Model Answer:
A suitable predicted value is +14 kJ mol-1.
Explanation: Reaction B is the reverse of reaction A, so the enthalpy change has the same magnitude but opposite sign.
Q1c(iii).
Structured 2 marks

Sketch a labelled energy level diagram for reaction B. Use relevant values from (c)(i) and (c)(ii).

Model Answer:
Energy level diagram with reactants and products labelled, a peak showing the activation energy for reaction B, and an overall enthalpy change of +14 kJ mol-1.
Explanation: Reaction B is endothermic, so products are higher in energy than reactants by 14 kJ mol-1.
Q1d(i).
Short Answer 2 marks

Explain what is meant by the term enthalpy change of combustion of sucrose.

Model Answer:
The enthalpy change when 1 mol of sucrose is completely burned in excess oxygen under standard conditions, with all reactants and products in their standard states.
Explanation: This is the standard definition of enthalpy change of combustion.
Q1d(ii).
Structured 3 marks

Use the Data Booklet to calculate the enthalpy change, in kJ mol-1, for the combustion of sucrose. Assume that all of the heat energy produced is transferred to the water. Show your working.

Model Answer:
q = mcΔT = 250 × 4.18 × (40.7 - 25.0) = 1.64 × 10^4 J = 16.4 kJ Molar mass of sucrose, C12H22O11 = 342 g mol-1 Moles of sucrose = 1.00 / 342 = 2.92 × 10^-3 mol ΔHc = -16.4 / (2.92 × 10^-3) = -5.62 × 10^3 kJ mol-1
Explanation: The heat gained by water equals heat released by combustion; the negative sign shows combustion is exothermic.
Q4a(i).
Short Answer 1 mark

Suggest the identity of X.

Model Answer:
X is chlorine.
Explanation: A cream precipitate with silver nitrate indicates AgCl.
Q4a(ii).
Short Answer 1 mark

Write an ionic equation to describe the formation of the cream precipitate. Include state symbols.

Model Answer:
Ag+(aq) + Cl-(aq) -> AgCl(s)
Explanation: Silver ions react with chloride ions to form cream silver chloride precipitate.
Q4a(iii).
Short Answer 2 marks

Describe a further test which would confirm the identity of X. test expected result

Model Answer:
Test the precipitate with dilute aqueous ammonia; silver chloride dissolves in dilute ammonia to give a colourless solution.
Explanation: AgCl is soluble in dilute ammonia, unlike AgBr and AgI.
Q4b(i).
Short Answer 1 mark

Suggest the structural formula of the straight-chain halogenoalkane Q.

Model Answer:
CH3CH2CH2CH2CH2Cl
Explanation: A straight-chain C5H11Cl halogenoalkane that undergoes SN2 is 1-chloropentane.
Q4b(ii).
Short Answer 2 marks

Explain why the reaction tends to proceed via an SN2 mechanism rather than an SN1 mechanism.

Model Answer:
Because Q is a primary halogenoalkane, formation of a primary carbocation is unstable and unlikely, so substitution occurs in one step by nucleophilic attack and leaving group departure (SN2).
Explanation: Primary halogenoalkanes favour SN2 due to poor carbocation stability.
Q4c(i).
Short Answer 1 mark

Name the type of reaction occurring.

Model Answer:
Elimination.
Explanation: Alcoholic hydroxide and heat under reflux promote elimination of HCl from halogenoalkanes.
Q4c(ii).
Short Answer 1 mark

Write an equation, using molecular formulae, to represent the reaction occurring.

Model Answer:
C4H9Cl + NaOH -> C4H8 + NaCl + H2O
Explanation: Elimination of HCl from C4H9Cl gives methylpropene, molecular formula C4H8.
Q4c(iii).
Short Answer 1 mark

Draw the skeletal formula of methylpropene.

Model Answer:
2-methylpropene, skeletal formula: CH2=C(CH3)2
Explanation: Methylpropene is 2-methylpropene.
Q4c(iv).
Short Answer 2 marks

Give the names of P and R.

Model Answer:
P and R are 1-chlorobutane and 2-chlorobutane.
Explanation: Both isomers can eliminate to form methylpropene as the major product.
Q5a.
Structured 6 marks

Complete the table to give the name of the reaction mechanisms of reactions 1 and 6 and the reagents and conditions required for reactions 1, 2 and 6.

Model Answer:
Reaction 1 mechanism: electrophilic addition; reagent/conditions: HBr(g) or hydrogen bromide, room temperature. Reaction 2 reagent/conditions: steam, phosphoric acid catalyst, high temperature and pressure. Reaction 6 mechanism: nucleophilic substitution; reagent/conditions: aqueous NaOH, heat under reflux.
Explanation: Ethene commonly undergoes electrophilic addition with hydrogen halides, hydration with steam, and haloalkane hydrolysis by nucleophilic substitution.
Q5b(i).
Structured 3 marks

In reaction 3 the organic molecule reacts with HCN and a KCN catalyst. Complete the diagram to show the mechanism of the reaction occurring. Include all relevant dipoles, lone pairs and curly arrows in your answer.

Model Answer:
Mechanism: CN- attacks the δ+ carbon of the carbonyl group; the C=O pi bond moves to oxygen to form an alkoxide intermediate; then the alkoxide is protonated by HCN to form the hydroxynitrile.
Explanation: This is nucleophilic addition to a carbonyl compound.
Q5b(ii).
Short Answer 2 marks

Name the functional groups present in the product of reaction 3.

Model Answer:
Hydroxyl (alcohol) and nitrile.
Explanation: HCN addition to a carbonyl gives a hydroxynitrile containing both groups.
Q5c.
Short Answer 1 mark

Draw the structure of the organic molecule W formed in reaction 4.

Model Answer:
W is the alcohol formed by reduction of the carbonyl compound in reaction 4; structure: CH3CH(OH)CH3 if the starting carbonyl is propanone, or the corresponding reduced alcohol shown by the reaction scheme.
Explanation: Reaction 4 is reduction of a carbonyl compound to an alcohol.
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