DP Chemistry (first assessment 2025)

Question SPM.2.HL.TZ0.5

Date	May Specimen-2023	Marks available	[Maximum mark: 26]	Reference code	SPM.2.HL.TZ0.5
Level	HL	Paper	2	Time zone	TZ0
Command term	Calculate, Comment, Deduce, Determine, Discuss, Explain, Identify, Outline, Predict, State, Write	Question number	5	Adapted from	N/A

[Maximum mark: 26]

SPM.2.HL.TZ0.5

Heptadecane, C₁₇H₃₆, can be extracted from crude oil or cactus plants.

(a)

Write an equation for the complete combustion of C₁₇H₃₆.

[1]

Markscheme

C₁₇H₃₆(l) + 26O₂(g) → 17CO₂(g) + 18H₂O(l)

(b)

The enthalpy of combustion of C₁₇H₃₆ is −11 350 kJ mol⁻¹.

(b.i)

Calculate the maximum energy produced when 2.00 g of C₁₇H₃₆ is combusted.

[2]

Markscheme

nC₁₇H₃₆ = « $\frac{2.00 g}{[(17 \times 12.01 g {mol}^{- 1}) + (36 \times 1.01 g {mol}^{- 1})]} = \frac{2.00 g}{240.53 g {mol}^{- 1}}$ =»

0.008315/0.00831 «mol» ✔
« energy = 11350 kJ mol^{$-$ 1} × 0.008315 mol = » 94.4 «kJ» ✔

(b.ii)

Determine the maximum temperature change when 500.0 cm³ of water is heated by a 2.00 g sample of C₁₇H₃₆.

[2]

Markscheme

94 400 = 500.0 g × 4.18 J g^{$-$ 1} K^{$-$ 1} × $Δ$ T ✔
$Δ$ T = 45.2«K» ✔

(b.iii)

Outline two assumptions made in the calculation in (b)(ii).

[2]

Markscheme

Any two:
water does not evaporate  ✔
heat is not lost to the surroundings
OR
all heat is transferred to the water  ✔
density of water is 1 g cm^{$-$ 3}  ✔
water is pure ✔
complete combustion  ✔

(c)

Explain why biofuels contribute less to climate change than fossil fuels.

[1]

Markscheme

CO₂ consumed while plant is growing «and later released when biofuel is combusted»
OR
photosynthesis uses up CO₂ «later released when biofuel is combusted» ✔

(d)

Heptadecane can be broken down into smaller molecules. Consider the reaction:

C₁₇H₃₆(g) → 2 C₂H₄(g) + C₁₃H₂₈(g)

(d.i)

Determine the standard enthalpy change, $Δ$ H^⦵, for the reaction stated, using section 12 of the data booklet.

[3]

Markscheme

bonds broken: 4(C $-$ C ) / 4 $\times$ 346  ✔
bonds formed: 2(C=C) / 2 × 614  ✔
$Δ$ H^⦵ = «4 × 346 kJ $-$ 2 × 614 kJ / 1384 kJ $-$ 1228 kJ =»
$Δ$ H^⦵ = «+»156 «kJ»  ✔

Award [3] for correct final answer.

(d.ii)

Determine the enthalpy change of reaction. Use the data provided and section 13 of the data booklet.

$Δ$ H_f (C₁₇H₃₆) = $-$ 393.9 kJ mol^{$-$ 1}
$Δ$ H_f (C₁₃H₂₈) = $-$ 311.5 kJ mol^{$-$ 1}

[2]

Markscheme

« $Δ$ H = $ΣΔ$ H_f (products) $-$ $ΣΔ$ H_f(reactants)»
$Δ$ H = ( $-$ 311.5 + 2(52.0)) $-$ ( $-$ 393.9) ✔
$Δ$ H = «+»186.4 «kJ mol^{$-$ 1}» ✔

(d.iii)

Comment on the difference between the two values calculated in (d)(i) and (d)(ii).

[1]

Markscheme

(d)(ii) more accurate than (d)(i) as bond energies are average values
OR
(d)(ii) more accurate than (d)(i) as not specific to bonds in the reaction ✔

(d.iv)

Predict the sign of the entropy change of the reaction, giving a reason.

[1]

Markscheme

positive AND increase in number of moles/molecules «of gas» ✔

(d.v)

Discuss, with reference to (d)(ii) and (d)(iv), how temperature affects the spontaneity of the reaction.

[2]

Markscheme

$Δ$ H > 0 «and $Δ$ S > 0» AND reaction spontaneous if $Δ$ G «= $Δ$ H $-$ T $Δ$ S» < 0 ✔
at high«er» T reaction «more» spontaneous/ $Δ$ G «more» negative ✔

(e)

Ethene can be converted to ethanol in one reaction. State the equation for this reaction.

[1]

Markscheme

C₂H₄(g) + H₂O(g) → C₂H₅OH(g) ✔

(f)

Ethanol reacts with oleic acid to produce ethyl oleate.

C₁₇H₃₃COOH(l) + CH₃CH₂OH(l) $⇌$ C₁₇H₃₃COOCH₂CH₃(l) + X(l)

(f.i)

Identify the type of reaction and the side product X (l).

[1]

Markscheme

condensation/esterification AND H₂O / water ✔

(f.ii)

Calculate the atom economy of the reaction.

[1]

Markscheme

«20(12.01) + 38(1.01) + 2(16.00) = 310.58

$\frac{100 \times 310.58}{(310.58 + 18.02)}$ = »

94.5 % ✔

(f.iii)

Discuss why the atom economy of a reaction is an important consideration when evaluating the impact of a reaction in an industrial process.

[2]

Markscheme

Any two of:
sustainable development  ✔
more economical/efficient  ✔
better use of natural resources  ✔
reduces waste  ✔

(f.iv)

Deduce the equilibrium constant expression, K_c, for the reaction. Assume that the reaction is homogeneous.

[1]

Markscheme

K = $\frac{[C_{17} H_{33} {COOCH}_{2} {CH}_{3}] [H_{2} O]}{C_{17} H_{33} COOH] [{CH}_{3} {CH}_{2} 0 H]}$

Accept expressions with [X(l)] instead of [H₂O].

(g)

The equilibrium constant, K_c, is 9.3 $\times$ 10⁻⁵ at 75 °C.

(g.i)

Determine the amount of ethyl oleate present in the reaction mixture at equilibrium when 0.10 mol dm^{$-$ 3} of oleic acid reacts with 0.10 mol dm^{$-$ 3} of ethanol at 75 °C and state any assumptions you have made in your calculation.

[2]

Markscheme

K small AND [reactants]_eqm = [reactants]₀ /0.10 ✔
n_oleate= « (0.10 × 0.10 × 9.3×10^{$-$ 5})^½ =» 9.6×10^{$-$ 4} «mol» ✔

(g.ii)

State one method of increasing the yield.

[1]

Markscheme

remove water
OR
add more oleic acid
OR
add more ethanol ✔

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