R1.4 Entropy and spontaneity questions for IB Chemistry

97 questions on Entropy and spontaneity, written by Augment in the style of IB exam papers. Six of them are below in full, to answer on paper.

IB Chemistry
Part of R1 What drives chemical reactions?
HL only

Questions by paper and level

Paper	Questions
Paper 1A	42
Paper 1B	9
Paper 2	46
Every paper	97

Counted on 10 November 2026

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Written by Augment in the style of IB exams, not past papers. Answer them on paper, then check each one against its markscheme with a free account.

Question 1 · CHEM-01009Paper 1A · 1 mark

Which change results in a decrease in the entropy of the system?

A
$H_{2} O (l) \to H_{2} O (g)$
B
$N H_{4} C l (s) \to N H_{3} (g) + H C l (g)$
C
$2 S O_{2} (g) + O_{2} (g) \to 2 S O_{3} (g)$
D
$C a C O_{3} (s) \to C a O (s) + C O_{2} (g)$

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Question 2 · CHEM-01899Paper 1B · 12 marks

A student determined $Δ H^{⊖}$ and $Δ S^{⊖}$ for dissolving potassium nitrate from its solubility at different temperatures.

K N O_{3} (s) ⇌ K^{+} (a q) + N O_{3}^{-} (a q) K = [K^{+}] [N O_{3}^{-}] = s^{2}

where $s$ is the concentration of the saturated solution in mol dm⁻³.

Method:

A known mass of $K N O_{3}$ was added to 10.0 cm³ of water in a graduated boiling tube.
The tube was heated in a water bath until all the solid dissolved, and the total volume of solution was read.
The tube was removed and the solution stirred with a thermometer as it cooled. The temperature at which crystals first appeared was recorded.

Mass of KNO₃ / g ± 0.01 g	Volume of solution / cm³ ± 0.2 cm³	Crystallization temperature / °C ± 0.5 °C	$s$ / mol dm⁻³	$\ln K$	$\frac{1}{T}$ / $10^{- 3}$ K⁻¹
4.00	11.6	28.0	3.41	2.45	3.321
5.00	12.0	36.5	4.12	2.83	3.229
6.00	12.4	41.5
7.00	12.8	47.0	5.41	3.38	3.124
8.00	13.2	51.5	5.99	3.58	3.080

Combining $Δ G^{⊖} = - R T \ln K$ with $Δ G^{⊖} = Δ H^{⊖} - T Δ S^{⊖}$ gives:

\ln K = - \frac{Δ H^{⊖}}{R} (\frac{1}{T}) + \frac{Δ S^{⊖}}{R}

The graph shows all five results with a line of best fit.

best fit

ln K (y-axis) against 1/T in 10⁻³ K⁻¹ (x-axis) for dissolving KNO₃

(a)
Identify the independent and the dependent variable in this experiment.
1 mark
(b)
Explain why the temperature at which crystals first appear is used.
1 mark
(c)
Calculate the missing values of $s$ , $\ln K$ and $\frac{1}{T}$ for the 6.00 g sample. $M (K N O_{3}) = 101.11 {g mol}^{- 1}$ .
2 marks
(d)
Calculate the percentage uncertainty in $K$ for the 4.00 g sample, using section 23 of the data booklet.
2 marks
(e)
Determine $Δ H^{⊖}$ , in kJ mol⁻¹, for dissolving potassium nitrate, using the gradient of the line of best fit.
2 marks
(f)
Calculate $Δ G^{⊖}$ at 320.1 K, using the value of $\ln K$ for the 7.00 g sample, and hence determine $Δ S^{⊖}$ , in J K⁻¹ mol⁻¹.
2 marks
(g)
Explain the sign of $Δ S^{⊖}$ for dissolving potassium nitrate.
1 mark
(h)
Suggest one source of random error in the method and an improvement to reduce its effect.
1 mark

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Question 3 · CHEM-01095Paper 2 · 8 marks

Ammonium nitrate dissolves readily in water at room temperature, although the process is endothermic.

N H_{4} N O_{3} (s) \to N H_{4}^{+} (a q) + N O_{3}^{-} (a q)

When 4.00 g of ammonium nitrate was dissolved in 100.0 g of water at 298 K, the temperature fell by 2.9 °C.

(a)
Calculate the enthalpy change of solution of ammonium nitrate, in kJ mol⁻¹.
3 marks
(b)
Outline why the entropy change for this process is positive.
1 mark
(c)
The standard entropy change for the process is +109 J K⁻¹ mol⁻¹. Calculate ΔG^⦵, in kJ mol⁻¹, at 298 K. Use your answer to (a) and section 1 of the data booklet.
2 marks
(d)
Explain why the process is spontaneous although it is endothermic.
1 mark
(e)
Predict, with a reason, how increasing the temperature affects the spontaneity of the process.
1 mark

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Question 4 · CHEM-01010Paper 1A · 1 mark

Which substance has the greatest standard entropy, $S^{⊖}$ , at 298 K?

A
$C (s)$ (graphite)
B
$H_{2} O (l)$
C
$C H_{3} O H (l)$
D
$C H_{4} (g)$

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Question 5 · CHEM-01900Paper 1B · 10 marks

Iron(III) ions react with thiocyanate ions to form a blood-red complex ion.

F e^{3 +} (a q) + S C N^{-} (a q) ⇌ F e S C N^{2 +} (a q)

Equilibrium mixtures were placed in a colorimeter cell held in a thermostatted water bath. The equilibrium concentration of $F e S C N^{2 +}$ was found from the absorbance using a calibration curve, and used to calculate $K$ at each temperature.

$T$ / K ± 0.5 K	288.0	298.0	308.0	318.0	328.0
$K$	209	158	123	113	77
$Δ G^{⊖}$ / kJ mol⁻¹	−12.79	−12.54		−12.49	−11.84

The graph shows $Δ G^{⊖}$ against $T$ with a line of best fit that excludes the result at 318.0 K, extrapolated to 0 K.

best fit

ΔG° / kJ mol⁻¹ (y-axis) against T / K (x-axis)

(a)
Explain why colorimetry is a suitable method for finding the concentration of $F e S C N^{2 +}$ in these mixtures.
1 mark
(b)
Using section 15 of the data booklet, suggest a suitable wavelength, in nm, for the measurements.
1 mark
(c)
Calculate the missing value of $Δ G^{⊖}$ at 308.0 K.
2 marks
(d)
Identify the anomalous result and suggest a reason for it, other than a calculation error.
2 marks
(e)
Determine $Δ S^{⊖}$ , in J K⁻¹ mol⁻¹, from the gradient of the line.
2 marks
(f)
Determine $Δ H^{⊖}$ , in kJ mol⁻¹, from the graph.
1 mark
(g)
Outline why the value of $Δ H^{⊖}$ obtained from this graph is less reliable than the value of $Δ S^{⊖}$ .
1 mark

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Question 6 · CHEM-01357Paper 2 · 10 marks

Hydrogen and iodine react in a sealed vessel to form hydrogen iodide.

H_{2} (g) + I_{2} (g) ⇌ 2 H I (g)

0.500 mol of hydrogen and 0.400 mol of iodine were sealed in a 2.00 dm³ flask and held at 700 K until equilibrium was reached. The equilibrium mixture contained 0.640 mol of hydrogen iodide.

(a)
Identify the limiting reactant and state the amount, in mol, of hydrogen iodide that would form if the reaction went to completion.
2 marks
(b)
Calculate the percentage yield of hydrogen iodide at equilibrium.
1 mark
(c)
Determine the equilibrium amounts, in mol, of hydrogen and iodine.
2 marks
(d)
Calculate the value of the equilibrium constant, K_c, at 700 K.
2 marks
(e)
Calculate the standard Gibbs energy change, ΔG^⦵, in kJ mol⁻¹, for the reaction at 700 K. Use sections 1 and 2 of the data booklet.
2 marks
(f)
Explain why the percentage yield of hydrogen iodide cannot reach 100 % in a sealed vessel, even if no product is lost.
1 mark

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