OCR A Level Chemistry A Periodic table, elements and physical chemistry June 2025 question paper H432/01

The table shows the number of protons and neutrons in four different atoms.

Which two atoms are isotopes of the same element?

A W and X

B X and Y

C X and Z

D Y and Z

11.895 g of hydrated cobalt(II) chloride, CoCl₂•xH₂O, is heated to remove the water of crystallisation.

After heating to constant mass, the mass of the anhydrous salt was 6.495 g.

What is the value of x ?

A 3

B 5

C 6

D 7

What volume, in cm³, of 0.250 mol dm^–3 barium hydroxide solution is required to exactly neutralise 25.0 cm³ of 0.115 mol dm^–3 hydrochloric acid?

A 5.75

B 11.50

C 23.00

D 54.35

The graph shows the volume of hydrogen released over time in a chemical reaction.

What is the rate of reaction, in cm³ s^–1, at 30 seconds?

A 0.8

B 1.2

C 2.1

D 2.4

Methane reacts with oxygen as shown in the equation:

The table shows some bond enthalpies.

What is the bond enthalpy, in kJ mol^–1, of the C–H bond in methane?

A +109

B +367

C +435

D +568

The table below shows the boiling points of some hydrogen halides.

What is the best explanation for the trend in boiling points from HC l to H I ?

A The strength of the covalent bonds increases.

B The strength of the hydrogen bonds increases.

C The strength of the induced dipole–dipole interactions (London forces) increases.

D The strength of the permanent dipole–dipole interactions increases.

The burette readings from an experiment are shown:

The burette has an uncertainty of ±0.05 cm³.

What is the percentage uncertainty of the titre?

A 0.17%

B 0.19%

C 0.34%

D 0.37%

The overall equation for a hydrogen–oxygen fuel cell is:

The half-equation at the negative electrode is:

What is the half-equation at the positive electrode?

Two redox systems are shown below.

Which species is the strongest reducing agent?

A Mg²⁺(aq)

B Mg(s)

C Pb²⁺(aq)

D Pb(s)

The rate constant, k, for a first order reaction is 6.19 × 10^–3 s^–1.

What is the half-life, in s, for the reaction?

A 4.29 × 10^–3

B 49

C 112

D 323

A mixture of ethanoic acid and ethanol is left to reach equilibrium.

A student determines the amount of ethanoic acid remaining in the equilibrium mixture by titrating the mixture with a solution of sodium hydroxide.

Which row of the table shows a suitable indicator for the titration?

Barium is in Group 2 of the periodic table.

Which statement(s) about barium is / are correct?

1 Its outer shell electrons are in an s-orbital.

2 It reacts more vigorously with water than magnesium.

3 It has a higher first ionisation energy than magnesium.

A 1, 2 and 3

B Only 1 and 2

C Only 2 and 3

D Only 1

Chromium forms the complex ions [Cr(NH₃)₆]³⁺ and [Cr(OH)₆]^3–.

Which statement(s) about the complexes is / are correct?

1 Chromium is in the same oxidation state in each complex ion.

2 The complex ions are different colours.

3 The complex ions have different coordination numbers.

A 1, 2 and 3

B Only 1 and 2

C Only 2 and 3

D Only 1

This question is about trends in the periodic table.

The graph shows the melting points for Period 3 elements from sodium to silicon.

Complete the graph to show approximate melting points for P, S, C l and Ar.

Explain why silicon has a higher melting point than aluminium.

Use ideas about structure and bonding in your answer.

The table shows the first ionisation energies for the elements in Period 3.

Write an equation to represent the first ionisation energy of sulfur. Include state symbols.

The table shows that there is a general increase in first ionisation energies across Period 3, but that this trend is not followed by phosphorus to sulfur.

Explain why the first ionisation energy of sulfur is lower than the first ionisation energy of phosphorus.

A student adds an aqueous solution of bromine, Br₂(aq), to two test tubes.

The student adds aqueous sodium iodide, NaI(aq), to the Br₂(aq) in one test tube, and aqueous sodium chloride, NaCl (aq), to the Br₂(aq) in the other test tube.

Complete the table to show the student's observations.

Write ionic equation(s) for any reaction(s) that take place.

Ionic equation(s)........

* The hydrides of silicon, phosphorus and sulfur have simple molecular structures, but their molecules have different shapes.

Explain why molecules of SiH₄, PH₃ and H₂S have different shapes.

Your answer should include ‘dot-and-cross’ diagrams (outer shells only), the names of the shapes and an explanation of why the bond angles are different.

This question is about carbon monoxide and carbon as reducing agents in industry.

Iron(III) oxide, Fe₂O₃, can be reduced by carbon monoxide to form iron.

Some standard enthalpy change of formation, _fH, and standard entropy, S, values are shown in the table.

Substance	Fe2O3(s)	CO(g)	Fe(s)	CO2(g)
Δf H / kJ mol–1	–824	–111	0	–394
S / J K–1 mol–1	87	198	To calculate in part (a)(iv)	214

Explain why the standard enthalpy change of formation of Fe has a value of 0 kJ mol^–1.

Calculate the enthalpy change, _rH, for the forward reaction in Equilibrium 17.1.

H =........ kJ mol^–1

Explain why the standard entropy of CO₂ is greater than the standard entropy of Fe₂O₃.

The standard entropy change, S, for the forward reaction in Equilibrium 17.1 has a value of +15 J K^–1 mol^–1.

Use data from the table to calculate the standard entropy, S, of Fe.

S =........ J K^–1 mol^–1

Carbon can also reduce Fe₂O₃(s) to iron as shown in Equilibrium 17.2.

The forward reaction is only feasible at high temperatures.

• Show that the forward reaction is not feasible at 100 °C.

• Calculate the minimum temperature, in K, for the forward reaction to be feasible.

Give the minimum temperature to the nearest whole number.

Steam can be reduced by carbon monoxide in a reversible reaction.

At 700 K the value of K_p is 8.13.

Explain why K_p for Equilibrium 17.3 has no units.

An equal number of moles of steam and carbon monoxide are left at 700 K in a sealed container until equilibrium is reached.

At equilibrium, both H₂ and CO₂ have a partial pressure of 211 kPa.

Calculate the partial pressures of H₂O and CO at equilibrium.

At a second temperature the value of K_p for Equilibrium 17.3 is 129.

Explain whether the second temperature is higher or lower than 700 K.

Nitrogen monoxide, NO, reacts with oxygen to form nitrogen dioxide, NO₂.

The rate of reaction increases in the presence of a heterogeneous platinum catalyst.

Explain why platinum is an example of a heterogeneous catalyst for Reaction 18.1.

Explain how the platinum catalyst increases the rate of reaction.

Include a labelled sketch of the Boltzmann distribution on the grid.

In industry, catalysts have great economic importance and benefits for increased sustainability.

Explain how the use of catalysts is beneficial to the environment.

The following data was obtained in a series of experiments on the rate of the reaction for Reaction 18.1 at a constant temperature.

Determine the orders with respect to NO and O₂. Explain your reasoning.

Determine the rate equation and calculate the rate constant, k, including units.

Complete the table by adding the missing value of ln k.

This question is about monobasic acids and buffer solutions.

Nitric acid, HNO₃, is a strong monobasic acid.

Calculate the pH of 0.150 mol dm^–3 HNO₃.

Give your answer to 2 decimal places.

Nitrous acid, HNO₂, is a weak monobasic acid.

Predict whether the pH of 0.150 mol dm^–3 HNO₂ is higher, lower or the same as the pH of 0.150 mol dm^–3 nitric acid, HNO₃.

Explain your answer.

Prediction........

Explanation........

When heated, aqueous HNO₂ disproportionates. The equation is shown below.

• What is meant by disproportionation?

• Use oxidation numbers to show that disproportionation has taken place.

* A student plans to prepare a buffer solution with a pH of 4.85 using propanoic acid, CH₃CH₂COOH, and sodium propanoate, CH₃CH₂COONa.

K_a for CH₃CH₂COOH = 1.32 × 10^–5 mol dm^–3.

The student mixes 125 cm³ of 0.180 mol dm^–3 CH₃CH₂COOH(aq) with a calculated mass of CH₃CH₂COONa(s). The student assumes that the volume of the solution does not change.

• Calculate the mass of CH₃CH₂COONa needed to make 125 cm³ of a buffer solution with a pH of 4.85.

• Explain why this solution resists changes in pH when a small amount of alkali is added.

A student sets up a standard cell based on redox systems 2 and 4.

Draw a labelled diagram to show how this cell could be set up to measure its standard cell potential.

Include the conditions required to measure the standard cell potential.

standard conditions........

Calculate the standard cell potential of this cell.

A student predicts that cobalt metal will reduce acidified VO²⁺ ions to form V²⁺ ions in two stages.

Explain why this two-stage reduction should happen in terms of electrode potentials and equilibria shifts.

Include overall equations for the predicted reactions.

This question is about reactions of compounds of iron and copper.

Complete the electron configuration of

an Fe atom: 1s²........

a Cu²⁺ ion: 1s²........

Solutions of iron(II) sulfate, FeSO₄, and copper(II) sulfate, CuSO₄, can be distinguished by adding aqueous sodium hydroxide.

Describe what would be observed when NaOH(aq) is added to each solution.

Observation with FeSO₄(aq)........

Observation with CuSO₄(aq)........

The cyclic organic compound pyridine, C₅H₅N, can act as a monodentate ligand.

The structure of pyridine is shown below.

What is meant by a monodentate ligand ?

Aqueous copper(II) sulfate reacts with salicylic acid to form a complex ion which has two stereoisomers.

One isomer of the complex ion is shown:

Draw the other stereoisomer of the complex.

Identify the type of stereoisomerism shown by the complexes.

Type of stereoisomerism........

Calculate the percentage by mass of CuSO₄•5H₂O in Mixture A.

Give your answer to 3 significant figures.

Percentage of CuSO₄•5H₂O in Mixture A =........ %

The student repeats their method, but after carrying out Step 1 they leave the solution exposed to the air for a few days. Some of the iron(II) sulfate is oxidised to iron(III) sulfate.

The student then continues with Step 2, Step 3 and Step 4 of the method.

This mean titre is greater than 31.15 cm³.

Suggest why this mean titre is greater than 31.15 cm³.