OCR A Level Chemistry A Unified chemistry June 2019 question paper H432/03

These short questions are from different areas of chemistry.

Explain why a CF₄ molecule has polar bonds but does not have an overall dipole.

Explain why a small proportion of molecules in water have a relative molecular mass of 20.

What is the partial pressure of O₂ (in Pa) in a gas mixture containing 21% O₂ by volume and with a total pressure of 1.0 × 10⁵ Pa?

What mass of carbon dioxide (in g) is formed by the complete combustion of 42.0 m³ (measured at RTP) of propane?

What is the number of oxygen atoms in 4.26 g of P₂O₅?

In Step 1, sodium carbonate, Na₂CO₃, makes the reaction mixture alkaline.

Write an ionic equation to show how carbonate ions form an alkaline solution in water.

In Step 2, explain why the mixture must be acidified so that crystals of benzoic acid appear.

Write the overall equation for the preparation of benzoic acid from phenylmethanol.

Use [O] for the oxidising agent.

Calculate the percentage yield of benzoic acid.

Give your answer to 3 significant figures.

In Step 3, describe how the student can recrystallise the impure crystals to obtain pure benzoic acid.

This question is about elements and compounds in Group 14 (Group 4) of the periodic table.

There are four oxides of lead: PbO, PbO₂, Pb₂O₃ and Pb₃O₄.

A student carries out an experiment to identify an unknown lead oxide, which is one of the four oxides of lead shown above.

The student plans to reduce the unknown lead oxide to lead by heating the lead oxide in a stream of methane gas, CH₄. The apparatus is shown below.

Student’s method

• Weigh an empty dish. Add the lead oxide to the dish and reweigh.

• Set up the apparatus and pass methane gas through the tube as shown. Heat the dish for 10 minutes.

• Pass cold air through the tube to cool the dish and contents.

• Weigh the dish and contents.

Write the equation for the reduction of Pb₂O₃ with CH₄.

The student uses safety glasses and a lab coat.

State, with a reason, one other important safety precaution the student should take when carrying out this experiment.

The student was not sure that all the oxygen had been removed from the lead oxide.

Suggest two modifications that the student could make to their method to be confident that all the oxygen had been removed. Explain your reasoning.

The student makes suitable modifications to the method and repeats the experiment to obtain the accurate results shown below.

Calculate the empirical formula of the lead oxide.

SiO₂ and CO₂ are oxides of other Group 14 (Group 4) elements.

Solid SiO₂ melts at 2156 °C. Solid CO₂ melts at −56 °C.

Suggest the type of lattice structure in solid SiO₂ and in solid CO₂ and explain the difference in melting points in terms of the types of force within each lattice structure.

Structure in SiO₂(s)........

Structure in CO₂(s)........

Explanation........

Dettol ® is a disinfectant containing the antiseptic chloroxylenol, shown below.

Chloroxylenol is a weak Brønsted–Lowry acid.

What is the systematic name of chloroxylenol?

Predict the number of peaks in a¹³C NMR spectrum of chloroxylenol.

Name the functional group responsible for the acidity of chloroxylenol and describe a simple test which would confirm the presence of this group.

Functional group........

Test........

A student measures the pH of the contents in a bottle of Dettol ® as 5.14.

The label on the bottle shows the percentage of chloroxylenol in Dettol ® as 4.80% i.e. 100 cm³ of Dettol^® contains 4.80 g of chloroxylenol.

Assume the following:

• Chloroxylenol is the only acidic component in Dettol ®.

• Chloroxylenol is a weak monobasic acid.

• The density of Dettol ® is 1.00 g cm −3.

Write the equation, using molecular formulae, for the acid dissociation of chloroxylenol.

Calculate the acid dissociation constant, K_a, for chloroxylenol.

K_a =........ mol dm⁻³

Dettol^® contains other chemicals including -terpineol, shown below.

-Terpineol is a chiral compound.

Show with an asterisk, (*), the chiral centre(s) in the structure of -terpineol.

-Terpineol meets the requirements for E / Z isomerism. However, only one E / Z isomer of -terpineol exists.

Explain

• why -terpineol meets the requirements for E / Z isomerism

• whether -terpineol is an E- or Z- isomer

• why only one E / Z isomer of -terpineol exists.

-Terpineol contains two functional groups.

For each functional group, choose a reagent that reacts with that group only. Draw the structures for the organic products of the reactions.

Show structures for organic compounds.

Reagent(s)........

Name of functional group that reacts........

Structure of organic product

Reagent(s)........

Name of functional group that reacts........

Structure of organic product

This question is about copper(II) sulfate, CuSO₄, and sodium thiosulfate, Na₂S₂O₃.

* Calculate ∆_solH(CuSO₄(s)) for reaction 5.2 and determine the enthalpy change of reaction 5.1, ∆_rH.

Assume that the specific heat capacity, c, of the solution is the same as for water.

Show your working, including an energy cycle linking the enthalpy changes.

The standard enthalpy change of reaction, ∆_r , and the standard free energy change, ∆ for converting anhydrous sodium thiosulfate to hydrated sodium thiosulfate are shown below.

Standard entropies are given in the table.

Determine the standard entropy, , of anhydrous sodium thiosulfate, Na₂S₂O₃(s).

Give your answer to 3 significant figures.

Sodium thiosulfate contains the thiosulfate ion, S₂O₃²⁻. The displayed formula of S₂O₃²⁻ can be shown as below.

Predict the O–S–S bond angle and name of the shape of the thiosulfate ion.

Bond angle........

Name of shape........

In some of its reactions, the thiosulfate ion forms the tetrathionate ion, S₄O₆²⁻.

The S₄O₆²⁻ ion is a ‘dimer’ of S₂O₃²⁻.

Draw a displayed formula for the S₄O₆²⁻ ion.

This question is about reactions of iron compounds.

In the boxes, write the formulae of A and B.

The student thinks that the reaction of iron(II) sulfide with H⁺ / H₂O is a redox reaction.

Explain, with reasons, whether the student is correct.

Write the equation for the reaction of [Fe(H₂O)₆]²⁺(aq) with Cl₂(g).

Construct an equation for the reaction of H₂S(g) with H⁺(aq)/MnO₄⁻(aq).