OCR A Level Chemistry A Unified chemistry June 2018 question paper H432/03

Select an element from the first three periods that fits each of the following descriptions.

The element that forms a 1– ion with the same electron configuration as helium.

The element with the highest first ionisation energy.

The element in Period 3 which has the successive ionisation energies shown below.

The element which forms a compound with fluorine that has octahedral molecules.

An element which reacts with water to form an acidic solution.

The element X, which forms a compound with hydrogen, XH₃, with a molar mass of 34.0 g mol^–1.

An element which forms a compound with hydrogen in which the element has an oxidation number of –4.

The element which has a density of 1.33 × 10^–3 g cm^–3 at room temperature and pressure.

Table 1.1 shows some properties of Period 3 chlorides.

Explain the properties shown in Table 1.1 in terms of bonding and structure.

This question looks at reactions of hydrogen peroxide and of cobalt(II) ions.

Aqueous hydrogen peroxide decomposes as shown in equation 2.1.

The reaction is catalysed by manganese(IV) oxide, MnO₂.

A student investigates the decomposition of a hydrogen peroxide solution as outlined below.

• The student adds 50.00 cm³ of H₂O₂(aq) to a conical flask.

• The student adds a small spatula measure of MnO₂ and quickly connects the flask to a gas syringe.

• The student measures the volume of oxygen every 200 seconds.

The student allows the reaction in equation 2.1 to proceed until no more gas is evolved. The volume of O₂ in the syringe is now 55 cm³, measured at RTP.

Calculate the initial concentration of the H₂O₂.

Give your answer to two significant figures.

initial concentration of H₂O₂ =........ mol dm^–3

Hydrogen peroxide can act as an oxidising agent or as a reducing agent.

Some standard electrode potentials are shown below.

Use this information to write an equation for a reaction in which hydrogen peroxide acts as a reducing agent.

Cobalt(II) forms complex ions with water ligands and with chloride ligands.

• With water ligands, cobalt(II) forms a pink octahedral complex ion, [Co(H₂O)₆]²⁺.

• With chloride ligands, cobalt(II) forms a blue tetrahedral complex ion.

A student dissolves cobalt(II) sulfate in water in a boiling tube. A pink solution forms.

Experiment 1 The student places the boiling tube in a water bath at 100 °C. Concentrated hydrochloric acid is added dropwise. The colour of the solution changes from pink to blue.

Experiment 2 The student places the boiling tube from experiment 1 in an ice/water bath at 0 °C. The colour of the solution changes from blue to pink.

Write the equilibrium equation for the reaction that takes place when the colour of the solution changes.

Explain the observations and predict whether the formation of the blue colour is exothermic or endothermic.

Ethanedioic acid, (COOH)₂, is present in rhubarb leaves.

A student carries out a redox titration using aqueous cerium(IV) sulfate, Ce(SO₄)₂(aq), to determine the percentage, by mass, of ethanedioic acid in rhubarb leaves.

In the titration, Ce⁴⁺(aq) ions oxidise ethanedioic acid in hot acid conditions:

Ce⁴⁺(aq) ions have a yellow colour. Ce³⁺(aq) ions are colourless.

The student weighs 82.68 g of rhubarb leaves and extracts ethanedioic acid from the leaves.

The ethanedioic acid is added to dilute sulfuric acid to form a colourless solution which is made up to 250.0 cm³ with distilled water.

The student heats 25.00 cm³ of this solution to 70 °C and titrates this volume with 0.0500 mol dm^–3 Ce(SO₄)₂ from the burette.

The student repeats the titration to obtain concordant (consistent) titres.

Titration results The trial titre has been omitted.

This titration is self-indicating and the student does not need to add an indicator.

What colour change would the student observe at the end point?

Colour change from........ to........

Calculate the percentage, by mass, of ethanedioic acid in the rhubarb leaves.

Give your answer to an appropriate number of significant figures.

This question is about two compounds used in medicine.

Cis-platin, PtCl₂(NH₃)₂, is a complex of platinum which is used in cancer treatment.

What is the oxidation number of platinum in cis-platin?

Cis-platin is prepared in a ligand substitution reaction which takes place in multiple steps.

The equation for the final step forming cis-platin is shown below.

cis-platin

In the box, outline the mechanism for the formation of cis-platin from [PtCl₃(NH₃)]^–. Use curly arrows and lone pairs where appropriate.

cis-platin

Paracetamol is a solid organic compound used in tablets as a painkiller.

Name the functional groups present in paracetamol.

A student carries out two experiments in the laboratory based on succinic acid (butanedioic acid), (CH₂COOH)₂.

Succinic acid is esterified by ethanol, C₂H₅OH, in the presence of an acid catalyst to form an equilibrium mixture.

The equilibrium constant, K_c, for this equilibrium can be calculated using the amounts, in moles, of the components in the equilibrium mixture, using expression 5.1.

A student carries out an experiment to determine the value of K_c for this equilibrium.

• The student mixes together 0.0500 mol of succinic acid and 0.150 mol of ethanol, with a small amount of an acid catalyst.

• The mixture is allowed to reach equilibrium.

• The student determines that 0.0200 mol of succinic acid are present in the equilibrium mixture.

Which technique could be used to determine the equilibrium amount of succinic acid?

Write the equation for the equilibrium reaction that takes place.

Draw the skeletal formula of the ester present in the equilibrium mixture.

K_c is the equilibrium constant in terms of equilibrium concentrations.

Why can expression 5.1 be used to calculate K_c for this equilibrium?

Calculate the value of K_c for this reaction.

Show your working.

This question is about organic reactions.

Give the systematic name for compound A.

What type of reaction has taken place?

A similar reaction takes place when propanone, (CH₃)₂CO, is mixed with OH^–(aq) ions.

Draw the structure of the organic product of this reaction.

* Many organic reactions use electrophiles as reagents.

Explain the role of electrophiles in organic chemistry.

Your answer should include one reaction of an aliphatic compound and one reaction of an

aromatic compound, including relevant mechanisms.