Questions

Section A

For each of the questions only one of the lettered responses (A – D) is correct.

Select the correct response in each case and mark its code letter by connecting the dots as illustrated on the answer sheet.

1 Which one of the following conversions is an oxidation?

A Mn²⁺ MnCl₂

B MnO₄^- MnO₄^2-

$C Mn(OH)₂ MnO₂

D Mn³⁺ Mn₂O₃

2 One stage in preparing chrome alum from potassium dichromate is represented by the equation:

K₂Cr₂O₇ + H₂ SO₄ + 3SO₂ = K₂ SO₄ + Cr₂(SO₄)₃ + H₂O

Which one of the following statements is true?

A The mixture represented by the right-hand side of the equation is orange in colour.

B The oxidation number of chromium changes from +7 to +3.

C Sulphur dioxide acts as a reducing agent.

D The formula for chrome alum crystals is KCr(SO₄)₂

2 Which one of the following colours corresponds to the hexa-aqua complex of the ion given?

$A Co²⁺ pink

B Cr³⁺ orange

C Al³⁺ red

D Cu²⁺ green

Section B

Answer all questions in the spaces provided.

1. Ammonia is a monodentate ligand whereas edta is a chelating ligand.

Explain the difference between the two types of ligand.

[3]

NI98Sk

2. Putrescine is a diamine formed by the putrefaction (rotting) of the proteins in flesh.

NH₂CH₂CH₂CH₂CH₂NH₂

putrescine

Putrescine can act as a bidentate ligand forming a complex with the nickel(II) ion.

(i) What structural feature does putrescine possess that allows it to act as a ligand?

[1]

(ii) Draw a possible structure for the complex formed between putrescine and nickel(II) ions.

[2]

NI98S

3 Several cannon were discovered from the wreck of the Girona, the Spanish galleon wrecked off the Antrim coast. If a fragment of a cannon is dissolved in concentrated nitric acid, and a student attempts to show the presence of (i) manganese and (ii) cobalt, describe what tests should be used, and state what would be observed if the metals were present.

(a)(i) manganese ions

[3]

(ii) cobalt ions

[3]

(b) Iron is the predominant metal present and can be identified by adding a solution of thiocyanate ions. State the formula of the complex formed.

[1]

CHM3J8

3 Describe, giving practical details including safety precautions, how a sample of chromium(III) nitrate may be converted to potassium dichromate. (Equations are not required).

(Up to 2 marks may be obtained for the quality of the language in this part.)

[9]

(b) What are the formulae and colours of the following ions in aqueous solution?

formula colour in aqueous solution

(i) chromium(III) ions [2]

(iii) dichromate ions

[2]

(c) Name a gas which may be detected by the colour change of dichromate to chromium (III).

[1]

(d) Write an equation for the reaction between dichromate ions and this gas.

[2]

CHM3J8