7.4.4 Equilibrium

Section A

Section B

1. Write an equation for the dissociation of water and use it to define K_w, including units where appropriate.

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2. Calculate the pH of the solution produced when 0.0006 g of magnesium oxide is dissolved in 100 cm³ of water.

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3. Vanadium pentoxide, a red-brown solid, is used as a catalyst in the Contact Process for the formation of sulphur trioxide. 2SO₂ + O₂ 2SO₃

(a) (i) Write an equilibrium expression, K_p, for the formation of sulphur trioxide and calculate its value for an equilibrium mixture at 2 atmospheres pressure which contains 96 g of sulphur dioxide, 120 g of oxygen and 80 g of sulphur trioxide.

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(ii)Explain the effect, if any, of vanadium pentoxide on the value of K_p.

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(b) Vanadium exists in several oxidation states some of which are coloured. Complete the following table.

Oxidation state	Formula	Colour
+5	VO₂⁺	yellow
+4	VO²⁺
+3	V³⁺
+2	V²⁺	violet

E^o /V

Mg²⁺ (aq) + 2e^- Mg (s) -2.37

Zn²⁺ (aq) + 2e^- Zn (s) -0.76

Fe²⁺ (aq) + 2e^- Fe (s) -0.44

V³⁺ (aq) + e^- V²⁺ (s) -0.26

VO²⁺ (aq) + 2H⁺ (aq) + e^- V³⁺ (aq) + H₂O (l) +0.34

Fe³⁺ (aq) + e^- Fe²⁺(aq) +0.77

VO₂⁺ (aq) + 2H⁺ (aq) + e^- VO²⁺ (aq) + H₂O (l) +1.00

(i) Which species, in the table, is the strongest reducing agent ?

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(ii) Predict what would be observed when the following mixtures are prepared. Write an equation for any reaction taking place.

A solution containing V²⁺ (aq) is mixed with acidified solution containing VO²⁺ (aq).

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A piece of zinc was dipped into a solution containing V³⁺ (aq).

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(d) If solutions of vanadium(III) chloride, VCl₃, and potassium cyanide, KCN, in water are mixed and alcohol is added a complex cyanide, K₃V(CN)₆, is precipitated.

(i) The cyanide ion acts as a ligand. Explain the term ligand.

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(ii) Suggest the coordination number of vanadium in the complex. Explain your answer.

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(iii) Explain the formation of the complex from the aqueous vanadium ion in terms of ligand replacement.

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