(a)(i) Define in terms of electron transfer I. oxidizing agent; II. reducing agent.
(ii) Write a balanced equation to show that carbon in a reducing agent.
(iii) State the change in oxidation number of the specie that reacted with carbon in (a)((ii).
(b) A gas X has a vapour density of 32. It reacts with sodium hydroxide solution to form salt and water only. It decolourizes acidified potassium tetraoxomanganate (VII) solution and reacts with H\(_2\)S to form sulphur. Using the information provided:
(i) identify gas X; (ii) state two properties exhibited by X;
(iii) give two uses of X.
(c) Consider the following substances: sodium; lead (II) iodide; hydrogen; magnesium; oxygen. Which of the substances
(i) conducts electricity?
(ii) is produced at the cathode during electrolysis of H\(_2\)SO\(_{4(aq)}\)?
(iii) corresponds to the molecular formula A\(_2\)?
(iv) is an alkaline earth metal?
d)(i) Define the term salt.
(ii) Mention two types of salt
(iii) Give an example of each of the salts mentioned in (d)(ii) above.
(e) In a neutralization reaction, dilute tetraoxosulphate (VI) acid completely reacted with sodium hydroxide solution.
(i) Write a balanced equation for the reaction
(ii) How many moles of sodium hydroxide would be required for the complete neutralization of 0.50 moles of tetraoxosulphate (VI) acid?
(a) Redox in terms of electron transfer
- (i) I. Oxidizing agent: a substance that accepts (gains) electrons from another species during a reaction, and is itself reduced.
- II. Reducing agent: a substance that donates (loses) electrons to another species during a reaction, and is itself oxidized.
- (ii) Carbon acting as a reducing agent: \[2CuO + C \to 2Cu + CO_2\] Here carbon removes oxygen from (reduces) copper(II) oxide.
- (iii) The species that reacted with carbon is copper in CuO. Its oxidation number changes from +2 to 0 (a decrease of 2, i.e. it is reduced).
(b) Gas X (vapour density 32, so molar mass \(= 2\times32 = 64\ \text{g mol}^{-1}\)). It reacts with NaOH to give salt and water only (acidic oxide), decolourizes acidified KMnO\(_4\) (it is a reducing agent) and reacts with H\(_2\)S to give sulphur.
- (i) Gas X is sulphur(IV) oxide, SO\(_2\) (molar mass 64).
- (ii) Two properties: it is a colourless gas with a choking, irritating smell; it is acidic and a strong reducing agent (bleaching action; turns moist blue litmus red).
- (iii) Two uses: as a bleaching agent (for straw, paper and wool); as a food preservative/fumigant and in the manufacture of sulphuric acid (contact process).
(c) From sodium, lead(II) iodide, hydrogen, magnesium, oxygen
- (i) Conducts electricity: sodium and magnesium (metals conduct in the solid state).
- (ii) Produced at the cathode during electrolysis of dilute H\(_2\)SO\(_4\): hydrogen.
- (iii) Corresponds to formula A\(_2\): hydrogen (H\(_2\)) and oxygen (O\(_2\)) are diatomic molecules.
- (iv) Alkaline earth metal: magnesium (Group 2).
(d) Salts
- (i) A salt is a compound formed when the ionisable hydrogen ion(s) of an acid are wholly or partly replaced by a metallic ion or the ammonium ion.
- (ii) Two types: normal (neutral) salt and acid salt (also basic salt, double salt, complex salt).
- (iii) Examples: normal salt, e.g. Na\(_2\)SO\(_4\) (sodium sulphate); acid salt, e.g. NaHSO\(_4\) (sodium hydrogen sulphate).
(e) Neutralization of H\(_2\)SO\(_4\) with NaOH
- (i) \[H_2SO_4 + 2NaOH \to Na_2SO_4 + 2H_2O\]
- (ii) From the equation, 1 mole of H\(_2\)SO\(_4\) needs 2 moles of NaOH. Therefore for 0.50 mole of acid: \[0.50 \times 2 = 1.0\ \text{mole of NaOH}\]
(a) Redox in terms of electron transfer
- (i) I. Oxidizing agent: a substance that accepts (gains) electrons from another species during a reaction, and is itself reduced.
- II. Reducing agent: a substance that donates (loses) electrons to another species during a reaction, and is itself oxidized.
- (ii) Carbon acting as a reducing agent: \[2CuO + C \to 2Cu + CO_2\] Here carbon removes oxygen from (reduces) copper(II) oxide.
- (iii) The species that reacted with carbon is copper in CuO. Its oxidation number changes from +2 to 0 (a decrease of 2, i.e. it is reduced).
(b) Gas X (vapour density 32, so molar mass \(= 2\times32 = 64\ \text{g mol}^{-1}\)). It reacts with NaOH to give salt and water only (acidic oxide), decolourizes acidified KMnO\(_4\) (it is a reducing agent) and reacts with H\(_2\)S to give sulphur.
- (i) Gas X is sulphur(IV) oxide, SO\(_2\) (molar mass 64).
- (ii) Two properties: it is a colourless gas with a choking, irritating smell; it is acidic and a strong reducing agent (bleaching action; turns moist blue litmus red).
- (iii) Two uses: as a bleaching agent (for straw, paper and wool); as a food preservative/fumigant and in the manufacture of sulphuric acid (contact process).
(c) From sodium, lead(II) iodide, hydrogen, magnesium, oxygen
- (i) Conducts electricity: sodium and magnesium (metals conduct in the solid state).
- (ii) Produced at the cathode during electrolysis of dilute H\(_2\)SO\(_4\): hydrogen.
- (iii) Corresponds to formula A\(_2\): hydrogen (H\(_2\)) and oxygen (O\(_2\)) are diatomic molecules.
- (iv) Alkaline earth metal: magnesium (Group 2).
(d) Salts
- (i) A salt is a compound formed when the ionisable hydrogen ion(s) of an acid are wholly or partly replaced by a metallic ion or the ammonium ion.
- (ii) Two types: normal (neutral) salt and acid salt (also basic salt, double salt, complex salt).
- (iii) Examples: normal salt, e.g. Na\(_2\)SO\(_4\) (sodium sulphate); acid salt, e.g. NaHSO\(_4\) (sodium hydrogen sulphate).
(e) Neutralization of H\(_2\)SO\(_4\) with NaOH
- (i) \[H_2SO_4 + 2NaOH \to Na_2SO_4 + 2H_2O\]
- (ii) From the equation, 1 mole of H\(_2\)SO\(_4\) needs 2 moles of NaOH. Therefore for 0.50 mole of acid: \[0.50 \times 2 = 1.0\ \text{mole of NaOH}\]
