Redox Reactions: Chemistry 0620 (Cambridge O Level / IGCSE)

Tungsten metal, used to make the filaments inside some specialist lamps, is manufactured by heating tungsten(VI) oxide in a stream of hydrogen gas:

$\text{WO}_3 + 3\text{H}_2 \rightarrow \text{W} + 3\text{H}_2\text{O}$

Which substance is reduced in this reaction?

A school technician demonstrates the extraction of silicon from sand. A mixture of silicon(IV) oxide (the main compound in sand) and magnesium powder is ignited in a crucible, and the reaction proceeds vigorously:

$\text{SiO}_2 + 2\text{Mg} \rightarrow \text{Si} + 2\text{MgO}$

(a) State, in terms of oxygen, what happens to the silicon(IV) oxide in this reaction. [1]

(b) State, in terms of oxygen, what happens to the magnesium in this reaction. [1]

(c) Identify which substance is the reducing agent in this reaction, and give a reason for your answer in terms of oxygen transfer. [2]

(d) State the term used to describe a reaction, like this one, in which both oxidation and reduction happen at the same time. [1]

Sodium chlorate, $\text{NaClO}_3$, is used industrially as a weedkiller and contains the chlorate ion, $\text{ClO}_3^-$.

What is the oxidation number of chlorine in the chlorate ion, $\text{ClO}_3^-$?

A food scientist investigates the redox behaviour of vitamin C (ascorbic acid) using the two standard test reagents for oxidising and reducing agents.

Test 1: a few drops of acidified aqueous potassium manganate(VII) are added to a vitamin C solution. The purple colour disappears.

Test 2: a few drops of aqueous potassium iodide are added to a fresh sample of the vitamin C solution. There is no colour change.

(a) State what test 1 shows about the redox behaviour of vitamin C, and identify which species, vitamin C or the manganate(VII) ions, is reduced in test 1. [2]

(b) Use the result of test 2 to explain why vitamin C is not acting as an oxidising agent. [2]

(c) State the term used to describe a substance, like vitamin C, that reduces another species and is itself oxidised. [1]

(d) The oxidation number of manganese in the manganate(VII) ion, $\text{MnO}_4^-$, is $+7$. In test 1 the manganese ends up as $\text{Mn}^{2+}$ ions. State the change in the oxidation number of manganese during test 1, and use it to confirm that the manganese has been reduced. [2]

Excess iron filings are added to aqueous tin(II) chloride, $\text{SnCl}_2$. Iron displaces tin from the solution:

$\text{Fe(s)} + \text{SnCl}_2(aq) \rightarrow \text{FeCl}_2(aq) + \text{Sn(s)}$

(a) Write the ionic equation for this reaction, omitting any spectator ions. [1]

(b) Using your ionic equation, write separate half-equations for the oxidation process and for the reduction process taking place. [2]

(c) Identify the oxidising agent in this reaction, and explain your answer in terms of electron transfer. [2]

(d) State the change in the oxidation number of iron, and the change in the oxidation number of tin, during this reaction. [2]

In the extraction of lead, lead(II) oxide is reduced by reaction with carbon monoxide gas:

$\text{PbO} + \text{CO} \rightarrow \text{Pb} + \text{CO}_2$

Which substance is oxidised in this reaction?

A student carries out two separate tests using the standard test reagents for oxidising and reducing agents.

Test 1: sulfur dioxide gas, $\text{SO}_2$, is bubbled through a few $\text{cm}^3$ of acidified aqueous potassium manganate(VII).

Test 2: a few drops of chlorine water are added to a few $\text{cm}^3$ of aqueous potassium iodide.

(a) State the colour change observed in test 1. [1]

(b) State whether sulfur dioxide is acting as an oxidising agent or a reducing agent in test 1, giving a reason based on your answer to (a). [2]

(c) State the colour change observed in test 2, and name the coloured product responsible for this colour change. [2]

(d) State whether chlorine is acting as an oxidising agent or a reducing agent in test 2, giving a reason based on your answer to (c). [2]

Aqueous iron(II) sulfate, $\text{FeSO}_4$, is oxidised by an excess of acidified aqueous potassium manganate(VII). In the resulting solution, the oxidation number of iron has changed to $+3$.

What is the correct name of the iron-containing product formed?

Chlorine gas is bubbled into aqueous sodium bromide, and the following reaction occurs:

$\text{Cl}_2 + 2\text{NaBr} \rightarrow 2\text{NaCl} + \text{Br}_2$

(a) State the oxidation number of chlorine in $\text{Cl}_2$ and in $\text{NaCl}$. [2]

(b) State the oxidation number of bromine in $\text{NaBr}$ and in $\text{Br}_2$. [2]

(c) Use your answers to (a) and (b) to identify which element is oxidised and which is reduced in this reaction. [2]

(d) Identify the oxidising agent in this reaction, and explain your answer in terms of electron transfer. [2]

A student tests three unknown colourless aqueous solutions, X, Y and Z, using the two standard test reagents for oxidising and reducing agents. The results are shown in the table.

(a) Identify which solution acts as an oxidising agent, and explain your answer using the results in the table. [2]

(b) Identify which solution acts as a reducing agent, and explain your answer using the results in the table. [2]

(c) State what the results for solution Z show about its behaviour with these two reagents. [1]