Iodine reacts with nitric acid to form nitrogen dioxide, NO₂.

Balance the following equation for this reaction.

I₂ + 10HNO₃   ➔   ......HIO₃  +  ......NO₂  +  ......H₂O 

Give the oxidation state of iodine in I₂ and HIO₃.

Two half equations involving iodine are shown below.  Use these two half equations to write an overall equation for the reaction.

IO₃⁻ + 5e⁻ + 6H⁺ ➔ 3H₂O +  $\frac{1}{2}$ I₂ 

I⁻ ➔ $\frac{1}{2}$ I₂  + e⁻ 

Iodide ions can behave as a reducing agent in the reaction with chlorine.  State the meaning of the term 'reducing agent'.

Write the simplest ionic equation for the reaction of chlorine and iodide ions. 

Ammonium sulfate, (NH₄)₂SO₄, is a salt formed from the neutralisation of ammonia by sulfuric acid.

Write a balanced equation for the formation of ammonium sulfate from ammonia and sulfuric acid. State symbols are not required.

Explain why ammonium sulfate is an example of a salt.

How is ammonia acting as a base in this neutralisation?

A student wants to determine the number of moles of ammonia in a 400 cm³ solution using a titration. They add 25.0 cm³ of the ammonia solution to a conical flask and titrate with 0.100 mol dm^-3 H₂SO₄. The titre recorded was 31.75 cm³ of H₂SO₄. 

Calculate the number of moles of ammonia in the 400 cm³ solution, giving your answer to 3 significant figures.

Ammonia can be used to produce hydrazine, N₂H₄, as shown in the reaction below.

2NH₃ + NaClO ➔ N₂H₄ + NaCl + H₂O

Use oxidation numbers to show that this is a redox reaction.

Ethanoic acid is a weak acid found in vinegar.

What is meant by the term weak acid?

The equation for the reaction between ethanoic acid and magnesium is shown below.

Mg_(s)  +  2CH₃COOH_(aq)  ➔  (CH₃COO)₂Mg_(aq)  +  H_2(g)

Deduce, using oxidation states, which element has been oxidised and which element has been reduced.

A student uses a titration to determine the concentration of ethanoic acid in a sample of vinegar. 

The student adds 25.0 cm³ of the vinegar to a volumetric flask and makes the solution up to 250 cm³ before transferring to a burette. They then add 25.0 cm³ of 5.00 x 10^-2 mol dm^-3 Ba(OH)₂ to a conical flask and titrate this with the vinegar solution, recording a mean titre of 24.17 cm³.

The equation for the reaction is shown below.

2CH₃COOH_(aq) + Ba(OH)_2(aq) ➔ (CH₃COO)₂Ba_(aq) + 2H₂O_(l)

Calculate the concentration of CH₃COOH in the vinegar, giving your answer to 3 significant figures.

Iodine can be used on a small scale to purify water. When iodine reacts with water, the following equilibrium is set up.

I₂ + H₂O ⇌ HI + HIO

Using oxidation numbers, explain why this equilibrium is an example of disproportionation.

Chlorine is also used to purify water but on a much larger scale. Chlorine can be manufactured by the reaction below.

MnO_2(s) + 4HCl_(aq) ➔ MnCl_2(aq) + 2H₂O_(l) + Cl_2(g)

Use oxidation numbers to show which substance has been oxidised and which has been reduced. 

Nitrite ions, NO₂⁻, react with chlorate (V) ions, ClO₃⁻, to produce chlorine as shown in the reaction below.

2ClO₃⁻ + 5NO₂⁻ + 2H⁺ ➔ Cl₂ + 5NO₃⁻ + H₂O 

In a titration experiment, 25.0 cm³ of sodium nitrite solution was completely reacted with 26.79 cm³ of 0.0200 mol dm^-3 potassium chlorate(V) solution. 

Calculate the concentration, in g dm^-3, of sodium nitrite to 1 decimal place.

Chlorine is an important chemical and has a number of applications including sterilising water and extracting bromine from seawater. Chlorine can be formed by the oxidation of chloride ions.

In terms of electrons, what is meant by the term oxidation?

Write a half equation to show the oxidation of chloride ions to chlorine. 

In the extraction of bromine from seawater, chlorine acts as an oxidising agent. 

What is meant by the term oxidising agent?

Chlorine can also oxidise water to form oxygen. 

Write an equation for this reaction and give the oxidation state of chlorine in the chlorine-containing product that is formed.

This question is about redox reactions.

State the meaning of the term redox as applied to a reaction.

SO₃²⁻ ions can be oxidised by Cr₂O₇²⁻ ions in acidic conditions to form Cr³⁺ and SO₄²⁻.  Write the following equations:

• A half equation for the oxidation of SO₃²⁻ to SO₄²⁻
• A half equation for the reduction of Cr₂O₇²⁻ to Cr³⁺
• An overall equation for the reaction 

Salts can be prepared in redox reactions of metals with acids.  What is meant by a salt?

The aluminium sulfate salt can be prepared by reacting aluminium with sulfuric acid as shown in the equation below.

2Al_(s) + 3H₂SO_4(aq) ➔ Al₂(SO₄)_3(aq) + 3H_2(g)

Use oxidation numbers to show which element has been oxidised and which element has been reduced.

This question is about the redox reactions of chlorine.

Chlorine and water react in a redox reaction according to the equation below.

Cl₂ + H₂O ⇌ HClO + HCl

Write two half equations to show the redox processes happening in this reaction.

Hydrochloric acid can be oxidised with sodium chlorate (I) (NaClO) solution. Write the following equations:

• A half equation for the oxidation of chloride ions to chlorine
• A half equation for the reduction of chlorate ions (ClO⁻) to chlorine in acidic conditions 
• An overall equation for the reaction 

ClO⁻ ions are found in bleach and their concentration can be found by the reaction with iodide ions.

ClO⁻ + 2I⁻ + 2H⁺ ➔ I₂ + Cl⁻ + H₂O 

10 cm³ of bleach, containing ClO⁻ ions, was reacted with 69.5 mg of potassium iodide. 

Find the concentration of ClO⁻ ions in the bleach solution, giving your answer to 3 significant figures.

Copper can be extracted from CuS by reaction with nitric acid to produce a solution of copper sulfate, as shown in the reaction below.

3CuS_(s) + 8HNO_3(aq) ➔ 3CuSO_4(aq) + 8NO_(g) + 4H₂O_(l)

Give the oxidation states of nitrogen in HNO₃ and NO.

Deduce a half equation for the reduction of the nitrate ion in acidic conditions to form NO and H₂O.

Deduce a half equation for the oxidation of the sulfide ion to form the sulfate ion and H⁺ ions in solution.

NO can also be formed when sodium nitrite (NaNO₂) is reacted with iodide ions. 

Give the oxidation state of nitrogen in NO₂⁻ and NO, and write a half equation for the conversion of NO₂⁻ to NO.

During this reaction, iodide ions are converted to iodine. 

Write an overall equation for the reaction, and state the role of NO₂⁻ in this reaction.

The processes of oxidation and reduction can be defined in terms of the transfer of electrons.

Define the term reduction in terms of electrons

The following reaction between copper and nitric acid can be used to produce NO.

3Cu + 8H⁺ + 2NO₃⁻ ➔ 3Cu²⁺ + 4H₂O + 2NO

If the nitric acid is concentrated and warm, then NO₂ can be produced instead of NO.

Give the oxidation states of nitrogen in NO₃⁻, NO and NO₂.

Write a half equation for the formation of the NO₂ from NO₃⁻ and state whether oxidation or reduction is occurring.

Write a half equation for the formation of NO from NO₃⁻, in acidic conditions, and state whether oxidation or reduction is occurring.

The dichromate ion, Cr₂O₇²⁻, can be used to oxidise iodide ions to iodine.

What is the oxidation state of chromium in Cr₂O₇²⁻?

Write the following equations:

• A half equation for the oxidation of iodide ions to iodine
• A half equation for the reduction of Cr₂O₇²⁻ to Cr³⁺ in acidic conditions 
• An overall redox equation for the reaction 

MnO₂ is another oxidising agent that can be used to oxidise iodide ions to iodine.

2I⁻ + MnO₂ + 4H⁺ ➔ Mn²⁺ + 2H₂O + I₂

Give the oxidation state changes of Mn and I and show that this is a redox reaction.

The percentage purity of a 0.850 g sample of impure iodine can be determined using a titration. The iodine is dissolved in potassium iodide solution and titrated with sodium thiosulfate according to the reaction below.

2S₂O₃²⁻ + I₂ ➔ 2I⁻ + S₄O₈²⁻ 

The titration requires 28.85 cm³ of 0.200 mol dm⁻³ sodium thiosulfate.

Calculate the percentage purity of the sample of iodine, giving your answer to 3 significant figures.