1.1.4 - Redox

Question 1

What is an oxidation number?

Answer 1

Oxidation number - the number of electrons that an atom uses to bond with atoms of another element.

Question 2

Fill in the gaps:

Metals usually form ______ ions by losing electrons and therefore having an ______ in oxidation number.

Answer 2

Metals usually form positive ions by losing electrons and therefore having an increase in oxidation number.

Question 3

______ generally react by gaining ______ with a decrease in _______ number to form negative ions.

Answer 3

Non-metals generally react by gaining electrons with a decrease in oxidation number to form negative ions.

Question 4

1. Write a balanced equation for the reaction of aluminium and sulfuric acid to form the salt Al₂(SO₄)₃ and hydrogen gas.
2. What has been oxidised and what has been reduced?

Answer 4

1. 2Al (s) + 3H₂SO₄ (aq) —> Al₂(SO₄)₃ (aq) + 3H₂(g)
2. The Al has been oxidised.
3. The H₂ has been reduced.

Question 4

Here are some redox reactions below:

1. Cl₂ + 2KBr —> Br₂ + 2KCl
2. 2SO₂ + O₂ —> 2SO₃
3. 2HBr + H₂SO₄ —> SO₂ + Br₂ + 2H₂O

For each reaction, use oxidation numbers to find out what has been oxidised and what has been reduced.

Answer 4

1. The Cl₂ has been reduced whilst the Br₂ has been oxidised.
2. The S has been oxidised whilst the O₂ has been reduced.
3. Br₂ has been oxidised whilst H₂ has been reduced.

Question 5

What is the oxidation state of O in H₂O?

Answer 5

Oxidation state of O: -2

Question 5

Magnesium reacts with chlorine to form magnesium chloride:

Mg + Cl₂ —> MgCl₂

1. Write one half equation for magnesium and one for the chlorine.
2. Using the half equations, state which is the reducing agent and which is the oxidising agent and give a reason for your answer.

Answer 5

1. Mg —> Mg²⁺ + 2e^-
   Cl₂ + 2e^- —> 2Cl^-
2. Mg is the reducing agent. This is because it has reduced the Cl₂ by donating two of its electrons.
3. Cl₂ is the oxidising agent. This is because it has oxidised the Mg by removing two electrons from the Mg.

Question 6

What is the oxidation state of O₂?

Answer 6

Oxidation state of O_{2 - 0}

Question 6

Write down the oxidation number of Fe in the compunds:

• Iron (II) chloride
• FeCl₃

_​Write down the oxidation number of Cu in the compunds:

• Cu₂O
• Copper (II) oxide

Answer 6

• Iron (II) chloride +2
• FeCl₃ +3
• Cu₂O +1
• Copper (II) oxide +2

Question 8

When a fluorine atom is combined with another atom from another element, what is the oxidation number of the combined fluorine?

Answer 8

Combined fluorine: -1

Question 9

Describe the term ‘reduction.’

Answer 9

Reduction - the gain of electrons or a decrease in the oxidation number.

Question 11

What is the oxidation state of Na?

Answer 11

Oxidation state of Na - 0

Question 13

Describe the term ‘oxidation.’

Answer 13

Oxidation - the loss of electrons or an increase in the oxidation number.

Question 14

Work out the oxidation number of CO_3.

Answer 14

CO₃

_{_____________}

O = -2

O = -2

O = -2

C = +4

___________

Overall = 2-

Question 15

Reactive metals react with many acids in redox reactions.

1. Write the balanced equation for the reaction between magnesium and dilute hydrochloric acid. Make sure to include the state symbols.
2. Identify what has been oxidised and what has been reduced.

Answer 15

1. Mg (s) + 2HCl (aq) —> MgCl₂ (aq) + H₂ (g)
2. The Mg has been oxidised.
3. The H₂ has been reduced.

Question 16

What is the oxidation state of N in NH₃?

Answer 16

Oxidation state of N in NH₃ - -3