Chapter 4: Redox reactions

Question 1

Redox reaction

Answer 1

chemical reaction involving the transfer of electrons between species
* oxidation is the loss of electron, but the gain of oxygen
A → A⁺ + e–

• reduction is the gain of electron, but the loss of oxygen
  B⁺ + e– → B

A conjugate redox pair is usually written as oxidant / reductant

Question 2

Oxidising agents

Answer 2

oxidants causes another species to be oxidised
- the oxidant must accept electrons to cause another species to lose electrons
- therefore it itself is reduced

Question 3

Reducing agents

Answer 3

reductant causes another species to be reduced
- the reductant must lose electrons to cause another species to gain electrons
- therefore it itself is oxidised

Question 4

Rules for oxidation state

Answer 4

• Oxidation state of a free element (an element by itself or bonded to itself) is always zero
• Oxidation state of a simple ion is equal to the charge of the ion
• The sum of oxidation states in a neutral compound is 0
• The sum of oxidation states in a polyatomic ion is equal to the charge of the ion
• The normal oxidation state for hydrogen in a compound is +1*
• The normal oxidation state for oxygen in a compound is −2*

Change in oxidation state
* Oxidation involves an increase in number (becomes more positive)
* Reduction shows a reduction in number (becomes more negative)
* No change = no redox reaction

Question 5

Balancing in acidic solution

Answer 5

1. Key elements need to be balanced (all elements expect H and O)
2. Oxygen atoms need to be balanced with water molecules, H2O (l)
3. Hydrogen atoms need to be balanced with hydrogen ions, H+ (aq)
4. Electrons need to be added to balance charge on each side of the equation
5. States need to be included

Question 6

Balancing in basic solution

Answer 6

1. Key elements need to be balanced (all elements expect H and O)
2. Oxygen atoms need to be balanced with water molecules, H2O (l)
3. Hydrogen atoms need to be balanced with hydrogen ions, H+ (aq)
4. Hydrogen ions need to be neutralised with hydroxide, OH–(aq)neutralisation of hydroxide and hydrogen ions forms water, H2O(l)these water molecules can be cancelled
5. Electrons need to be added to balance charge on each side of the equation
6. States need to be included

Question 7

Balancing overal reactions

Answer 7

1. Manipulate the half-equations to have the same amount of electrons
2. Combine the reactants and products from both half-equations
3. Cancel out the species that appear on both sides of the equation