Redox

Question 1

What is oxidation?

Answer 1

The loss of electrons.

Question 2

What is reduction?

Answer 2

The gain of electrons.

Question 3

What are the rules for assigning oxidation states?

Answer 3

1. An uncombined element has an oxidation state of zero.
2. Some elements have the same oxidation state in all of their compounds.
3. For a compound, the oxidation state of each atom in the compound’s formula counts separately and the numerical sum is zero.
4. For an element existing as an ion, the oxidation state is the charge on the ion.
5. For compound ions, the sum of oxidation states of the atoms is the charge on the ion.

Question 4

What elements have the same oxidation state in all their compounds?

Answer 4

• Group 1 elements (+1).
• Group 2 elements (+2).
• Group 3 elements (+3).
• Hydrogen (+1) except for in metal hydrides (-1).
• Fluorine (-1).
• Oxygen (-2) except for in peroxides (-1) or when bonded to fluorine (+2).
• Cl, Br, I (-1) except for when combined with oxygen (varies).

Question 5

How are oxidation states used to make compounds and ions?

Answer 5

Roman numerals in chemical names are used to indicate the oxygen state of the elements e.g. iron (III) oxide (Fe is +2).

Question 6

What is oxidation and reduction in terms of oxidation states?

Answer 6

• Oxidation is an increase in oxidation state.
• Reduction is a decrease in oxidation state.

Question 7

What is an oxidising agent?

Answer 7

A species that accepts electrons. It contains an atom which decreases oxidation states in a reaction.

Question 8

What is a reducing agent?

Answer 8

A species that donates electrons. It contains an atom which increases oxidation states in a reaction.

Question 9

What are the rules for constructing half equations?

Answer 9

1. Write the formula for the reactants and products and balance the atoms undergoing redox.
2. Balance any oxygen atoms by adding water.
3. Balance hydrogen atoms by adding H+ ions.
4. Balance the charges by adding electrons.

Question 10

How do you combine half equations?

Answer 10

1. One equation must show reduction and the other must show oxidation.
2. The number of electrons being transferred must be the same.
3. They can then be added and any species that appears on both sides of the equation must be cancelled.