physical 7 - redox

Question 1

What is oxidation?

Answer 1

Oxidation is the process of electron loss and oxidising agents are electron acceptors.

Question 2

What is reduction?

Answer 2

Reduction is the process of electron gain and reducing agents are electron donors.

Question 3

What are the rules for assigning oxidation states?

Answer 3

1. In elements, the oxidation state is always zero.
2. On simple ions, the oxidation state is the charge of the ion.
3. The total of all the oxidation states must always equal the overall charge on the species.
4. In molecules and more complex ions, the more electronegative element is assaulted to be the negative ion.

Question 4

What info is necessary about the range of oxidation states?

Answer 4

It is limited by the position of an element in the Periodic Table. For example, N is in group 5 and could lose 5 electrons (oxidation state +5) or gain 3 electrons (oxidation state -3) before another shell of electrons is affected.

With the transition metals, they usually have positive oxidation states as they are metals, losing electrons until they reach a group 0 noble gas electron configuration, but not usually higher than +7.

Question 5

How do you write half equations?

Answer 5

1. Balance the elements.
2. Sort out O’s. For every O gained/lost, add/remove a water molecule.
3. Sort out H’s. For every H gained/lost, add/remove one hydrogen ion.
4. Add electrons where necessary in order to balance the charge on each side of the equation.

Question 6

How do you combine half equations?

Answer 6

1. The two half equations are added together so that the electrons cancel out.
2. To do this multiply each side by however much necessary.
3. Sometimes hydrogen ions and/or water molecules may appear on both sides of the combined equation - these should be cancelled out.

Question 7

What is a redox reaction?

Answer 7

In any reaction where one species is oxidised (loses electrons), another must be reduced (gain electrons). Both reduction and oxidation occur and so they are called redox reactions.

Question 8

How do you define oxidising agent in terms of electrons?

Answer 8

The oxidising agent is the species that removes electrons from another and in doing so is itself reduced as it gains electrons.

Question 9

How do you define reducing agent in terms of electrons?

Answer 9

The reducing agent is the species that gives electrons to another and so is itself oxidised as it loses electrons.

Question 10

What is a disproportionation reaction?

Answer 10

A disproportionation reaction is a reaction where a species is simultaneously oxidised and reduced.