Redox I

Question 1

What is a redox reaction?

Answer 1

A redox reaction is a reaction in which both oxidation and reduction occur.

Question 2

How is oxidation defined?

Answer 2

Oxidation is defined as the loss of electrons.

Question 3

How is reduction defined?

Answer 3

Reduction is defined as the gain of electrons.

Question 4

What is the acronym for remembering oxidation and reduction?

Answer 4

OILRIG (oxidation is loss reduction is gain (of electrons))

Question 5

What is an oxidising agent?

Answer 5

An oxidising agent is a species that oxidises another species by removing one or more electrons. When an oxidising agent reacts, it gains electrons and therefore, is reduced.

Question 6

What is a reducing agent?

Answer 6

A reducing agent is a species that reduces another species by adding one or more electrons. When a reducing agent reacts, it loses electrons and is therefore, oxidised.

Question 7

What is an oxidation number?

Answer 7

Oxidation number is the charge that an ion has or the charge that it would have if the species was fully ionic (in the case of molecules).

Question 8

What are the rules for determining oxidation number?

Answer 8

• the oxidation number of an unconvinced element is zero (e.g. Mg or Cl2)
• the sum of the oxidation numbers of all the elements in a neutral compound is zero (e.g. in MgO, Mg is +2, O is -2, which sum to 0)
• the sum of the oxidation numbers in an ion is equal to the charge on the ion (e.g. in OH-, O is -2, H is +1, which sums to -1)
• the more electronegative element in a substance is always given the negative oxidation number
• the oxidation number of fluorine is always -1
• The oxidation number of hydrogen is always +1, except when combined with a less electronegative element, when it is -1
• the oxidation number of oxygen is -2, except in peroxides, where it is -1 and combined with fluorine it is +2.

Question 9

How can oxidation numbers be used to classify reactions?

Answer 9

An element is oxidised when its oxidation number increases. An element is reduced when its oxidation number decreases.

Question 10

What is a disproportionation reaction?

Answer 10

Disproportionation is the simultaneous oxidation and reduction of an element (within the same species) in a single reaction.

Question 11

What are the systematic names of chemicals?

Answer 11

The systematic names use Roman numerals when a compound could have multiple oxidation states. E.g. FeCl2 is iron(II) chloride, while FeCl3 is iron(III) chloride because the oxidation number of iron here is +3. However, systematic names are not always used, e.g. Na2SO3 is technically sodium sulphate(IV), but it is often called sodium sulphite.

Question 12

How do you work out the formula of a compound from the oxidation numbers of the species within it?

Answer 12

The oxidation numbers can be treated as ions, and then the crossover rule can be used to work out the formula. E.g. if you are told to work out the formula of iron (III) sulphate, you know that that is Fe3+ and SO42-, so the formula is Fe2(SO4)3.

Question 13

When metals form ions, are they generally oxidised or reduced?

Answer 13

Generally, metals lose electrons to form positive ions, and so are oxidised.

Question 14

When non-metals form ions, are they generally oxidised or reduced?

Answer 14

Generally, non-metals gain electrons to form negative ions, so are reduced.

Question 15

How do you write half equations?

Answer 15

When writing half equations, you need to write out the reactants on the left and the products on the right, then balance first with water to balance oxygen, then hydrogen ions to balance hydrogens, then electrons to balance charges.

Question 16

How do you write full ionic equations from balanced symbol equations?

Answer 16

To write full ionic equations from balanced symbol equations, split any ionic, aqueous compounds into the ions they contain and cancel any spectator ions, then write out the equation.

Question 17

How do you write full ionic equations from half equations?

Answer 17

First, ensure that the number of electrons on either side of the equation is the same, so they will cancel out (if not, multiply one or more equations to make them equal). Write the reactants on the left and the products on the right, balance with water and/or hydrogen ions if necessary. Cancel anything which is the same on both sides and write out the finished equation.