GCSE OCR Gateway C3.3a-b Redox Reactions

New OCR Gateway specification from September 2016 Higher tier: grades 9 to 4: 

In this and subsequent posts I’m simply going to explain and illustrate each learning objective as they come up in the topics in the new GCSE specification. 

I’m giving you my notes from each lesson.

You can really get ahead of your class if you follow this blog and all the posts that will appear here about the new GCSEs over the coming months. 

This rejigging of the specification is just that: there is nothing really new here it has all been with us for the past half century at least.
 
That written in italics is for the higher tier paper only.

Redox reactions

C3.1c use the names and symbols of common elements from a supplied periodic table to write formulae and balanced chemical equations where appropriate for the first 20 elements, Groups 1, 7, and 0 and other common elements included within the specification.

C3.3a explain reduction and oxidation in terms of loss or gain of oxygen, identifying which species are oxidised and which are reduced

If an element or compound gains oxygen in a chemical change it is being oxidized.

And if in the same reaction an element or compound loses oxygen it is being reduced.  

Oxidation is the gain of oxygen reduction is the loss of oxygen

Example 1:

Copper oxide +  hydrogen    =     copper    +    water

 Note that a colour change also occurs in this reaction from black copper oxide to reddish-pink copper.

GCSE Higher tier material follows:

C3.3b explain reduction and oxidation in terms of gain or loss of electrons, identifying which species are oxidised and which are reduced

C3.1b use the names and symbols of common elements and compounds and the principle of conservation of mass to write formulae and balanced chemical equations and half equations

Most redox reactions involve elements other than oxygen.

So reduction and oxidation are better defined in terms of the loss and gain of electrons

Reduction is the gain of electrons by a species in a chemical change.

Oxidation is the loss of electrons in a chemical change.

A simple way to remember this is to use the term OILRIG

O       Oxidation

I        Is the

L       Loss of electrons

R       Reduction

I        Is the

G       Gain of electrons

If you prefer to understand this inductively(!) as one of my students did then just put the whole definition into reverse and remember GIRLIO!!

G       Gain of electrons

I        Is

R       Reduction

L       Loss of electrons

I        Is

O       Oxidation

Either way here are a couple of non oxygen containing examples of redox reactions commonly requiring you to give an explanation in GCSE higher tier papers.

Example 2:

The reaction between a reactive metal like magnesium and copper sulfate solution (sometimes called a displacement reaction) is redox reaction. 

This reaction is a template for many other reactions in solution between a reactive metal and a less reactive metal salt solution.

Identifying the species in the reaction:

Magnesium atoms are oxidised, they are the reducing agent
Copper ions are reduced, they are the oxidising agent
The sulphate ions are spectator ions

Observations:
Note the colour change that happens in this reaction .

Copper sulfate is a blue solution but it turns colourless because magnesium sulfate is a colourless solution.

A brown precipitate forms in the solution and this is the copper forming.

Example 3:

Another typical redox reaction is that between a halogen and a halide salt such as chlorine and its reaction with potassium iodide solution.

Other reactions between halogens and halides ions of a halogen lower in the periodic table follow a similar pattern to this example:

Identifying the species in the reaction:
The iodide ions are oxidised and are the reducing agent
Chlorine molecules are reduced they are the oxidising agent
Potassium ions are spectator ions

Observations:
Note that there is colour change in this reaction from colourless to a brown solution.

It is the iodine solution formed that is brown in colour.
Aqueous chlorine and potassium salt solutions are colourless.