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Oxidation and Reduction
When an oxidation reaction and a reduction reaction occur simultaneously, this is called a
Redox Reaction.
Oxidation and reduction can be defined in two different ways:

Oxidation Reduction
Is Is
Loss Gain

An oxidising agent removes electrons

A reducing agent gives electrons to something else

By the…

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When we heat a piece of copper in oxygen, the surface of the copper becomes coated with
a black solid.

The copper's surface is oxidised ­ it gains oxygen.

The product, copper ll oxide is an ionic compound:

Cu (s) + ½ O (g) Cu 2+ O 2- (s)…

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Displacement Reactions
When chlorine is bubbled through potassium iodide solution, a displacement reaction takes

The colourless solution turns brown as iodine is produced.

We say that the chlorine Cl2 has displaced the Iodine, I2, from the solution.

The overall equation for the reaction is:

Cl2 (g) + 2Kl (aq)…

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Oxidation State
Oxidation states simplify the whole process of working out what is being oxidised and what
is being reduced in redox reactions.

Oxidation state shows the total number of electrons which have been removed from an
element ­ a positive oxidation state or added to an element ­ a…

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Something is Oxidised If Something is Reduced if
It gains oxygen It loses oxygen

It loses electrons It gains electrons

Its oxidation number increases Its oxidation number decreases

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Assign the oxidation states for each element in the following compounds:

Step 1: O is -2
There are two Os so the total contribution of O to the oxidation
state is 2 x (-2) = -4

Step 2: To make the total of the oxidation states add up…

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Oxidation states of elements

Element Oxidation state of Element
O -2
H +1
F -1
Cl -1

Compound Oxidation state of Elements
H2O H +1 O -2
CH4 H+1 C-4
BrF3 F -1 Br +3
SO2 O -2 S +4
PCl3 Cl ­ 1 P +3

Ion Oxidation states of…

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Using Oxidation States to identify what's been oxidised and what's
been reduced


Oxidation involves an increase in oxidation state

Reduction involves a decrease in oxidation state

Example 1

This is the reaction between magnesium and hydrochloric acid or
hydrogen chloride gas:

Mg + 2HCl MgCl2 + H2

The oxidation…

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Example 2

The reaction between sodium hydroxide and hydrochloric acid is:

NaOH + HCl NaO + H2O

Checking all the oxidation states:

Nothing has changed.

This isn't a redox reaction.

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Example 3

The reaction between chlorine and cold dilute sodium hydroxide
solution is:

2NaOH + Cl2 NaCl + NaO O + H 2O

The chlorine has changed oxidation state because it has ended up in
compounds starting from the original element.

Checking all the oxidation states shows:

The chlorine is…


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