OCR AS Chemistry F332: Oxidation and Reduction

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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 end of a Redox reaction:
The oxidising agent has been reduced
The reducing agent has been oxidised

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Example
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)
The reaction is composed of two half-reactions which can be described by two half
equations.…read more

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Displacement Reactions
When chlorine is bubbled through potassium iodide solution, a displacement reaction takes
place.
The colourless solution turns brown as iodine is produced.
We say that the chlorine Cl2 has displaced the Iodine, I2, from the solution.…read more

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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 negative oxidation state.
Each atom in a molecule or ion is assigned and oxidation state to show how much it is
oxidised or reduced.…read more

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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…read more

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Example
Assign the oxidation states for each element in the following compounds:
C02
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 to 0
C must be +4
CH4
Step 1: H is +1
There are four Hs so the total contribution of H to the oxidation
state is 4 x (+1) = + 4
Step 2:…read more

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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 elements
NH4+ H +1 N -3
ClO3- O -2 Cl +5
VO2+ O -2 V +4
Summary of rules for assigning oxidation states:
The oxidation state of atoms in elements is 0
In compounds, the…read more

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Using Oxidation States to identify what's been oxidised and what's
been reduced
Remember:
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 states have changed.
There are two elements which are in compounds on one side of the
equation and as uncombined elements on the other.
The magnesium's oxidation state has increased ­ it has been
oxidised.…read more

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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.…read more

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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 the only thing to have changed oxidation state.
It has been oxidised and reduced.
One atom has been reduced because its oxidation state has fallen.
The other has been oxidised.…read more

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