Oxidation and Reduction
Oxidation and Reduction:
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Key words:
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Oxidation is the loss of electrons during a reaction by a substance and reduction is the gain of electrons by a substance.
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An element is oxidised when its oxidation number is increased.
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An element is reduced when its oxidation number is decreased.
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An oxidation agent: is a substance which oxidizes something else (they give oxygen to other substances) which means an oxidation agent must gain electrons in a reaction
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A reducing agent: Removes oxygen from another substance and therefore it needs to lose electrons in a reaction to do so.
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Ways to remember these Keywords:
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Oxidisation
Is
Loss of electrons
Reduction
Is
Gain of electrons.
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Oscar Ate Green Oranges = Oxidising Agents Give Oxygen
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Ruby Ate Red Oranges = Reducing Agents Remove Oxygen
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Oxidation state:
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It's important to know which elements have been oxidised and which ones have been reduced in reactions as they show electron transfer in compounds. This can be known by knowing the oxidation state of a compound.
Rules for calculating the oxidation state:
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· Workout the oxidation state of the known elements
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· Workout the oxidation state of the unknowns
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Finding oxidation states of known compounds:
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(H2O) in this compound the hydrogen atom loses 1 electron when reacting and oxygen needs to gain two electrons in order to have reacted therefor two hydrogen atoms give one electron each to the oxygen atom and therefore the hydrogen oxidation state is +1 and oxygen is +2 (you can work this out by using the periodic table and the elements electrons arrangements which should be familiar)
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There may be exam questions where you need to work out the oxidation state of a curtain substance in a species
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An exam type of question could be “workout the oxidation state of chlorine in this species HCLO4.
Steps to working out this question.
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Look at the other elements in the compound (H and O)
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Use your knowledge of electron arrangement and the periodic table to workout how many electrons these species need to gain or lose in the reaction and this will be their oxidation state i.e. for hydrogen it needs to lose 1 electron (+1) and for oxygen it needs to gain 2 electrons (-2)
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Notice that it's o4 which means there are 4 oxygen atoms.
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This means in total this reaction needs to gain 8 oxygen electrons and lose 1 hydrogen electron.
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Therefore the overall oxidation state for chlorine is 8 - 1 = 7 (hydrogen has been oxidised and oxygen has been reduced)
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Example and explanation
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Magnesium is in group 2 and therefore needs to lose electrons when it reacts therefore it’s been oxidised and acts as the reducing agent (because it loses electrons) and oxygen is in group 6 and needs to gain electrons when it therefore acts as an oxidising agent.
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