3. Redox (OXIDATION & REDUCTION)
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Describe what is meant by oxidation
Loss of electrons Loss of hydrogen Gain of oxygen
Describe what is meant by reduction
Gain of electrons Gain of hydrogen Loss of oxygen
Describe what is meant by the oxidation number
Oxidation number of an element represents the number of electrons lost or gained
Will the oxidation number of an element increase or decrease if it is oxidised
The oxidation number will increase if its been oxidised as it has lost electrons Oxidation number will decrease if its been reduced and gained electrons
What is an oxidising agent
A species that oxidises another species by accepting electrons from them so it itself is reduced
What is a reducing agent
A species that reduces other species by donating electrons to them so it itself is oxidised
Give the rules for assigning oxidation numbers
• oxidation number of a free element is always 0 • oxidation numbers of elements in a compound total 0 • oxidation number of a polyatomic ion is equal to ionic charge
Give the oxidation numbers for the following metals: Group 1 metals Group 2 metals Aluminium
Group 1 = +1 Group 2 = +2 Aluminium = +3
Give the oxidation numbers for the following non metals: Oxygen Hydrogen Fluorine Chlorine, Bromine, Iodine
Oxygen = -2 (except peroxide , H202, and compounds with fluorine -1 as Fluorine is more electronegative so it needs to be negative) Hydrogen = +1 except with metal hydrides eg NaH where its -1 Fluorine = -1 Chlorine, Bromine, Iodine = -1 except in compounds with oxygen and fluorine as they are more electronegative so chlorine, bromine and iodine will need to be positive
Where are the electrons in a reduction half equation
The left TIP: redox –> “reduction” is on the left hand side of the word Cu2+ + 2e- –> Cu
Where are the electrons in a oxidation half equation
The right 2Cl –> Cl2 + 2e-
What is a disproportionation reaction
Where a species is simultaneously oxidised and reduced at the same time
Give the rules to describe how to form more complex half equations
• add H2O in the products to balance the oxygen • add H+ (if solution is acidic) or OH- (if solution is alkaline) in the reactants to balance the hydrogen • add electrons to balance the charge
Describe how to combine half equations
1) multiply half equations to get equal amounts of electrons on each side 2) add them together, getting rid of excess H2O and H+ on each side