1
Q
what is an oxidising agent
A
this is the species that is reduced in a reaction and causes another species to be oxidised
2
Q
what is a reducing agent
A
is the species that is oxidised in a reaction and causes another species to be reduced
3
Q
what is a redox
A
- it is essentially two reactions that occur simultaneously
- the reduction of a species
- the oxidation of a species
4
Q
write out half equations for
Mg + 2HCL = MgCL2 + H2
A
Mg = Mg2+ + 2e- (oxidation)
2H+ + 2e- = H2 (reduction)
5
Q
species you need to add in order to balance the equation
A
H+ - when reactions are carried out in acidic conditons
OH- - usually when reactions are carried out in alkaline conditions
H20 - usually when the equation needs extra O and H to be added
6
Q
describe redox titrations
A
- can be used to determine the amounts of species being oxidised and reduced, this is an redox titration
- a known concentration of either a reducing agent or oxidising agent is placed in burette and titrated against an unknown concentration of the chemical that is being oxidised or reduced respectively
- dont need a indiciator as many of teh species self indicate due to changing colour in different oxidation states
7
Q
descirbe redox titrations between Fe2+ and MnO4-
A
- MnO4- is a common oxidising agent, it has a deep purple colour but becomes colourless when it is reduced from +7 to +2 oxidation state
- this usually occurs in the presence of H+ ions
- MnO4- is commonly used to oxidise solutions containing iron(II) but can be used as an oxidising agent in many other chemical reactions
- MnO4- + 8H+ + 5Fe2+ = Mn2+ + 5Fe3+ + 4H2O
- end point is seen when excess MnO4- ions are present as a faint pink colour appears, this ocucrs because all the Fe2+ ions have reacted an the MnO4- can no longer be reduced to the colourless Mn2+
- can be used to determine the concentration of iron in an unknown solution or indeed other ions that can be oxidised by MnO4- or to calculate the percentage composition of a metal in a solid sample of a compound or alloy
- when calculating the mass of iron in a substance the molar mass of an Fe2+ ion is taken to be the same as the molecular mass of Fe therefore the titration results can be used directly to covert the number molecules into the mass present
8
Q
redox titrations between I2 and S2O32-
A
- iodine is a useful substance to use in redox titrations as it has a dark blue black colour in the presence of starch but when it is reduced to iodide ions the colour disappears
- I2 = 2I- + 2ei
- the reduction of iodine to iodidie ions will occur presence of thiosulfate ions
2S2O32- + I2 = S4O62- + 2I- - use aqueous iodide ions and aqueous thiosulfate to determine the concentration of unknown reducible species
- often invovles an initial reaction between the unknown oxidising agent and iodide ions ( such as potassium iodide) which has iodine as a product
For example - Cl2 + 2I- = 2Cl- + I2
- the iodine then goes on to react with thiosulfate ions from a solution with a known concentration being added added form a burette
- ## in presence of starch a blue balck colour will remain present as long as there is any iodine, once this has all reacted with thiosulfate ions this will disappear marking the end point of reaction
