redox

Question 1

what is an oxidising agent

Answer 1

this is the species that is reduced in a reaction and causes another species to be oxidised

Question 2

what is a reducing agent

Answer 2

is the species that is oxidised in a reaction and causes another species to be reduced

Question 3

what is a redox

Answer 3

• it is essentially two reactions that occur simultaneously
• the reduction of a species
• the oxidation of a species

Question 4

write out half equations for

Mg + 2HCL = MgCL2 + H2

Answer 4

Mg = Mg2+ + 2e- (oxidation)

2H+ + 2e- = H2 (reduction)

Question 5

species you need to add in order to balance the equation

Answer 5

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

Question 6

describe redox titrations

Answer 6

• 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

Question 7

descirbe redox titrations between Fe2+ and MnO4-

Answer 7

• 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

Question 8

redox titrations between I2 and S2O32-

Answer 8

• 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