3.2 redox reactions

Question 1

how do you construct half equations

Answer 1

1. write reagents and products
2. balance atoms present
   3, add 2 hydrogens to combine with each oxygen to form water
3. find the total charges on both sides to find number of electrons needed.

Question 2

how do you combine half equations

Answer 2

Write the two redox equations with one beneath the other so that the arrows align:

Multiply one or both of the half-equations so that the number of electrons is the same in both half-equations.

Combine the half-equations into a single equation with all the reactants on the left hand side and the products on the right hand side. You do not need to include the electrons as they will cancel each other out

Question 3

why is there no need for an indicator in a redox titration

Answer 3

As one of the reactants in a redox titration invariably changes colour, then there is usually no need for an indicator.

Question 4

what is meant by titre concordant results

Answer 4

Titre results are said to be concordant if they are within 0.20 cm3 of each other

Question 5

write out the half equation for the reduction of acidified manganate and the colour change

Answer 5

MnO4 - + 8H+ + 5e- —> Mn2+ + 4H2O
purple to pale pink

Question 6

write the half equation for the reduction of dichromate and its colour change

Answer 6

Cr2O7- + 14H + 6e —-> 2Cr3+ + 7H2O
Orange to green

Question 7

write the half equation for iron(II) and its colour change

Answer 7

Fe2+ —-> Fe3+ + e-
pale green to pale yellow

Question 8

write down the half equation for oxidation iodine

Answer 8

I2 + 2e —–> 2I-
Brown to colourless

Question 9

write the oxidation of thiosulphate

Answer 9

S2O3 2- —> S4O6 2- +2e
no colour change

Question 10

describe how you would carry out a redox titration

Answer 10

Carried out the same way as a acid-base titration.

Question 11

whats the differance between acid base titration and a redox titration

Answer 11

Redox titrations follow the same procedure as acid-base titrations, the only difference being the type of reaction that takes place.

Question 12

describe the redox titration for copper (II) ions

Answer 12

1. measure 25cm^3 of Cu2+ into a conical flask
2. add excess iodide ions e.g. KI to Cu2+ solution to ensure all Cu2+ reacts to produce I2.
   It will form a cloudy brown solution.

3.titrate the I2 against sodium thiosulphate until the mixture is straw coloured. This tells us that the most the iodine has react leaving a little remaining.
4. add starch indicator which goes blue black and continue until the colour vanishes. the mixture is now flesh-coloured.