3.2- Redox reactions

ion/ half equations

Elements where the oxidation state of a single atom is changing only requires electrons to balance

eg. Mg→ Mg2+ + 2e-, Fe3+ +e-→ Fe2+

compound ions require oxygen, water, hydrogen and electrons to balance

eg. MnO4- + 8H+ +5e-→ Mn2+ + 4H2O

Combining half equations

• If needed multiply equations so that the electrons are balanced

• cancel out electrons on both sides add elements if match

• can simplify if you can

Redox titrations

Involves 25cm3 of solution measured using a volume metric pipette and placed in a conical flask. Second solution is added a little at a time from burette swirling the mixture during addition. Continued until desired color change is seen.

Many redox titrations do not need an indicator as colours of reactants frequently allow the endpoint to be seen

Acidified Manganate (VII) ions with iron (II) ions

purple sol of

burette

Purple solution of oxidizing agent is added from the burette. When it reacts with species to be oxidized it forms Mn2+ (colourless). The Ependpoint is reached when solution goes pale pink, this is because a very small amount of purple manganate remains appearing pink when dilute.

1. MnO4- + 8H+ +5e- → Mn2+ + 4H2O

the colour change of solution Fe2+ (pale green) is oxidised to Fe3+ (yellow) is usually difficult to see a solutions are dilute colour change associated with manganate is used for endpoint. iron ions are oxidized:

2. Fe2+ → Fe3+ + e-

Acidified dichromate (VI) ions with iron (II) ions

Potassium dichromate in acid solution oxidizes Fe2+ to Fe3+ with a color change from Dark orange Cr2O72- to green solution of Cr3+

1. Cr2O72- + 14H+ +6e- → 2Cr3+ + 7H2O

iron is again oxidized

2. Fe2+ → Fe3+ + e-

Acid in the dichromate titration

Acid must be present for this reaction to occur usually sulfuric acid

if the pH of a solution Rises too high then dichromate ion is broken up into two chromate ions:

Cr2O72- + H2O = 2CrO42- + 2H+

dark orange. Yellow

chromium always has an oxidation state of plus six so no redox reactionchromium always has an oxidation state of +6 so no redox reaction

Equilibrium if acid is added

Equilibrium will shift to the left and to the right if base is added

Redox titration for copper (II) ions

Addition of colourless solution containing iodide ions to a blue solution of copper (II) ions lead to the formation of a cloudy brown solution. Cu2+ ions in solution react with iodide ions generating a brown solution of iodine and are reduced to Copper (I) in the precipitate of CuI.

1. 2Cu2+ +4I- →2CuI + I2

iodine is titrated with Sodium thiosulfate to work out how much copper was present originally. Sodium thiosulfate is a common reducing agent.

2. 2S2O32- → S4O62- + 2e-

3. I2 +2e- = 2I-

Brown. Colourless

Experiment details

• Adding excess iodide ions to Cu2+ ensures all Cu2+ reacts to produce I2. Mixture formed is a cloudy Brown solution

• titrate the brown I2 against sodium thiosulfate until mixture is straw coloured

• adding a start indicator goes blue black and continue until colour vanishes

Finding the molar mass of a redox titration

• Find out the number of moles

• workout in either 25cm3 or 250cm3

• ratio between compounds

• work out the other moles

• workout the concentration then the molar mass