Redox Reactions.

Consider the two oxidising agents chlorate ions, ClO-, and iron (III) ions, Fe3+. Their reduction can be represented as follows:

Add a little potassium chlorate (I) solution to a solution of iron (II) ions.

Observation:

When the colourless solution of potassium chlorate was added to a pale green solution of Fe2+ ions the solution darkened slightly with the formation of a brown precipitate. This brown precipitate is Fe3+ ions.

Ionic Equation For This Reaction:

Inference:

Since the ClO- has oxidised the Fe2+ to Fe3+, then the order of oxidising strength is:

ClO- > Fe3+

Experiment 3

Consider the two oxidising agents bromine, Br2, and chlorate ions, ClO-. Their reduction can be represented as follows:

Add a little of potassium chlorate (I) ions to a solution of bromine ions.

Observation:

When the colourless potassium chlorate (I) solution was added to the colourless solution of bromide ions, there was no change in appearance so the mixture was colourless.

Ionic Equation For This Reaction:

Inference:

Had bromine molecules been formed, an orange/red precipitate would have formed. Therefore, since there was no reaction this suggests that ClO- ions could not oxidise the Br- to Br2, so the order of oxidising strength is:

Br2 > ClO-

Experiment 4

Consider the two oxidising agents chlorate ions, SO42-, and iodine, I2. Their reduction can be represented as follows:

Add a little of a saturated solution of sulphur dioxide (this contains sulphuric (IV) acid) to a solution containing iodine.

Observation:

When the colourless solution of sulphur dioxide was added to a pale yellow/orange solution of I2 the solution quickly decolourised.

Ionic Equation For This Reaction:

Inference:

The change in colour implies that iodine ions were formed. Since the I2 had oxidised the H2SO3 to SO42-, then the order of oxidising strength is:

I2 > SO42-

Experiment 5

Consider the two oxidising agents chlorine, Cl2, and bromine, Br2. Their reduction can be represented as follows:

Add a solution of chlorine in water to a solution containing bromide ions.

Observation:

When the colourless chlorine water was added to the colourless solution of bromide ions, the mixture turned a pale yellow. Addition of hexane (a colourless inert solvent) formed a layer above the reaction mixture. This layer turned a pale orange when the test tube was shaken.

Ionic Equation For This Reaction:

Inference:

The result from the addition of the hexane showed that bromine had been liberated from the reaction mixture. Therefore, it can be said that Cl2 has oxidised the Br- to Br2, so the order of oxidising strength is:

Cl2 > Br2

SUMMARY OF EXPERIMENTS 1-5

INTERPRETATION OF RESULTS

From the results a list of the oxidising agents in order of decreasing oxidising ability can be formed:

Cl2 > Br2 > ClO- > Fe3+ > I2 > SO42-

ACIDIFIED MANGANATE (VII)

(Experiment 6)

Acidified manganate (VII), MnO4-, is reduced according to the redox half equation:

Firstly, MnO4- should be compared with the strongest oxidising agent on the list above - Cl2 - whose reduction can be represented as:

Add a solution of chloride ions to acidified manganate (VII) [ this is produced by adding some dilute sulphuric acid to potassium manganate (VII) solution].

Observation:

When the colourless chloride ion solution was added to the purple solution of acidified manganate (VII), the mixture turned colourless.

Ionic Equation For This Reaction:

Inference:

Since the MnO4- has oxidised the Cl- to Cl2 then the order of oxidising strength is:

MnO4- > Cl2

This means that the addition of MnO4- to the experimental data changes the list to:

MnO4- > Cl2 > Br2 > ClO- > Fe3+ > I2 > SO42-