Determine Oxidation State of Unknown Species from Experimental Data

Sometimes in redox questions we are required to determine the oxidation state of an unknown species.

Let's take a look at an example.

determine oxidation state from expt data question

From the question we know that chlorine is reduced to chloride ions, and thiosulphate undergoes oxidation.

But the sulphur-containing product is not known, hence we need to determine the oxidation state of sulphur in the product, then decide which is the answer.

We can use the following 4 steps to determine oxidation state of an unknown species given experimental data.

determine oxidation state from expt data guideline

Let's put these steps to work!

1. Determine number of moles of reactants

This is straightforward as the information will be available in the question for us to calculate amount of both reactants.

determine oxidation state from expt data determine amount of reactants

2. From known half equation, determine number of moles of electrons transferred

We can write out the 2 half equations first.

determine oxidation state from expt data half equations

Notice the reduction half equation from Cl2 to Cl- can be balanced so the number of moles of electrons is known.

But for the oxidation half equation, since the product is unknown, we cannot balance this half equation and determine the moles of electrons.

We can use the known half equation to calculate the moles of electrons gained by Cl2 first.

Since electrons are totally transferred in a redox reaction, the moles of electrons lost by S2O32- will be equal to moles of electrons gained by Cl2.

determine oxidation state from expt data determine moles of electron transferred

3. Determine mole ratio of electron to unknown

We can now find mole ratio of electron lost to thiosulphate which will work out to be 8:1.

determine oxidation state from expt data determine mole ratio of electron to unknown

4. Determine oxidation state of unknown

Each S2O32- loses 8 electrons, and since there are 2 S in each thiosulphate ion, then each S will lose 4 electrons.

So the oxidation state of sulphur will increase by 4 units.

Finally we can determine oxidation state of S in S2O32- to be +2, and deduce that oxidation state of S in the product will be +6.

determine oxidation state from expt data determine oxidation state of unknown

Comparing the options we can conclude the answer has to be option A where the oxidation state of sulphur in HSO4- is +6.

determine oxidation state from expt data answer

Topic: Redox Reactions, Physical Chemistry, A Level Chemistry, Singapore

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