2.1.5 (b) & (c) Writing formulae using Oxidation Numbers

Syllabus

(b) writing formulae using oxidation numbers

(c) use of a Roman numeral to indicate the magnitude of the oxidation number when an element may have compounds/ions with different oxidation numbers.

{Examples should include, but not be limited to, Iron(II) and Iron(III).

{Learners to write formulae from names such as Chlorate(I) & Chlorate(III) & vice versa.}

{Note that 'Nitrate’ and ‘Sulphate’, with no shown oxidation number, are assumed to be NO₃^– and SO₄^2–.}

What does this mean?

According to the OCR if you read "Sulphate ion" you should assume they mean SO₄^2-.

And if you read "Nitrate ion" you should assume they mean NO₃^-.

But these are not the only Sulphate or Nitrate ions.

i) Sulphates

Strictly speak SO₄^2-is a Sulphate (VI) ion.

This is because we can assume the Oxygen atoms are -2.

Giving us a total of -8

Of which the Sulphur cancels only 6, leaving the -2 charge.

So this ion contains a +6 Sulphur atom

Hence, Sulphate (VI) but other Sulphates exist.

According to the old system you just had to learn the formulas of other Sulphates

The structure and bonding of the sulfate ion

So, what would we call these ions now?

SO₃^2-

- Assume O = -2, giving a total negative charge of -6

-2 is left over, so the Sulphur atom is cancelling only 4

Therefore, it is a +4 Sulphur

And this is Sulphate (IV)

SO₅^2-

- Assume O = -2, giving a total negative charge of -10

-2 is left over, so the Sulphur atom is cancelling only 8

Therefore, it is a +8 Sulphur

And this is Sulphate (VIII)

Canonical resonance structures for the nitrate ion

ii) Nitrates

Strictly speak NO₃^-is a Nitrate (V) ion.

This is because we can assume the Oxygen atoms are -2.

Giving us a total of -6

Of which the Nitrogen cancels only 5, leaving the -1 charge.

So this ion contains a +5 Nitrogen atom- Hence, Nitrate (V)

NO₂^- used to be called the Nitrite ion.

Assume O = -2, giving a total negative charge of -4

-1 is left over, so the Nitrogen atom is cancelling only 3

Therefore, it is a +3 Nitrogen

And this is Nitrate (III)

Other examples.

Questions tend to ask "suggest a formula for" - this gives you some leeway.

Eg. Suggest a formula for the Chlorate (III) ion

From its name this ion must contain only Chlorine and Oxygen
It must be overall negative
The Oxidation number of the Chlorine is +3
So add -2 Oxygen atoms until the ion just becomes negative

So, with a +3 Cl

Adding one -2 Oxygen atom would give us ClO⁺ which isn't negative
Adding another gives us ClO₂^- which is what would be on the markscheme
Although they would also accept ClO₃^2- and potentially other mathematically correct suggestions

Eg. Suggest a formula for the Chlorate (V) ion

From its name this ion must contain only Chlorine and Oxygen
It must be overall negative
The Oxidation number of the Chlorine is +5
So add -2 Oxygen atoms until the ion just becomes negative

So, with a +5 Cl

Adding one -2 Oxygen atom would give us ClO³⁺ which isn't negative
Adding another gives us ClO₂⁺ still isn't negative
But adding another gives us ClO₃^-

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