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 SO42–.}
What does this mean?
According to the OCR if you read "Sulphate ion" you should assume they mean SO42-.
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 SO42-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
So, what would we call these ions now?
SO32-
- 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)
SO52-
- 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)
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 ClO32- 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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