M2,S1 Oxidation Numbers

Question 1

What do oxidation numbers show?

Answer 1

How many electrons an atom has

ox. no.s show you how many electrons an atom has accepted or donated to form an ion, or to form part of a compound.

Question 2

What is the first rule you need to remember when you assign oxidation numbers?

Answer 2

Uncombined elements have an oxidation number of 0. This means they haven’t accepted or donated any electrons. Elements that are bonded to identical atoms also have an oxidation number of 0.

Question 3

What’s the second rule you need to remember when you assign oxidation numbers?

Answer 3

The oxidation number of a simple monotomic ion (an ion consisting of just one atom) is the same as its charge.

Question 4

What’s the third rule you need to remember when you assign oxidation numbers?

Answer 4

For molecular ions, the sum of the oxidation numbers is the same as the overall charge of the ion.

Question 5

What’s the fourth rule you need to remember when you assign oxidation numbers?

Answer 5

For a neutral compound, the overall oxidation number is 0. If the compound is made up of more than one element, each element will have its own oxidation number.

Question 6

What oxidation numbers do you need to learn?

Answer 6

Oxygen nearly always has an oxidation number of -2, except in peroxides (O₂^2-) where it’s -1 and molecular oxygen, O₂, where it’s 0.

Hydrogen always has an oxidation number of +1, except in metal hydrides (MH_x, where M is a metal) where it’s -1, and in molecular hydrogen, H₂, where it’s 0.

Question 7

Why do we use roman numerals to express oxidation numbers?

Answer 7

If an element can have multiple oxidation numbers or isn’t in its ‘normal’ oxidation state, the oxidation number can be shown by using roman numerals, e.g. (III) = 3+.

The roman numerals are written after the name of the element they correspond to.

e.g. Iron (II) sulfate, iron has an oxidation number of +2, Formula FeSO₄

In iron(III) sulfate iron has an oxidation number of +3, fromula Fe₂(SO₄)₃

Question 8

What do -ate compounds contain?

Answer 8

Ions with the names ending in -ate (e.g. sulphate, carbonate, nitrate, chlorate) contain oxygen and another element.

Question 9

What do you need to remember about the formula of -ate compounds?

Answer 9

Sometimes the ‘other’ element in the ion can exist with multiple oxidation numbers, and so form different -ate ions.

In these cases, the oxidation number is attached as a roman numeral after the name of the ate compound.

The roman numerals correspond to the ‘other’ element (the non-oygen element) in the -ate compound.

e.g. in sulfate(VI), the sulfur has an oxidation number of +6 - this forms an SO₄^2- ion

In a sulfate(IV) ion, the sulfur has an oxidation number of +4 - this form an SO₃^2- ion.

In a nitrate(III) ion, nitrogen has an oxidation number of +3 - this forms an NO₂^- ion.

Question 10

kIn sulfate(VI), the sulfur has an oxidation number of +6 - this forms an SO₄^2- ion

In a sulfate(IV) ion, the sulfur has an oxidation number of +4 - this form an SO₃^2- ion.

Why has changing the oxidation number changed the formula of the ion?

Answer 10

1. The formula must change because, in molecular ions, the sum of the oxidation numbers is equal to the overall charge of the ion.
2. The no. of oxygen atoms can no longer be four (SO₄^2-) as this would make the ox. no. of S +6.
3. If the oxidation number of S is +4, and the overall charge of the molecule is -2, the total charge of the oxygens must be:

? +4 = -2

-6 +4 = -2

4. If the total charge of the oxygens is -6, and the total charge of one oxygen is -2, the no. of oxygen atoms must be:
   - 6 ÷ -2 = 3

Question 11

What should you assume when there are no oxidation numbers shown?

Answer 11

nitrate = NO<sub>3</sub><sup>-</sup>
sulphate = SO<sub>4</sub><sup>2-</sup>

Question 12

calc. the oxidation no. of Fe in Fe

Answer 12

single, uncombined elements have an ox. no. of 0

Question 13

calc. the oxidation no. of Fe in FeCl₃

Answer 13

Not charged - neutral compounds have an ox. no of 0.

Cl has a -1 charge so the overall charge of Cl:

-1 x 3 = -3

so for there to be 1 Fe atom, it must have an ox. no of +3, as +3 + -3 =0 (the overall charge of the molecule)

Question 14

What is the ox. no. of Fe in K₂FeO₄ ?

Answer 14

Not charged - neutral compounds have an ox. no of 0.

the overall charge of the potassium is +1 x 2 = +2

the overall charge of the oxygen is -2 x 4 = -8

To get a neutral molecule (with a charge of 0) find what the two known charges add up to:

-8 + 2 = -6

Find what would need to be added to give you the charge of the molecule (0).

• 6 + _ = 0
• 6 + 6 = 0

so the ox. no. is +6

Question 15

What is the ox. no. of Fe in [Fe(H₂O)₆]²⁺?

Answer 15

the charge outside the brackets applies to everything inside, so the ox. no. of Fe is +2.

Question 16

What is the ox. no. of Cl in Cl₂O₇?

Answer 16

Not charged - neutral compounds have an ox. no of 0.

Combined oxygen has an ox. no. of -2 so the overall charge of the oxygen is -2 x 7 = -14

so the total ox. no. of Cl must be the difference between the total charge of the oxygens and the overall charge (0).

-14 + 14 = 0

you need to ÷ by two as there are two oxygen atoms

14 ÷ 2 = 7

so the ox. no. of Cl is +7

Question 17

Why is the ox. number of a sulphate ion with 4 oxygen atoms +6?

SO₄^2-

Answer 17

Different ox. no.s affect the formula of the ion.

For the ox. no. of S to be +6, there has to be 4 oxygen atoms. this is because:

In a molecular ion the sum of the ox. no.s = the overall charge.

total charge of oxygen: -2 x 4 = -8

the charge of the sulphur atom must be the difference in the total charge of oxygen and the overall charge of the molecule:

• 8 + ? → -2
• 8 + 6 → -2

In a sulfate(IV) ion, the sulfur has an oxidation number of +4 - this form an SO₃^2- ion.

Question 18

What is the ox. no. of Cl in ClO₃^- ?

Answer 18

It is charged so it is a molecular ion -it’s oxidation no. will be the same as the overall charge of the ion (-1).

combined oxygen has an oxidation number of -2, so the total charge of the oxygens is -2 x 3 = -6

so the oxidation no. of Cl must be (the diff. in the total charge of the oxygens and the overall charge)

• 6 + _ = -1
• 6 + 5 = -1

so the ox. no. of Cl is +5

Question 19

How do you work out the formula of a molecule from its systematic name?

e.g. What is the formula of iron(III) sulphate.

Answer 19

1. Find the oxidation number of the non ‘-ate’ ion from the systematic name (it’s the roman numeral number).
2. You need to find the overall charge of the -ate ion. If the ‘-ate’ ion dosen’t have an oxidation number, assume the formula of the -ate ion. (e.g. if there’s no oxidation no. shown, assume nitrate = NO₃^- and sulphate is SO₄^2- ). Use this as its overall charge.
3. The overall charge of the compound will be 0, so find how many of each ion are needed to balance out the charges.

Question 20

How do you work out the systematic name of a compound given its formula?

Answer 20

1. Look at the oxygen atom(s) it contains that aren’t oxygen - which element is bonded to the oxgen to form the ‘ate’ ion. (may or may not also need to know: which isn’t?)
2. Oxygen usually exists with an oxidation number of -2.
3. Find the total charge of the oxygen(s) - the charge, -2, multiplied by the no. of oxygen atoms.
4. Look at the overall charge of the molecule - what do you need to add to this to get the total charge of the oxygens? This gives you the oxidation number of the -ate ion.