Ionic Equations (Edexcel A Level Chemistry): Revision Note
Writing Chemical Formulae
Oxidation numbers are a useful tool for naming compounds as some elements can exist with more than one oxidation number
For compound with two elements it is straight forward to name the compound
For example
PCl3 is phosphorus(III) chloride or phosphorus trichloride
PCl5 is phosphorus(V) chloride or phosphorus pentachloride
OF2 is oxygen difluoride
O2F2 is dioxygen difluoride
In order to name a more complete compound we use Roman numerals for the element that has a variable oxidation number
K2CrO4 potassium chromate(VI)
Worked Example
Can you name these metal compounds?
Cu2O
MnSO4
Na2CrO4
KMnO4
Na2Cr2O7
Answer:
Answer 1: copper(I) oxide:
The ox. no. of 1 O atom is -2 and Cu2O has overall no charge so the ox. no. of Cu is +1
Answer 2: manganese(II) sulfate:
The charge on the sulfate ion is -2, so the charge on Mn and ox. no. is +2
Answer 3: sodium chromate(VI):
The ox. no. of 2 Na atoms is +2 so CrO4 has an overall -2 charge, so the ox. no. of Cr is +6
Answer 4: potassium manganate(VII):
The ox. no. of a K atom is +1 so MnO4 has overall -1 charge, so the ox. no. of Mn is +7
Answer 5: sodium dichromate(VI):
The ox. no. of 2 Na atoms is +2 so Cr2O7 has an overall -2 charge, so the ox. no. of Cr is +6. To distinguish it from CrO4 we use the prefix di in front of the anion
Ionic Half-Equations
Balancing full ionic equations
Balancing equations using redox principles is a useful skill and is best illustrated by following an example
It is important to follow a methodical step-by-step approach so that you don't get lost:
Worked Example
Writing overall redox reactions
Manganate(VII) ions (MnO4- ) react with Fe2+ ions in the presence of acid (H+) to form Mn2+ ions, Fe3+ ions and water
Write the overall redox equation for this reaction
Answer
Step 1: Write the unbalanced equation and identify the atoms which change in oxidation number
Step 2: Deduce the oxidation number changes
Step 3: Balance the oxidation number changes
Step 4: Balance the charges
Step 5: Finally, balance the atoms
Metals & Non-metals
Metals
Metals, in general, will form positive ions by losing electrons
Therefore, they are oxidised and the oxidation number increases
Example 1:
When sodium reacts with water, sodium hydroxide and hydrogen gas is formed
2Na (s) + H2O (l) → 2NaOH (aq) + H2 (g)
The oxidation number of sodium changes from 0 to +1
Example 2:
When magnesium reacts with hydrochloric acid, magnesium chloride and hydrogen gas is formed
Mg (s) + 2HCl (l) → MgCl2 (aq) + H2 (g)
The oxidation number of magnesium changed from 0 to +2
Non-metals
Non-metals, in general, will form negative ions by gaining electrons
Therefore, they are reduced and the oxidation number decreases
Example:
When sodium reacts with oxygen, sodium oxide is formed
4Na (s) + O2 (g) → Na2O (s)
The oxidation number of oxygen changes from 0 to -2
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