Compounds, Formulae & Equations (OCR A Level Chemistry A): Revision Note
Exam code: H432
Chemical formulae
Ionic compounds are formed from a metal and a non-metal bonded together
Ionic compounds are electrically neutral
This means that the positive charges equal the negative charges
Positive ions
Some non-metal ions can have a positive charge, e.g.:
Hydrogen, H+
All metals form positive ions
Group 1 metals:
Lose 1 electron
Have a 1+ charge, eg. K+
Group 2 metals:
Lose 2 electrons
Have a 2+ charge, eg. Ca2+
Group 3 metals:
Lose 3 electrons
Have a 3+ charge, eg. Al3+
The charge on the ions of the transition elements can vary
Roman numerals are used to indicate their charge
Mn(II) represents the Mn2+ ion
Mn(VII) represents the Mn7+ ion
Roman numerals are used in some compounds formed from transition elements to show the charge (or oxidation state) of metal ions
In copper(II) oxide, CuO, the copper ion has a 2+ charge
In copper(I) oxide, Cu2O, the copper ion has a 1+ charge
Negative ions
The non-metals in group 15 to 17 have:
A negative charge
The ‘ide’ suffix, e.g. nitride, sulfide, chloride
Group 17 elements:
Gain 1 electron
Have a 1- charge, eg. bromide, Br-
Group 16 elements:
Gain 2 electrons
Have a 2- charge, eg. oxide, O2-
Group 15 elements:
Gain 3 electrons
Have a 3- charge, eg. nitride, N3-
Polyatomic ions
Polyatomic ions are ions that are made of more than one type of atom
For A-level Chemistry:
The most common positive polyatomic ion is:
Ammonium, NH4+
There are several polyatomic negative ions:
Hydroxide, OH-
Nitrate, NO3-
Sulfate, SO42-
Ion formulae table
Ion | Formula and charge |
|---|---|
Silver(I) | Ag+ |
Ammonium | NH4+ |
Zinc(II) | Zn2+ |
Hydroxide | OH- |
Nitrate | NO3- |
Sulfate | SO42- |
Carbonate | CO32- |
Hydrogen carbonate | HCO3- |
Phosphate | PO43- |
Worked Example
Formulae
Determine the formulae of the following ionic compounds
Magnesium chloride
Iron(III) oxide
Aluminium nitrate
Answers:
Magnesium chloride = MgCl2
Magnesium is in Group 2 so has a charge of 2+
Chlorine is in group 17 so has a charge of 1-
Magnesium needs two chloride ions for each magnesium ion to be balanced
Therefore, the formula is MgCl2
Iron (III) oxide = Fe2O3
The Roman numeral states that iron has a charge of 3+
Oxygen is in group 16 so has a charge of 2-
The charges need to be equal so 2 iron ions to 3 oxide ions will balance electrically
Therefore, the formula is Fe2O3
Aluminum nitrate = Al(NO3)3
Aluminium is in group 13 so has a charge of 3+
Nitrate is a polyatomic ion and has a charge of 1-
The polyatomic ion needs to be placed in a bracket if more than 1 is needed
Therefore, the formula of aluminium nitrate is Al(NO3)3
Balancing chemical equations
A symbol equation is a shorthand way of describing a chemical reaction using chemical symbols to show the number and type of atoms involved
A word equation uses only words to describe the reactants and products of a chemical reaction
Balancing equations
Atoms are not created or destroyed in chemical reactions
Therefore, the number of atoms of each element must be the same on both sides of the equation
To balance equations:
Do not change any chemical formulae
Add numbers in front of formulae to balance atoms
In combustion reactions, balance carbon first, then hydrogen, and finally oxygen
Step-by-step method
Write the correct formulas for the reactants and products
Count the number of each type of atom on both sides
Add numbers in front of formulas to balance the atoms one at a time
Add appropriate state symbols:
(s) = solid
(l) = liquid
(g) = gas
(aq) = aqueous (dissolved in water)
Worked Example
Write the balanced symbol equation for the following reaction:
magnesium + oxygen → magnesium oxide
Answer:
Step 1: Write out the symbol equation showing reactants and products
Mg + O2 → MgO
Step 2: Count the numbers of atoms in each reactant and product
Mg | O | |
|---|---|---|
Reactants | 1 | 2 |
Products | 1 | 1 |
Step 3: Balance the atoms one at a time until all the atoms are balanced
2Mg + O2 → 2MgO
This is now showing that 2 moles of magnesium react with 1 mole of oxygen to form 2 moles of magnesium oxide
Step 4: Use appropriate state symbols in the fully balanced equation
2Mg (s) + O2 (g) → 2MgO (s)
Ionic equations
In aqueous solutions, ionic compounds dissociate into their ions
However, not all ions in the solution actually take part in the chemical reaction
The ions that do not react are called spectator ions
An ionic equation shows only the species that are involved in the chemical change
It leaves out the spectator ions
Worked Example
1. Write the balanced symbol equation for the following reaction:
zinc + copper(II) sulfate → zinc sulfate + copper
2. Write the ionic equation for the above reaction.
Answer 1:
Step 1: To balance the equation, write out the symbol equation showing reactants and products
Zn + CuSO4 → ZnSO4 + Cu
Step 2: Count the numbers of atoms in each reactant and product.
Zn | Cu | S | O | |
|---|---|---|---|---|
Reactants | 1 | 1 | 1 | 4 |
Products | 1 | 1 | 1 | 4 |
The equation is already balanced
Step 3: Use appropriate state symbols in the equation
Zn (s) + CuSO4 (aq) → ZnSO4 (aq) + Cu (s)
Answer 2:
Step 1: The full chemical equation for the reaction is
Zn (s) + CuSO4 (aq) → ZnSO4 (aq) + Cu (s)
Step 2: Break down reactants into their respective ions
Zn (s) + Cu2+ + SO42- (aq) → Zn2++ SO42- (aq) + Cu (s)
Step 3: Cancel the spectator ions on both sides to give the ionic equation
Zn (s) + Cu2+ + SO42- (aq) → Zn2++ SO42- (aq) + Cu (s)
So, the final ionic equation is:
Zn (s) + Cu2+(aq) → Zn2+ (aq) + Cu (s)
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