Full & Ionic Equations (Edexcel International AS Chemistry): Revision Note
Writing & Balancing Equations
A symbol equation is a shorthand way of describing a chemical reaction using chemical symbols to show the number and type of each atom in the reactants and products
A word equation is a longer way of describing a chemical reaction using only words to show the reactants and products
Balancing equations
During chemical reactions, atoms cannot be created or destroyed
The number of each atom on each side of the reaction must therefore be the same
E.g. the reaction needs to be balanced
When balancing equations remember:
Not to change any of the formulae
To put the numbers used to balance the equation in front of the formulae
To balance firstly the carbon, then the hydrogen and finally the oxygen in combustion reactions of organic compounds
When balancing equations follow the following the steps:
Write the formulae of the reactants and products
Count the numbers of atoms in each reactant and product
Balance the atoms one at a time until all the atoms are balanced
Use appropriate state symbols in the equation
The physical state of reactants and products in a chemical reaction is specified by using state symbols
(s) solid
(l) liquid
(g) gas
(aq) aqueous
Formulae for Ionic Compounds
The formulae of simple ionic compounds can be calculated if you know the charge on the ions
Below are some common ions and their charges:
Common Ions & Their Charges Table
For ionic compounds you have to balance the charge of each part by multiplying each ion until the sum of the charges = 0
Example: what is the formula of aluminium sulfate?
Write out the formulae of each ion, including their charges
Al3+ SO42-
Balance the charges by multiplying them out:
Al3+ x 2 = +6 and SO42- x 3 = -6; so +6 – 6 = 0
So the formula is Al2(SO4)3
Examiner Tips and Tricks
Another method that also works is to 'swap the numbers'.
In the example above the numbers in front of the charges of the ions (3 and 2) are swapped over and become the multipliers in the formula (2 and 3).
Easy when you know how!
Worked Example
Balance the following equation:
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
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
Many chemical reactions in aqueous solutions involve ionic compounds, however only some of the ions in solution take part in the reactions
The ions that do not take part in the reaction are called spectator ions
An ionic equation shows only the ions or other particles taking part in a reaction, and not the spectator ions
Worked Example
1. Balance the following equation
zinc + copper(II) sulfate → zinc sulfate + copper
2. Write down 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. 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)
Zn (s) + Cu2+(aq) → Zn2+ (aq) + Cu (s)
Experimental Observations & Equations
Chemical equations give information about the reaction that is taking place
Balanced equations show the number of particles participating in the reaction and the number of products being formed
Balanced equations can be used to calculate the number of moles involved in reactions
Balanced equations can, also, be used to calculate masses and volumes involved in reactions
Ionic equations only show the reacting particles
Ionic equations allow you to identify spectator ion
Types of reaction
Chemical equations can be used to determine the type of reaction taking place
Displacement reactions
Br2 (aq) + 2KI (aq) → I2 (aq) + 2KBr (aq)
In this reaction, the more reactive bromine displaces the less reactive iodide in potassium iodide
This can also be seen in the ionic equation for the reaction
Br2 (aq) + 2I- (aq) → I2 (aq) + 2Br- (aq)
Examiner Tips and Tricks
The use of chemical equations can help identify risks and hazards in the reaction and suggest appropriate precautions where necessary
For example, the use of aqueous bromine in the above example should suggest the potential use of a fume cupboard and nitrile gloves because:
Bromine liquid is toxic, corrosive and harmful to the environment
Bromine water with a concentration of 0.2 mol dm3 is corrosive
With a concentration of between 0.06 mol dm3 and 0.2 mol dm3, bromine water is an irritant
Only below concentrations of 0.06 mol dm3 is bromine water considered a low hazard
Neutralisation reactions
These can be identified by the presence of reactant acids and bases as well as the formation of a neutral solution salt and water (and sometimes other compounds such as carbon dioxide)
HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (l)
Na2CO3 (aq) + 2HNO3 (aq) → 2NaNO3 (aq) + H2O (l) + CO2 (g)
The ionic equations can more clearly demonstrate the neutralisation of an acid and a base:
H+ (aq) + OH- (aq) → H2O (l)
2H+ (aq) + CO32- (aq) → H2O (l) + CO2 (g)
Examiner Tips and Tricks
For neutralisation reactions, the main hazards are linked to:
The concentration of the acid
The type of acid
This could mean strong, e.g. HCl, or weak, e.g. CH3COOH
This could also mean monoprotic, e.g. HNO3, diprotic, e.g. H2SO4, or triprotic, e.g. H3PO4
The concentration of the base
The strength of the base
The physical state of the base, e.g. NaOH (s) is arguably considered more corrosive than a high concentration solution of NaOH (aq)
Precipitation reactions
These are shown by the reaction of two aqueous solutions to form products which include one solid
BaCl2 (aq) + Na2SO4 (aq) → BaSO4 (s) + 2NaCl (aq)
The ionic equation shows the precipitation reactions more clearly as there are no other products considered
Ba2+ (aq) + SO42- (aq) → BaSO4 (s)
Examiner Tips and Tricks
A precipitation reaction is a clear example of where consideration for further practical procedures is most obvious
The formation of a solid product should tell you that any purification of the product should include filtering or decanting as a minimum
You've read 0 of your 5 free revision notes this week
Sign up now. It’s free!
Did this page help you?