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
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)
Exam Tip
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)
Exam Tip
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)
Exam Tip
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
