Equations (AQA GCSE Chemistry)
Revision Note
Author
StewartExpertise
Chemistry Lead
Word Equations
- These show the reactants and products of a chemical reaction using their full chemical names
- The reactants are those substances on the left-hand side of the arrow and can be thought of as the chemical ingredients of the reaction
- They react with each other and form new substances
- The products are the new substances which are on the right-hand side of the arrow
- The arrow (which is spoken as “goes to” or “produces”) implies the conversion of reactants into products
- Reaction conditions or the name of a catalyst (a substance added to make a reaction go faster) can be written above the arrow
- An example is the reaction of sodium hydroxide (a base) and hydrochloric acid producing sodium chloride (common table salt) and water:
- Sodium hydroxide + hydrochloric acid ⟶ sodium chloride + water
Symbol Equations
- A symbol equation uses the formulae of the reactants and products to show what happens in a chemical reaction
- A symbol equation must be balanced to give the correct ratio of reactants and products:
S + O2 → SO2
- This equation shows that one atom of sulfur (S) reacts with one molecule of oxygen (O2) to make one molecule of sulfur dioxide (SO2)
Nothing created - nothing destroyed
- New substances are made during chemical reactions
- However, the same atoms are always present before and after reaction
- They have just joined up in different ways
- Atoms cannot be created or destroyed, so if they exist in the reactants then they absolutely must be in the products!
- Because of this the total mass of reactants is always equal to the total mass of products
- This idea is known as the Law of Conservation of Mass
Balancing Equations
- The Law of Conservation of Mass enables us to balance chemical equations, since no atoms can be lost or created
- You should be able to:
- Write word equations for reactions outlined in these notes
- Write formulae and balanced chemical equations for the reactions in these notes
- 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
- During chemical reactions as 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
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)
Exam Tip
Chemical equations do not contain an equals sign between the left and right-hand sides but are written with an arrow instead. The arrow means that the reactants have reacted together and formed the product(s).
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