Word & Chemical Equations (Edexcel IGCSE Chemistry)
Revision Note
Author
StewartExpertise
Chemistry Lead
Writing Equations
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
Conservation of Mass
- 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
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 produces sodium chloride (common table salt) and water:
sodium hydroxide + hydrochloric acid ⟶ sodium chloride + water
Representing reactions as equations
- Chemical equations use the chemical symbols of each reactant and product
- When balancing equations, there has to be the same number of atoms of each element on either side of the equation in accordance with the Law of Conservation of Mass
- 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)
- The following non-metals must be written as molecules: H2, N2, O2, F2, Cl2, Br2 and I2
- To balance an equation you work across the equation from left to right, checking one element after another
- If there is a group of atoms, for example a nitrate group (NO3–) that has not changed from one side to the other, then count the whole group as one entity rather than counting the individual atoms
- Examples of chemical equations:
- Acid-base neutralisation reaction:
NaOH (aq) + HCl (aq) ⟶ NaCl (aq) + H2O (l)
- Redox reaction:
2Fe2O3 (aq) + 3C (s) ⟶ 4Fe (s) + 3CO2 (g)
- In each equation there are equal numbers of each atom on either side of the reaction arrow so the equations are balanced
- Don't forget to add state symbols when writing balanced equations:
Balancing Equations
The best approach is to practice lot of examples of balancing equations
- By trial and error change the coefficients (multipliers) in front of the formulae, one by one checking the result on the other side
- Balance elements that appear on their own, last in the process
Worked example
Example 1:
Balance the following equation:
aluminium + copper(II)oxide ⟶ aluminium oxide + copper
Unbalanced symbol equation:
Al + CuO ⟶ Al2O3 + Cu
Answer:
Worked example
Example 2:
Balance the following equation:
magnesium oxide + nitric acid ⟶ magnesium nitrate + water
Unbalanced symbol equation:
MgO + HNO3 ⟶ Mg(NO3)2 + H2O
Answer:
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).
Writing ionic equations
- In aqueous solutions ionic compounds dissociate into their ions, meaning they separate into the component ions that formed them, e.g. hydrochloric acid and potassium hydroxide dissociate as follows:
HCl (aq) → H+ (aq) + Cl-(aq)
KOH (aq) → K+ (aq) + OH- (aq)
- It is important that you can recognise common ionic compounds and their constituent ions
- These include:
- Acids such as HCl and H2SO4
- Group I and Group II hydroxides e.g. sodium hydroxide
- Soluble salts e.g. potassium sulfate, sodium chloride
- Follow the example below to write ionic equations
Worked example
Write the ionic equation for the reaction of aqueous chlorine and aqueous potassium iodide.
Answer:
- Step 1: Write out the full balanced equation:
2KI (aq) + Cl2 (aq) → 2KCl (aq) + I2 (aq)
- Step 2: Identify the ionic substances and write down the ions separately
2K+ (aq) + 2I- (aq) + Cl2 (aq) → 2K+ (aq) + 2Cl- (aq) + I2 (aq)
- Step 3: Rewrite the equation eliminating the ions which appear on both sides of the equation (spectator ions ) which in this case are the K+ ions:
2I- (aq) + Cl2 (aq) → 2Cl- (aq) + I2 (aq)
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