Unit Chemical Reactions Writing Formula Equations Ws 1 Answer Key

Edexcel IGCSE Double Science: Chemistry

Revision Notes

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 thereactants andproducts of a chemical reaction using their full chemical names
The reactants are those substances on theleft-hand sideof the arrow and can be thought of as the chemical ingredientsof the reaction
They react with each other and form new substances
The products are the new substances which are on theright-hand side of the arrow
The arrow (which is spoken as "goes to" or "produces") implies theconversion of reactants into products
Reactionconditions or the name of acatalyst (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 thesame 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:

Using state symbols table, IGCSE & GCSE Chemistry revision notes

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

Balancing Equations WE1 1, downloadable IGCSE & GCSE Chemistry revision notes

Worked Example

Example 2

Balance the following equation:

magnesium oxide + nitric acid ⟶ magnesium nitrate + water

Unbalanced symbol equation:

MgO + HNO₃ ⟶ Mg(NO₃)₂ + H ₂ O

Answer

Balancing Equations WE2 1, downloadable IGCSE & GCSE Chemistry revision notes

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).

Author: Stewart

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