Chemical Equations (AQA GCSE Combined Science: Synergy: Physical Sciences): Revision Note
Exam code: 8465
Symbols and formulae
Elements
All substances are made from atoms
An element is a pure substance containing only one type of atom
There are about 100 different elements
Each element is represented by a chemical symbol, consisting of one or two letters
The first letter is always a capital
The second (if there is one) is always lowercase
For example:
O represents oxygen
Na represents sodium
Cl represents chlorine
All elements are arranged in the periodic table
Compounds
Compounds are formed when two or more elements chemically combine during a reaction
Atoms of different elements are joined together in fixed proportions
Compounds can be represented by formulae using the symbols of the atoms they contain
Compounds can only be separated back into elements by chemical reactions
They cannot be separated by physical methods like filtration or evaporation
Writing formulae
The formula of a compound shows which elements it contains and the ratio of atoms
Subscript numbers show how many atoms of each element are in one formula unit
Chemical | Formula | Elements present | Ratio of atoms |
|---|---|---|---|
Water | H2O | Hydrogen, oxygen | 2 hydrogen : 1 oxygen |
Carbon dioxide | CO2 | Carbon, oxygen | 1 carbon : 2 oxygen |
Sodium chloride | NaCl | Sodium, chlorine | 1 sodium : 1 chlorine |
Sulfuric acid | H2SO4 | Hydrogen, sulfur, oxygen | 2 hydrogen : 1 sulfur : 4 oxygen |
Subscript numbers after a bracket belong to everything inside the bracket
For example:
Ca(NO3)2
Outside the brackets:
1 calcium atom
Inside the brackets:
1 nitrogen and 3 oxygen
The brackets have a subscript 2 after them
This means that the contents of the bracket are doubled:
(1 nitrogen and 3 oxygen) x 2 = 2 nitrogen and 6 oxygen
Examiner Tips and Tricks
The symbols for elements always start with a capital letter.
Never write sodium as "NA" or cobalt as "CO". CO is carbon monoxide, which is a completely different substance.
You need to know the names and symbols of the first 20 elements, the elements in Groups 1, 7 and 0, and other common elements using a supplied periodic table.
Chemical reactions
Chemical reactions always involve the formation of one or more new substances
They often involve a detectable energy change (temperature rise or fall, light produced, etc.)
Reactions can be summarised using word equations
Word equations have:
Reactants written on the left
Products written on the right
An arrow to separate the reactants and products
magnesium + oxygen → magnesium oxide
Worked Example
Ammonia reacts with nitric acid to form the fertiliser ammonium nitrate. Write a word equation for the reaction taking place.
Iron(II) hydroxide and sodium sulfate are formed when iron(II) sulfate solution and sodium hydroxide react together. Write a word equation for the reaction taking place.
Carbon is the main element found in coal and burns in air to produce carbon dioxide. Write a word equation for the reaction taking place.
Answers:
Ammonia + nitric acid → ammonium nitrate
This question has all the information in the correct order
Ammonia reacts with nitric acid
This becomes ammonia + nitric acid
to form
This is the arrow in the equation
to form the fertiliser ammonium nitrate
This tells you that the product is ammonium nitrate
Iron(II) sulfate + sodium hydroxide → iron(II) hydroxide + sodium sulfate
Careful: This question has all the required information but the products are written first
Iron(II) hydroxide and sodium sulfate are formed
This becomes → iron(II) hydroxide + sodium sulfate
when iron(II) sulfate solution and sodium hydroxide react together
This becomes Iron(II) sulfate + sodium hydroxide →
Carbon + oxygen → carbon dioxide
Careful: Not all of the required information is given in the question
You are expected to know that burning in air means that the chemical is reacting with oxygen
Carbon... ...burns in air
This becomes carbon + oxygen
to produce
This is the arrow in the equation
to produce carbon dioxide
This tells you that the product is carbon dioxide
Balanced symbol equations
Symbol equations use the formulae of reactants and products instead of names
Symbol equations must be balanced
This means that the number of each type of atom must be equal on both sides
Atoms are neither created nor destroyed in a chemical reaction
This is the conservation of mass
Numbers, or coefficients, are used to balance equations
These are placed in front of the chemical formula
How to balance an equation
Balance the equation for hydrogen reacting with oxygen to form water:
Step 1: write the unbalanced equation
H2 + O2 → H2O
Step 2: count atoms on each side
Atom | Left | Right | Balanced |
|---|---|---|---|
H | 2 | 2 | Yes |
O | 2 | 1 | No |
Step 3: add coefficients to balance
2H2 + O2 → H2O
Step 4: recheck
Atom | Left | Right | Balanced |
|---|---|---|---|
H | 4 | 4 | Yes |
O | 2 | 2 | Yes |
Worked Example
Aluminium reacts with copper(II) oxide to produce aluminium oxide and copper. Balance the symbol equation for the reaction taking place.
Al (s) + CuO (s) ⟶ Al2O3 (s) + Cu (s)
Answer:
The balanced symbol equation is:
2Al (s) + 3CuO (s) ⟶ Al2O3 (s) + 3Cu (s)
Balancing aluminium atoms
There are 2 aluminium atoms on the product side, so 2 aluminium atoms are needed on the reactant side
2Al + CuO ⟶ Al2O3 + _Cu
Balancing oxygen atoms
There are 3 oxygen atoms on the product side, so 3 oxygen atoms are needed on the reactant side
This means that 3 CuO will be needed as we cannot change the chemical formula
2Al + 3CuO ⟶ Al2O3 + Cu
Balancing copper atoms
There are 3 copper atoms on the reactant side, so 3 copper atoms are needed on the product side
2Al + 3CuO ⟶ _Al2O3 + 3Cu
The equation is now balanced
Worked Example
When magnesium oxide, MgO, reacts with nitric acid, HNO3, it forms magnesium nitrate, Mg(NO3)2, and water.
Write a symbol equation for this reaction.
Answer:
The balanced symbol equation is:
MgO (s) + 2HNO3 (aq) ⟶ Mg(NO3)2 (aq) + H2O (l)
Write the unbalanced equation
Magnesium oxide, MgO, reacts with nitric acid, HNO3, it forms magnesium nitrate, Mg(NO3)2, and water
MgO + HNO3 ⟶ Mg(NO3)2 + H2O
The Mg and O atoms (not including the O in the NO3 group appear to be balanced), so we should focus on the H atoms and NO3 groups
Balancing hydrogen atoms
There are 2 hydrogen atoms on the product side, so 2 hydrogen atoms are needed on the reactant side
This means that 2HNO3will be needed as we cannot change the chemical formula
MgO + 2HNO3 ⟶ Mg(NO3)2 + H2O
This also balances the nitrate, NO3, groups
Checking the equation
The equation appears balanced so we need to check that it is
Reactant side:
1 Mg atom
1 O atom - not including those in the NO3 group
2 H atoms
2 NO3 groups - keep groups as a single entity if they are unchanged on both sides of the equation
Product side:
1 Mg atom
2 NO3 groups - keep groups as a single entity if they are unchanged on both sides of the equation
2 H atoms
1 O atom - not including those in the NO3 group
The equation is now balanced
Examiner Tips and Tricks
Careful: A common mistake when balancing symbol equations is to add, change or remove small numbers in the chemical formula of a substance
You cannot do this because it changes what the substance is
For example, if a product was water, H2O, and you added a second oxygen to make it H2O2 then it is no longer water
If you struggle to balance an equation, you can sometimes get a mark for writing the correct chemical formulae and leaving the equation unbalanced
State symbols
State symbols are added to equations to show the physical state of each substance
Symbol | State |
|---|---|
(s) | solid |
(l) | liquid |
(g) | gas |
(aq) | aqueous solution (dissolved in water) |
For example, when solid sodium is added to water and forms sodium hydroxide solution and hydrogen gas:
2Na (s) + 2H2O (l) → 2NaOH (aq) + H2 (g)
Examiner Tips and Tricks
State symbols are normally worth a mark, so you should always include them!
Half equations and ionic equations
Higher Tier Only
Ionic equations
Ionic equations show only the ions that actually change during a reaction
Ions that do not change are called spectator ions
Spectator ions are left out of ionic equations
For example, the halogen displacment reaction of chlorine with potassium bromide:
Cl2 (aq) + 2KBr (aq) → 2KCl (aq) + Br2 (aq)
Written out to show all the ions:
Cl2 (aq) + 2K+ (aq) + 2Br- (aq) → 2K+ (aq) + 2Cl- (aq) + Br2 (aq)
Both sides have K+ ions as spectator ions
So they are removed to give the ionic equation:
Cl2 (aq) + 2Br- (aq) → 2Cl- (aq) + Br2 (aq)
Examiner Tips and Tricks
For ionic equations, cancel any spectator ions. Leaving these in will typically lose a mark.
Half equations
Half equations show either the oxidation or the reduction step of a redox reaction separately
Electrons are shown as e⁻
For example, bromide ions lose electrons (oxidation) to form bromine:
2Br⁻ → Br2 + 2e⁻
For example, chlorine gains electrons (reduction) to form chloride ions:
Cl2 + 2e⁻ → 2Cl⁻
The number of electrons must balance between the two half equations
Examiner Tips and Tricks
To balance ionic and half equations, focus on the atoms first.
ABC - Atoms before charge
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