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Equations (AQA GCSE Chemistry: Combined Science)
Revision Note
How to write word equations
- Word equations show the reactants and products of a chemical reaction using their full chemical names
reactants → products
- The reactants are those substances on the left-hand side of the arrow
- They can be thought of as the chemical ingredients of the reaction
- They react with each other to form new substances, which are the products
- The products are on the right-hand side of the arrow
- The arrow (which is spoken as “to form” 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 to produce sodium chloride (common table salt) and water:
sodium hydroxide + hydrochloric acid ⟶ sodium chloride + water
Worked example
Word equations
- 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
- Ammonia reacts with nitric acid
- This question has all the information in the correct order
- 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 →
- Iron(II) hydroxide and sodium sulfate are formed
- Careful: This question has all the required information but the products are written first
- 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
- Carbon... ...burns in air
How to write balanced equations
How to write balanced equations
- A symbol equation uses the formulae of the reactants and products to show what happens in a chemical reaction
- When writing symbol equations, you should:
- Ensure reactants are on the left of the equation and products are on the right
- Write the following non-metals as molecules: H2, N2, O2, F2, Cl2, Br2 and I2
- Include state symbols
- Solid = (s)
- Liquid = (l)
- Gas = (g)
- Aqueous = (aq)
- You need to be confident using the state symbols (s), (l), (g) and (aq)
- You will not need to include them in all equations unless you are specifically asked to
- However, it is good practice to include state symbols in your equations so that you don't miss any marks
- A symbol equation must be balanced to give the correct ratio of reactants and products:
- For example, the combustion of sulfur:
S (s) + O2 (g)→ SO2 (g)
-
- This equation shows that one atom of sulfur, S, reacts with one molecule of oxygen, O2, to make one molecule of sulfur dioxide, SO2
Balancing equations
- When balancing equations, there must be the same number of atoms of each element on either side of the equation following the Law of Conservation of Mass
- 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 such as 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 balanced symbol / chemical equations include:
- 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
- The best approach is to practice lot of examples of balancing equations
- This can be by trial and error - changing the coefficients (numbers) in front of the formulae one by one and checking the result on the other side
- Balance elements that appear on their own, last in the process
Worked example
Symbol equations 1
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)
- Step 1 - 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
- Step 2 - 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
- Step 3 - 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
Symbol equations 2
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)
- Step 1 - writing 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
- Magnesium oxide, MgO, reacts with nitric acid, HNO3, it forms magnesium nitrate, Mg(NO3)2, and water
- Step 2 - 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 2 HNO3 will be needed as we cannot change the chemical formula
- MgO + 2HNO3 ⟶ Mg(NO3)2 + H2O
- This also balances the nitrate, NO3, groups
- Step 3 - 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 - remember to keep groups as a single entity if they are unchanged on both sides of the equation
- Product side:
- 1 Mg atom
- 2 NO3 groups - remember to 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
- Reactant side:
- The equation is now balanced
- The equation appears balanced so we need to check that it is
Examiner Tip
- 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
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