Conservation of Mass & Balanced Chemical Equations (AQA GCSE Chemistry)
Revision Note
The Law of Conservation of Mass
The Law of Conservation of Mass states that no matter is lost or gained during a chemical reaction.
Mass is always conserved, therefore the total mass of the reactants is equal to the total mass of the products, which is why all chemical equations must be balanced
The sum of the relative atomic/molecular masses of the reactants will be the same as the sum of the relative atomic/molecular masses of the products
A precipitation reaction is one in which two solutions react to form an insoluble solid called a precipitate
If the reaction flask is closed and no other substance can enter or leave the system, then the total mass of the reaction flask will remain constant
For example, the reaction between calcium chloride and sodium sulfate produces a precipitate of calcium sulfate.
If carried out in a closed system then the mass before and after the reaction will be the same
The balanced equation is:
CaCl2 (aq) + Na2SO4 (aq) ⟶ CaSO4 (s) + 2NaCl (aq)
Diagram showing the conservation of mass in a precipitation reaction
If the reaction flask is open and a gaseous product is allowed to escape, then the total mass of the reaction flask will change as product mass is lost when the gas leaves the system
For example, the reaction between hydrochloric acid and calcium carbonate produces carbon dioxide gas:
2HCl (aq) + CaCO3 (s) ⟶ CaCl2 (aq) + H2O (l) + CO2 (g)
Mass will be lost from the reaction flask unless it is closed
If the mass of a reaction flask is found to increase then it may be due to one of the reactants being a gas found in the air and all of the products are either solids or liquids
Examiner Tips and Tricks
Matter cannot be created or destroyed, so the total amount of matter before and after a reaction is the same. What changes is the chemical and physical properties of the reactants as they transform into products.
Representing reactions as equations
The numbers involved in chemical formulae and equations give a lot of information about the chemicals involved
In chemical formulae:
If there is no subscript number after an element, then there must be one of that particular element
e.g. CO contains one carbon and one oxygen atom
If there is a subscript number after an element, then that number belongs to the element just before it
e.g. CO2 contains one carbon atom and two oxygen atoms
If there is a subscript number after brackets, then that number belongs to all of the elements inside the bracket
e.g. Ca(OH)2 contains one calcium atom, two oxygen atoms AND two hydrogen atoms
The most complicated examples contain a subscript number inside the bracket as well as outside,
e.g. Ca(NO3)2 contains one calcium atom
Inside the bracket, there is one nitrogen atom and three oxygen atoms but the subscript 2 outside the bracket applies to the nitrogen and oxygen inside the bracket
This means that there are two nitrogen atoms and six oxygen atoms
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.
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.
Examiner Tips and Tricks
A large number before any chemical applies to that entire chemical
The last equation above starts with 2Fe2O3
The Fe2O3 suggests that there are two iron atoms and three oxygen atome
The large 2 in front applies to the whole Fe2O3, i.e. 2 x Fe2O3
Therefore, there are four iron atoms and six oxygen atoms involved in the reaction
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
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
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
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 are not confident balancing symbol equations, draw them out
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