balanced equations

Chemical reactions

Wednesday 8th Nov

• Chemical reactions happen all the time.

• They can be shown by a general equation:

Reactants Reactants Products Products• The same amount of chemicals must

be present on both sides of the reaction.

• The reactants are the chemicals that react together.

• The products are the new chemicals that are made.

What do the numbers mean?

• SO2– 1 sulphur– 2 oxygen

• MgCO3– 1 magnesium– 1 carbon– 3 oxygen

Identifying atomsCompound Element No of

atoms Element No of atoms

Salt (NaCl) Sodium 1 Chlorine 1

Carbon dioxide (CO2) Carbon Oxygen

Water (H2O) Hydrogen Oxygen

Methane (CH4) Carbon Hydrogen

Hydrogen sulphide (H2S)

Hydrogen Sulphur

Calcium carbonate (CaCO3)

Calcium Carbon

Oxygen




Carbon dioxide (CO2) Carbon 1 Oxygen 2

Water (H2O) Hydrogen Oxygen



Hydrogen Sulphur


Calcium Carbon

Oxygen





Water (H2O) Hydrogen 2 Oxygen 1



Hydrogen Sulphur


Calcium Carbon

Oxygen






Methane (CH4) Carbon 1 Hydrogen 4


Hydrogen Sulphur


Calcium Carbon

Oxygen








Hydrogen 2 Sulphur 1


Calcium Carbon

Oxygen








Hydrogen Sulphur


Calcium 1 Carbon 1

Oxygen 3

Now try these…• How many atoms of each element are in

the following molecules?1. H2SO4

2. Ca(OH)2

3. NaCl4. (NH3)3P04

5. 3H2O6. Al2(CO3)3

Representing reactions• When reactions occur the products

are turned into reactants.• This is written as a balanced

equation.• We also might need to be able to

draw this as the molecules, and in more detail as the atoms involved.

What is balanced in a chemical equation?

Three things are balanced in a chemical equation:1. Atoms2. Mass3. Charge

But how do we do it?Na + HCl NaCl + H2


1 x Na 1 x Na


1 x Na 1 x Na1 x H 2 x H


1 x Na 1 x Na1 x H 2 x H1 x Cl 1 x Cl



There are 2 H on the right and only 1 on the left… this isn’t balanced!



We need to add more Hydrogen on the left!



2 x H 2 x HNow we need to add a big number 2 in front of Hydrogen, as

we can’t change the compound only how many of the compound there are…



2 x H 2 x H2 x Cl 1 x Cl

This also means there are now 2 Cl, so we need 2 Cl on the right! Again we have to add a big number 2.




This also means there are now 2 Cl, so we need 2 Cl on the right! Again we have to add a big number 2.




1 x Na 2 x NaThis also means there are now 2 Na, so we need 2 Na on the

left! Again we have to add a big number 2.




2 x Na 2 x NaThis also means there are now 2 Na, so we need 2 Na on the

left! Again we have to add a big number 2.

Na + HCl NaCl + H2

2Na + 2HCl 2NaCl + H2

Balanced or unbalanced?

Remember the rulesand it will work like magic!

• A small number just talks about the element it is behind. E.g. O2 = 2 x O

• Small numbers outside the bracket mean multiply everything inside the bracket by that number E.g. (CO3)3 = 3 x C, 9 x O

•Big numbers mean multiply everything behind by it. E.g. 2 H2O = 4 x H, 2 x O

Try to balance these equations using the same method:

[1] Na + Cl2 NaCl[2] CH4 + O2 CO2 + H2O

[4] Al + O2 Al2O3

[3] Li + HNO3 LiNO3 + H2

[1] 2 Na + Cl2 2 NaCl[2] CH4 + 2 O2 CO2 + 2 H2O[4] 4 Al + 3 O2 2 Al2O3

[3] 2 Li + 2 HNO3 2 LiNO3 + H2

Here are the answers:

balanced equations

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