1. Chemical reactions
Wednesday 8th Nov

2. Reactants  Products Chemical reactions happen all the time.
They can be shown by a general equation:
Reactants  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.

3. What do the numbers mean?
SO2
1 sulphur
2 oxygen
MgCO3
1 magnesium
1 carbon
3 oxygen
The little number
tells us how many
atoms there are
of the element
it is behind

4. Identifying atoms Compound Element No of atoms Salt (NaCl) Sodium 1
Chlorine
Carbon dioxide (CO2)
Carbon
Oxygen
Water (H2O)
Hydrogen
Methane (CH4)
Hydrogen sulphide (H2S)
Sulphur
Calcium carbonate (CaCO3)
Calcium

5. Identifying atoms Compound Element No of atoms Salt (NaCl) Sodium 1
Chlorine
Carbon dioxide (CO2)
Carbon
Oxygen 2
Water (H2O)
Hydrogen
Methane (CH4)
Hydrogen sulphide (H2S)
Sulphur
Calcium carbonate (CaCO3)
Calcium

6. Identifying atoms Compound Element No of atoms Salt (NaCl) Sodium 1
Chlorine
Carbon dioxide (CO2)
Carbon
Oxygen 2
Water (H2O)
Hydrogen
Methane (CH4)
Hydrogen sulphide (H2S)
Sulphur
Calcium carbonate (CaCO3)
Calcium

7. Identifying atoms Compound Element No of atoms Salt (NaCl) Sodium 1
Chlorine
Carbon dioxide (CO2)
Carbon
Oxygen 2
Water (H2O)
Hydrogen
Methane (CH4) 4
Hydrogen sulphide (H2S)
Sulphur
Calcium carbonate (CaCO3)
Calcium

8. Identifying atoms Compound Element No of atoms Salt (NaCl) Sodium 1
Chlorine
Carbon dioxide (CO2)
Carbon
Oxygen 2
Water (H2O)
Hydrogen
Methane (CH4) 4
Hydrogen sulphide (H2S)
Sulphur
Calcium carbonate (CaCO3)
Calcium

9. Identifying atoms Compound Element No of atoms Salt (NaCl) Sodium 1
Chlorine
Carbon dioxide (CO2)
Carbon
Oxygen 2
Water (H2O)
Hydrogen
Methane (CH4) 4
Hydrogen sulphide (H2S)
Sulphur
Calcium carbonate (CaCO3)
Calcium 3

10. Now try these… 1. H2SO4 2. Ca(OH)2 3. NaCl 4. (NH3)3P04 5. 3H2O
How many atoms of each element are in the following molecules?
1. H2SO4
2. Ca(OH)2
3. NaCl
4. (NH3)3P04
5. 3H2O
6. Al2(CO3)3

11. 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.

12. What is balanced in a chemical equation?
Three things are balanced in a chemical equation:
1. Atoms
2. Mass
3. Charge

13. Now let's look at sodium - Na
But how do we do it? Na +
HCl 
NaCl H2
Start with an equation
Now let's look at sodium - Na

14. Now let's look at hydrogen- H
But how do we do it? Na +
HCl 
NaCl H2
1 x Na
Now let's look at hydrogen- H

15. Now let's look at chlorine - Cl
But how do we do it? Na +
HCl 
NaCl H2
1 x Na
1 x H
2 x H
Now let's look at chlorine - Cl

16. But how do we do it? Is it balanced? Are there the same number ofNa +
HCl 
NaCl H2
1 x Na
1 x H
2 x H
1 x Cl
Is it balanced?
Are there the
same number of
each atom on
both sides?

17. But how do we do it? NO! There aren't! Na + HCl  NaCl H2 1 x Na 1 x H
1 x Cl
NO! There aren't!

18. But how do we do it? What do we do? Na + HCl  NaCl H2 1 x Na 1 x H
1 x Cl
There are 2 H on the right and only 1 on the left… this isn’t balanced!
What do we do?

19. We need to add more Hydrogen on the left!
But how do we do it? Na +
HCl 
NaCl H2
1 x Na
1 x H
2 x H
1 x Cl
We need to add more Hydrogen on the left!

20. But how do we do it? 2 Na + HCl  NaCl H2 1 x Na 1 x H 2 x H 1 x Cl
Now 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

21. But how do we do it? 2 Na + HCl  NaCl H2 1 x Na 1 x H 2 x H 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. 2

22. But how do we do it? 2 2 Na + HCl  NaCl H2 1 x Na 1 x H 2 x H 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. 2 2

23. But how do we do it? 2 2 Na + HCl  NaCl H2 1 x Na 1 x H 2 x H 1 x Cl
This 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 2

24. But how do we do it? 2 2 2 Na + HCl  NaCl H2 1 x Na 1 x H 2 x H
1 x Cl
2 x Cl
2 x Na
This 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 2

25. Na +
HCl 
NaCl H2
UNBALANCED
2Na +
2HCl 
2NaCl H2
BALANCED

26. Balanced or unbalanced?

27. Remember the rules and 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

28. Try to balance these equations using the same method:
[1] Na + Cl2  NaCl
[2] CH4 + O2  CO2 + H2O
[3] Li + HNO3  LiNO3 + H2
[4] Al + O2  Al2O3

29. Here are the answers:
[1] 2 Na + Cl2  2 NaCl
[2] CH O2  CO H2O
[3] 2 Li + 2 HNO3  2 LiNO3 + H2
[4] 4 Al + 3 O2  2 Al2O3