Download Chemical reactions

Chemical reactions
Wednesday 8th Nov
• 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.
What do the numbers mean?
• SO2
– 1 sulphur
– 2 oxygen
• MgCO3
– 1 magnesium
– 1 carbon
– 3 oxygen
Identifying atoms
Compound
Salt (NaCl)
Element
Sodium
No of
atoms
1
Element
Chlorine
Carbon dioxide (CO2) Carbon
Water (H2O)
Hydrogen
Oxygen
Oxygen
Methane (CH4)
Hydrogen sulphide
(H2S)
Calcium carbonate
(CaCO3)
Carbon
Hydrogen
Hydrogen
Sulphur
Calcium
Carbon
Oxygen
No of
atoms
1
Identifying atoms
Compound
Salt (NaCl)
Element
Sodium
Carbon dioxide (CO2) Carbon
Water (H2O)
Hydrogen
Methane (CH4)
Hydrogen sulphide
(H2S)
Calcium carbonate
(CaCO3)
No of
atoms
Element
1
Chlorine
1
1
Oxygen
Oxygen
2
Carbon
Hydrogen
Hydrogen
Sulphur
Calcium
Carbon
Oxygen
No of
atoms
Identifying atoms
Compound
Salt (NaCl)
Element
Sodium
Carbon dioxide (CO2) Carbon
Water (H2O)
Hydrogen
Methane (CH4)
Hydrogen sulphide
(H2S)
Calcium carbonate
(CaCO3)
No of
atoms
Element
1
Chlorine
1
1
2
Oxygen
Oxygen
2
1
Carbon
Hydrogen
Hydrogen
Sulphur
Calcium
Carbon
Oxygen
No of
atoms
Identifying atoms
Compound
Salt (NaCl)
Element
Sodium
No of
atoms
Element
No of
atoms
1
Chlorine
1
Carbon dioxide (CO2) Carbon
Water (H2O)
Hydrogen
1
2
Oxygen
Oxygen
2
1
Methane (CH4)
Hydrogen sulphide
(H2S)
Calcium carbonate
(CaCO3)
1
Hydrogen
Sulphur
4
Carbon
Hydrogen
Calcium
Oxygen
Carbon
Identifying atoms
Compound
Salt (NaCl)
Element
Sodium
No of
atoms
Element
No of
atoms
1
Chlorine
1
Carbon dioxide (CO2) Carbon
Water (H2O)
Hydrogen
1
2
Oxygen
Oxygen
2
1
Methane (CH4)
Hydrogen sulphide
(H2S)
Calcium carbonate
(CaCO3)
1
2
Hydrogen
Sulphur
4
1
Carbon
Hydrogen
Calcium
Oxygen
Carbon
Identifying atoms
Compound
Salt (NaCl)
Element
Sodium
No of
atoms
Element
No of
atoms
1
Chlorine
1
Carbon dioxide (CO2) Carbon
Water (H2O)
Hydrogen
1
2
Oxygen
Oxygen
2
1
Methane (CH4)
Hydrogen sulphide
(H2S)
Calcium carbonate
(CaCO3)
Carbon
Hydrogen
1
Hydrogen
Sulphur
4
Calcium
1
Carbon
1
Oxygen
3
Now try these…
• How many atoms of each element are in the
following molecules?
1.
2.
3.
4.
5.
6.
H2SO4
Ca(OH)2
NaCl
(NH3)3P04
3H2O
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. Atoms
2. Mass
3. Charge
But how do we do it?
Na
+
HCl

NaCl
+
H2
But how do we do it?
Na
1 x Na
+
HCl

NaCl
1 x Na
+
H2
But how do we do it?
Na
1 x Na
+
HCl
1xH

NaCl
1 x Na
+
H2
2xH
But how do we do it?
Na
1 x Na
+
HCl
1xH
1 x Cl

NaCl
1 x Na
+
H2
2xH
1 x Cl
But how do we do it?
Na
1 x Na
+
HCl
1xH
1 x Cl

NaCl
1 x Na
+
H2
2xH
1 x Cl
But how do we do it?
Na
1 x Na
+
HCl

NaCl
1 x Na
+
H2
1xH
2xH
1 x Cl
1 x Cl
There are 2 H on the right and only 1 on the left… this isn’t
balanced!
But how do we do it?
Na
1 x Na
+
HCl

NaCl
1 x Na
+
1xH
1 x Cl
1 x Cl
We need to add more Hydrogen on the left!
H2
2xH
But how do we do it?
Na
1 x Na
+
HCl
1xH
1 x Cl

NaCl
1 x Na
+
H2
2xH
1 x Cl
2xH
2xH
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…
But how do we do it?
Na
1 x Na
+
HCl
1xH
1 x Cl

NaCl
1 x Na
+
H2
2xH
1 x Cl
2xH
2xH
2 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.
But how do we do it?
Na
1 x Na
+
HCl
1xH
1 x Cl

NaCl
1 x Na
+
H2
2xH
1 x Cl
2xH
2xH
2 x Cl
2 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.
But how do we do it?
Na
1 x Na
+
HCl
1xH
1 x Cl
2xH
2 x Cl

NaCl
1 x Na
+
H2
2xH
1 x Cl
2xH
2 x Cl
1 x Na
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.
But how do we do it?
Na
1 x Na
+
HCl
1xH
1 x Cl
2xH
2 x Cl

NaCl
1 x Na
+
H2
2xH
1 x Cl
2xH
2 x Cl
2 x Na
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.
Na
+
HCl

NaCl
+
2Na
+
2HCl

2NaCl
+
H2
H2
Balanced or unbalanced?
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
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
Here are the answers:
[1] 2 Na + Cl2  2 NaCl
[2] CH4 + 2 O2  CO2 + 2 H2O
[3] 2 Li + 2 HNO3  2 LiNO3 + H2
[4] 4 Al + 3 O2  2 Al2O3