Balanced Equations
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Transcript of 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
Identifying atomsCompound Element No of
atoms Element No of atoms
Salt (NaCl) Sodium 1 Chlorine 1
Carbon dioxide (CO2) Carbon 1 Oxygen 2
Water (H2O) Hydrogen Oxygen
Methane (CH4) Carbon Hydrogen
Hydrogen sulphide (H2S)
Hydrogen Sulphur
Calcium carbonate (CaCO3)
Calcium Carbon
Oxygen
Identifying atomsCompound Element No of
atoms Element No of atoms
Salt (NaCl) Sodium 1 Chlorine 1
Carbon dioxide (CO2) Carbon 1 Oxygen 2
Water (H2O) Hydrogen 2 Oxygen 1
Methane (CH4) Carbon Hydrogen
Hydrogen sulphide (H2S)
Hydrogen Sulphur
Calcium carbonate (CaCO3)
Calcium Carbon
Oxygen
Identifying atomsCompound Element No of
atoms Element No of atoms
Salt (NaCl) Sodium 1 Chlorine 1
Carbon dioxide (CO2) Carbon 1 Oxygen 2
Water (H2O) Hydrogen 2 Oxygen 1
Methane (CH4) Carbon 1 Hydrogen 4
Hydrogen sulphide (H2S)
Hydrogen Sulphur
Calcium carbonate (CaCO3)
Calcium Carbon
Oxygen
Identifying atomsCompound Element No of
atoms Element No of atoms
Salt (NaCl) Sodium 1 Chlorine 1
Carbon dioxide (CO2) Carbon 1 Oxygen 2
Water (H2O) Hydrogen 2 Oxygen 1
Methane (CH4) Carbon 1 Hydrogen 4
Hydrogen sulphide (H2S)
Hydrogen 2 Sulphur 1
Calcium carbonate (CaCO3)
Calcium Carbon
Oxygen
Identifying atomsCompound Element No of
atoms Element No of atoms
Salt (NaCl) Sodium 1 Chlorine 1
Carbon dioxide (CO2) Carbon 1 Oxygen 2
Water (H2O) Hydrogen 2 Oxygen 1
Methane (CH4) Carbon 1 Hydrogen 4
Hydrogen sulphide (H2S)
Hydrogen Sulphur
Calcium carbonate (CaCO3)
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
But how do we do it?Na + HCl NaCl + H2
1 x Na 1 x Na
But how do we do it?Na + HCl NaCl + H2
1 x Na 1 x Na1 x H 2 x H
But how do we do it?Na + HCl NaCl + H2
1 x Na 1 x Na1 x H 2 x H1 x Cl 1 x Cl
But how do we do it?Na + HCl NaCl + H2
1 x Na 1 x Na1 x H 2 x H1 x Cl 1 x Cl
But how do we do it?Na + HCl NaCl + H2
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!
But how do we do it?Na + HCl NaCl + H2
1 x Na 1 x Na1 x H 2 x H1 x Cl 1 x Cl
We need to add more Hydrogen on the left!
But how do we do it?Na + HCl NaCl + H2
1 x Na 1 x Na1 x H 2 x H1 x Cl 1 x Cl
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…
But how do we do it?Na + HCl NaCl + H2
1 x Na 1 x Na1 x H 2 x H1 x Cl 1 x Cl
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.
But how do we do it?Na + HCl NaCl + H2
1 x Na 1 x Na1 x H 2 x H1 x Cl 1 x Cl
2 x H 2 x H2 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 + HCl NaCl + H2
1 x Na 1 x Na1 x H 2 x H1 x Cl 1 x Cl
2 x H 2 x H2 x Cl 2 x Cl
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.
But how do we do it?Na + HCl NaCl + H2
1 x Na 1 x Na1 x H 2 x H1 x Cl 1 x Cl
2 x H 2 x H2 x Cl 2 x Cl
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: