chemical reactions. chemical reactions happen all the time. they can be shown by a general equation:...
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Chemical reactions
• 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 atoms
Compound ElementNo of
atomsElement
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 atoms
Compound ElementNo of
atomsElement
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 atoms
Compound ElementNo of
atomsElement
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 atoms
Compound ElementNo of
atomsElement
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 atoms
Compound ElementNo of
atomsElement
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 atoms
Compound ElementNo of
atomsElement
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 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. 3H2O
6.Al2(CO3)3
See CH10Mod03PP for answers
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. ChargeMore on this in Module 3