Writing Chemical Equations
Chemical equation
· The chemist’s shorthand for describing the course of chemical reaction.
Basic terms and symbols:
1. Reactants – the substance that undergoes change in a chemical reaction. They are always written on the left
2. Products – the substance produced in a chemical reaction. They are always written on the right.
3. (g) – for gas
4. (l) – for liquid
5. (S) – for solid substances
6. (aq) – for a substance in water solution
Heat
· Mercury (III) oxide ---------> mercury and oxygen
HgO ---------> Hg + O2
Chemical Reaction is the process by which chemical change takes place.
Types of Chemical Reactions
1. Syntheses/ Combination reaction
A + B ---------> AB
Examples:
CuS + S ---------> CuS
MgO + H2O -------> Mg(OH)2
2. Analysis/ Decomposition reaction
AB ---------> A + B
Examples:
H2O ---------> H2 + O2
KClO3 ---------> KCl + O2
3. Single replacement/ substitution
AX + B ---------> BX + A
AX + Y ---------> AY + X
Examples:
KI + Cl2 ---------> KCl + I2
Mg + H2SO4 ---------> MgSO4 + H2
4. Double Replacement / metathesis
AX + BY ---------> AY + BX
Examples:
AgNO3 + FeCl3 ---------> AgCl + Fe (NO3)3
Balancing Equations:
Consider the following equation:
N2 + H2 ---------> NH3
Steps:
1. Count the number of atoms of each element on the reactant side and product side.
Element Reactant Product
N 2 1
H 2 3
2. Find out which atoms are unbalanced.
3. Now balance each element at a time. Do this by placing a small whole number called coefficient in front of symbols or formulas.
N2 + 3H2 ---------> 2NH3
4. Check if the equation is completely balanced.
Element Reactant Product
N 2 2
H 6 6
Exercises:
Classify the following equations ant then balance.
1. C3H8 + O2 ---------> CO2 + H2O
2. Ca (ClO3)2 ---------> CaCl2 + O2
3. FeCl3 + Ca (OH)2 ---------> CaCl2 + Fe (OH)3
4. Hg (NO3)2 +Mg ---------> Mg (NO3)2 + Hg
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