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What is a chemical equation?

It is an equation used to describe what happensduring a chemical reaction.

The equation identifies the reactants (on the left ofthe equation) and products (on the right of theequation) of the reaction.

The chemicals can be represented by their names orby their chemical symbols.

What are reactants? Chemicals that are allowed to react (starting

materials)

Placed on left side of the equation

What are products? Chemicals produced in the reaction (resulting

substances)

Placed on right side of the equation

Step 1: Identify the reactants and products towrite the word equation.

Example: Formation of water

Reactants:

Product:

Oxygen and Hydrogen

Water

+LHS RHS

WordEquation

Step 2: Write the chemical formula under eachreactant and product.

Example: Formation of water

Oxygen + Hydrogen WaterO2 + H2 H2O

Step 3: Check if the equation is balanced.

Example: Formation of water

Oxygen + Hydrogen WaterO2 + H2 H2O

LHSNo. of O = 2No. of H = 2

RHSNo. of O = 1No. of H = 2

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Step 4: If it is not balanced, place coefficientsin front of each formula so that they are

balanced. (Recheck that the atoms are now balanced)

Example: Formation of water

Oxygen + Hydrogen WaterO2 + H2 H2O

LHSNo. of O = 2No. of H = 4

RHSNo. of O = 2No. of H = 4

2 2

Balanced!

The number of atoms of each element onboth sides of the equation must be the same.

Why must we balance the equation?

Law of conservation of matter Matter is neither lost nor gained in traditional

chemical reactions; it simply changes form

the number of atoms of an element in the LHSmust be the same as the RHS

implies that mass is also conserved during achemical reaction

Thus, theres a need to balance theequation since total mass isconserved!

Is this correct?

Example: Formation of water

Oxygen + Hydrogen WaterO2 + H2 H2O

LHSNo. of O = 2No. of H = 2

RHSNo. of O = 2No. of H = 2

2 Does thisstillrepresentwater?

Do not change the subscript of the formula as

it becomes a different compound!

Only change the number in front of theelement/compound. (Changing the ratio)

O2 + 2H2 2H2O

O2 + H2 H2O2

Step 5: Use standard abbreviations to write thephysical state next to each reactant andproduct.

(s)=solid, (l)=liquid, (g)=gaseous,(aq) =aqueous or dissolved in water

Example: Formation of water

Oxygen + Hydrogen WaterO2 + 2H2 2H2O(g) (g) (l)

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1) Formation of sodium chloride (table salt)

Na + Cl2 NaCl

2) Thermal Decomposition of silver carbonate

AgCO3 Ag + CO2 + O2

Step 1: Identify the reactants and products towrite the word equation.

Step 2: Write the chemical formula undereach reactant and product.

Step 3: Check if the equation is balanced. Step 4: If it is not balanced, place coefficients

in front of each formula so that they arebalanced. (Recheck that the atoms are now balanced)

All metals are denoted by their chemicalsymbol only (ie no subscripts)

eg Zn, Fe, Al, Mg, Na

All elements in Group VII are diatomic gasesthus the formula is X2.

eg F2, Cl2, Br2.

Other diatomic molecules : O2, N2, H2

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The formula of other molecules depend ontheir chemical names. eg sulfur dioxide,carbon monoxide, hydrogen sulfide

Common alkalis : NaOH , Ca(OH)2

Common acids : HCl, H2SO4, HNO3

Common names (used instead of chemicalnames): water , ammonia, methane

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Name Chemical formula

Sulfuric acid H2SO4

Hydrochloric acid HClNitric acid HNO

3

Carbonic acid H2CO

3

Sulfurous acid H2SO3

Nitrous acid HNO2

Phosphoric acid H3PO4

Phosphorus acid H3PO3

Acetic acid CH3COOH

Other typesof acids foryour generalknowledge

Mustknow foryear 2Chemistry

1) Magnesium metal burns in chlorine gas

to form magnesium chloride

2) Combustion of methane Methane gas (CH4) burns in oxygen to give carbon

dioxide and water

3) Iron (II) oxide (s) + carbon monoxide (g) iron (l) + carbon dioxide (g)

4) Aluminium + oxygen gas aluminiumoxide