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NSW Admission Board Rules 2015
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Chapter-Chemical reactions and equationsTopic- Chemical equations and balancing a chemical equation.
Chemical Equation-The method of representing a chemical reaction with the help
of name,symbols and formulae of the substances involved is known as chemical
equation.
Chemical equations are of two types:
i)Word Equation-The reactant and product species involved in a particular
reaction are written as words.For example- When magnesium ribbon burns in
oxygen to form a white powder of magnesium oxide may be written as follows:
Magnesium + Oxygen → Magnesium oxide
( reactants) ( product)
ii) Symbol Equation- a brief representation of a chemical reaction in terms of
symbols and formulae of the substances involved.For example-burning of
magnesium in oxygen to form magnesium oxide may be written as follows:
Mg + O2 → MgO
Making equation more informative:
i)(s) for solid, (l) for liquid, (g) for gaseous state and (aq) for solutions prepared in water
ii) gas evolved as( ↑) and precipitate as (↓)
iii) If heat evolved(exothermic reaction)-+ heat on the product side and if heat aborbes(endothermic
reaction)- + heat on the reactant side
iv) Conditions under which the reaction takes place- on the arrow between reactants and products.
Unbalanced and Balanced equations-
When the number of same atoms on both sides of the chemical equation are equal,it is said to be
balanced.For example:
Mg + O2 → MgO (unbalanced equation)
2Mg + O2 → 2MgO (balanced equation)
Balancing of chemical equations-
i)Write the chemical equation in the form of a word equation.
ii)Convert the word equation into the symbol equation by writing the symbols and formulae of all
the reactants and products.
, iii)Make the atoms of different elements on both sides of the equation by a suitable method.This is
known as balancing of equation.
iv)Do not change the formulae of the substances while balancing of equation.
Let us take one example-
Magnesium + Oxygen → Magnesium oxide
( reactants) ( product)
Symbol equation for the above reaction is as follows:
Mg + O2 → MgO
Let us count the number of atoms of all the elements in the reactants and products on both sides of
the equation.
Element Number of Number of atoms of products
atoms of (R.H.S)
reactants
( L.H.S)
Mg 1 1
O 2 1
The number of magnesium atoms are equal on both sides but the number of oxygen atoms are not
equal.To have two oxygen atoms on the right side,we multiply MgO by 2 and write 2MgO and
again count the number of various atoms on both sides-
Mg + O2 → 2MgO
Element Number of Number of atoms of products
atoms of (R.H.S)
reactants
( L.H.S)
Mg 1 2
O 2 2
Though the number of oxygen atoms has become equal but the number of magnesium atoms has
now become unequal.To have 2 magnesium atoms on the left hand side,we multiply Mg by 2 on the
left hand side and write 2Mg and again count the number of various atoms on both sides.
2Mg + O2 → 2MgO
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