CHEMICAL REACTION
AND EQUATION
CHANGE : A group of events or processes where final state differ from intial state is called Change
Feature Physical Change Chemical Change
New substance formed No Yes
Composition change No Yes
Reversibility Usually reversible (e.g., melt Usually irreversible (e.g.,
ice) burn paper)
Energy involved Little or none Heat, light, sound often
produced
Examples Boiling water, cutting paper Rusting iron, baking cake
CHEMICAL REACTION : A chemical reaction involves the transformation of one or more substances,
called reactants, into different substances known as products through the breaking and forming
of chemical bonds.
CHEMICAL EQUATION : The symbolic representation of a chemical reaction is called a chemical equation.
, CHARACTERISTICS OF CHEMICAL REACTION
Evolution of Gas: Bubbles form, like CO₂ from Na₂CO₃ + HCl → NaCl + H₂O + CO₂.
Precipitate Formation: Insoluble solid appears, e.g., AgNO₃ + NaCl → AgCl↓ + NaNO₃ (white ppt).
Color Change: Reactants alter hue, such as purple KMnO₄ turning colorless with citric acid.
Temperature Change: Exothermic (heat released, e.g., quicklime + water) or endothermic (heat absorbed)
State Change: Solid to gas/liquid, like candle wax burning: CₓHᵧ + O₂ → CO₂ + H₂O.
BALANCING OF CHEMICAL REACTION Balancing chemical equations means adjusting coefficients to ensure
equal numbers of each atom on reactant and product sides.
Step-by-Step Method
Start with the unbalanced equation and balance elements systematically:
Balance metals or less common (non metal ) elements first.
Balance oxygen and hydrogen last.
Use whole numbers; multiply all coefficients if fractions appear to clear them.
Here are 10 unbalanced equations ; balance them:
Fe + O₂ → Fe₂O₃
Al + HCl → AlCl₃ + H₂
Na + H₂O → NaOH + H₂ WHY ?
H₂ + N₂ → NH₃ We balance chemical equations so they
CaCO₃ →CaO + CO₂ obey the law of conservation of mass and
C₃H₈ + O₂ → CO₂ + H₂O correctly show how much of each substance
Zn + H₂SO₄ → ZnSO₄ + H₂
reacts and is formed.
NaOH + H₂SO₄ → Na₂SO₄ + H₂O
CuO + H₂ → Cu + H₂O
AgNO₃ + Cu → Cu(NO₃)₂ + Ag
Q. (3 Marks) Word problem on balanced reaction
A small welding workshop uses hydrogen gas obtained by reacting dilute sulphuric acid with zinc metal.
The word equation for the reaction is:
Zinc + Dilute sulphuric acid→ Zinc sulphate + Hydrogen
a) Write the skeletal chemical equation for this reaction using symbols and formulae.
b) Balance this chemical equation.
AND EQUATION
CHANGE : A group of events or processes where final state differ from intial state is called Change
Feature Physical Change Chemical Change
New substance formed No Yes
Composition change No Yes
Reversibility Usually reversible (e.g., melt Usually irreversible (e.g.,
ice) burn paper)
Energy involved Little or none Heat, light, sound often
produced
Examples Boiling water, cutting paper Rusting iron, baking cake
CHEMICAL REACTION : A chemical reaction involves the transformation of one or more substances,
called reactants, into different substances known as products through the breaking and forming
of chemical bonds.
CHEMICAL EQUATION : The symbolic representation of a chemical reaction is called a chemical equation.
, CHARACTERISTICS OF CHEMICAL REACTION
Evolution of Gas: Bubbles form, like CO₂ from Na₂CO₃ + HCl → NaCl + H₂O + CO₂.
Precipitate Formation: Insoluble solid appears, e.g., AgNO₃ + NaCl → AgCl↓ + NaNO₃ (white ppt).
Color Change: Reactants alter hue, such as purple KMnO₄ turning colorless with citric acid.
Temperature Change: Exothermic (heat released, e.g., quicklime + water) or endothermic (heat absorbed)
State Change: Solid to gas/liquid, like candle wax burning: CₓHᵧ + O₂ → CO₂ + H₂O.
BALANCING OF CHEMICAL REACTION Balancing chemical equations means adjusting coefficients to ensure
equal numbers of each atom on reactant and product sides.
Step-by-Step Method
Start with the unbalanced equation and balance elements systematically:
Balance metals or less common (non metal ) elements first.
Balance oxygen and hydrogen last.
Use whole numbers; multiply all coefficients if fractions appear to clear them.
Here are 10 unbalanced equations ; balance them:
Fe + O₂ → Fe₂O₃
Al + HCl → AlCl₃ + H₂
Na + H₂O → NaOH + H₂ WHY ?
H₂ + N₂ → NH₃ We balance chemical equations so they
CaCO₃ →CaO + CO₂ obey the law of conservation of mass and
C₃H₈ + O₂ → CO₂ + H₂O correctly show how much of each substance
Zn + H₂SO₄ → ZnSO₄ + H₂
reacts and is formed.
NaOH + H₂SO₄ → Na₂SO₄ + H₂O
CuO + H₂ → Cu + H₂O
AgNO₃ + Cu → Cu(NO₃)₂ + Ag
Q. (3 Marks) Word problem on balanced reaction
A small welding workshop uses hydrogen gas obtained by reacting dilute sulphuric acid with zinc metal.
The word equation for the reaction is:
Zinc + Dilute sulphuric acid→ Zinc sulphate + Hydrogen
a) Write the skeletal chemical equation for this reaction using symbols and formulae.
b) Balance this chemical equation.
