Every day,
many changes occur around us. Some are physical changes, while others are
chemical changes. Whenever a substance changes into a completely new substance
with different properties, a chemical reaction takes place.
Daily Life
Examples of Chemical Reactions
- Milk
turning sour
- Rusting
of iron
- Fermentation
of grapes
- Cooking
food
- Digestion
- Respiration
In all these
cases, new substances are formed.
HOW DO WE
KNOW A CHEMICAL REACTION HAS OCCURRED?
A chemical
reaction can be identified by observing:
- Change
in state
- Change
in colour
- Evolution
of gas
- Change
in temperature
ACTIVITY:
BURNING OF MAGNESIUM RIBBON
When
magnesium ribbon is burnt in air:
- It
burns with a dazzling white flame.
- White
ash of magnesium oxide is formed.
Reaction
Observation
- Bright
white flame
- White
powder formed
Important
Point
Magnesium
ribbon is cleaned before burning because it has a layer of magnesium oxide on
its surface which prevents proper burning.
CHEMICAL
EQUATIONS
A chemical
equation represents a chemical reaction using symbols and formulae.
Word
Equation
Magnesium +
Oxygen → Magnesium oxide
Chemical
Equation
Reactants
Substances
taking part in reaction.
Products
New
substances formed.
BALANCED
CHEMICAL EQUATION
According to
the Law of Conservation of Mass, atoms can neither be created nor
destroyed during a chemical reaction.
Therefore:
- Number
of atoms on LHS = Number of atoms on RHS
Such an
equation is called a balanced chemical equation.
STEPS FOR
BALANCING CHEMICAL EQUATIONS
Example:
Balanced
Equation:
Hit and
Trial Method
- Balance
one element at a time.
- Never
change chemical formula.
- Only
add coefficients.
PHYSICAL
STATES IN CHEMICAL EQUATIONS
Symbols
used:
- (s) →
solid
- (l) →
liquid
- (g) →
gas
- (aq) →
aqueous solution
Example:
TYPES OF
CHEMICAL REACTIONS
1.
COMBINATION REACTION
A reaction
in which two or more substances combine to form a single product.
Example 1:
Formation of Slaked Lime
Quick lime
reacts with water producing slaked lime and heat.
Observation
- Beaker
becomes hot.
- Heat is
released.
This is also
an exothermic reaction.
Whitewashing
Reaction
Slaked lime
reacts with carbon dioxide forming calcium carbonate.
Important
Point
- Calcium
carbonate gives shiny finish to walls.
- Marble
is also chemically calcium carbonate.
Other
Examples of Combination Reactions
Burning of
Coal
Formation of
Water
EXOTHERMIC
REACTIONS
Reactions
that release heat energy.
Example:
Burning of Methane
Respiration
– An Exothermic Reaction
Glucose
reacts with oxygen to release energy.
Important
Point
- Energy
released keeps living organisms alive.
2.
DECOMPOSITION REACTION
A reaction
in which a single compound breaks into simpler substances.
Activity:
Heating Ferrous Sulphate
Green
ferrous sulphate crystals lose water and decompose.
Observations
- Green
colour changes.
- Smell
of burning sulphur comes.
Reaction:
Thermal
Decomposition of Limestone
Important
Points
- Calcium
oxide = Quick lime
- Used in
cement manufacture
Heating Lead
Nitrate
Brown fumes
of nitrogen dioxide are produced.
Observation
- Brown
fumes observed
ELECTROLYTIC
DECOMPOSITION OF WATER
Electricity
decomposes water into hydrogen and oxygen.
Important
Point
Hydrogen
collected is double the volume of oxygen because water contains hydrogen and
oxygen in 2:1 ratio.
PHOTOCHEMICAL
DECOMPOSITION
Silver
Chloride in Sunlight
White silver
chloride turns grey.
Silver
Bromide Reaction
Applications
- Used in
black and white photography.
ENDOTHERMIC
REACTIONS
Reactions
that absorb energy.
Examples:
- Decomposition
reactions
- Electrolysis
- Photosensitive
reactions
3.
DISPLACEMENT REACTION
A more
reactive element displaces a less reactive element from its compound.
Iron and
Copper Sulphate Reaction
Observations
- Blue
colour fades.
- Iron
nail becomes brown.
Reaction:
Explanation
Iron is more
reactive than copper.
Other
Examples
Zinc
Displacing Copper
Lead
Displacing Copper
4. DOUBLE
DISPLACEMENT REACTION
Two
compounds exchange ions to form new compounds.
Sodium
Sulphate and Barium Chloride
White
precipitate of barium sulphate is formed.
Important
Point
- White
insoluble solid = precipitate
- Hence
called precipitation reaction.
LEAD NITRATE
+ POTASSIUM IODIDE REACTION
Yellow
precipitate of lead iodide forms.
Observation
- Bright
yellow precipitate formed.
5. OXIDATION
AND REDUCTION (REDOX REACTIONS)
OXIDATION
Addition of
oxygen OR removal of hydrogen.
Example
Copper forms
black copper oxide on heating.
Copper gains
oxygen → Oxidation
REDUCTION
Removal of
oxygen OR addition of hydrogen.
Example
Explanation
- CuO
loses oxygen → Reduced
- H₂
gains oxygen → Oxidised
OTHER REDOX
REACTIONS
Zinc Oxide
Reduction
Manganese
Dioxide Reaction
EFFECTS OF
OXIDATION IN DAILY LIFE
CORROSION
When metals
react with moisture, air or acids and get damaged.
Examples
- Rusting
of iron
- Green
coating on copper
- Black
coating on silver
Rusting
Iron reacts
with oxygen and moisture forming reddish brown rust.
Prevention
- Painting
- Greasing
- Galvanisation
RANCIDITY
Oxidation of
oils and fats causing bad smell and taste.
Prevention
- Use
airtight containers
- Add
antioxidants
- Refrigeration
- Flushing
nitrogen gas in chips packets
Important
Example
Nitrogen gas
is filled in chips packets to prevent oxidation.
