CHEMICAL REACTIONS AND EQUATIONS
B(caps)ASIC CONCEPT
PHYSICAL AND CHEMICAL CHANGES
Chemical change - one or
more new substances with new physical and chemical properties are formed.
Example: Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)
Here, when copper sulphate reacts with iron, two new substances,
i.e., ferrous sulphate and copper are formed.
Physical change - The type of change in which the physical properties of a material change. It can be changed in shape, state of matter,
color, density, size, odor, solubility, etc.
DETERMINE A CHEMICAL REACTION :
A chemical reaction can be
determined with the help of any of the following observations:
a) Evolution of a gas
b) Change in temperature
c) Formation of a precipitate
d) Change in colour
e) Change of state
CHEMICAL EQUATIONS :
when a
magnesium ribbon is burnt in oxygen, it gets converted to magnesium oxide.
The word-equation for the above reaction would be –
• Magnesium
+ Oxygen ® Magnesium oxide
(Reactants)
(Product)
The
substances that undergo chemical change in the above reaction magnesium and
oxygen, are the reactants. The new substance is magnesium oxide, formed during
the reaction, as a product.
A word-equation shows change of reactants to products through an
arrow placed between them. The reactants are written on the
left-hand
side (LHS) with a plus sign (+) between them. Similarly,
products are
written on the right-hand side (RHS) with a plus sign (+)
between them.
The arrowhead points towards the products, and shows the
direction of
the reaction.
TYPES OF CHEMICAL REACTIONS :
Few examples are:
● Combination
● Decomposition
● Single Displacement
● Double displacement
● Redox
● Endothermic
● Exothermic
● Precipitation
● Neutralisation
WORD EQUATION :
A word equation is a chemical
reaction expressed in words rather than chemical
formulas. It helps identify
the reactants and products in a chemical reaction.
For example: Sodium + Chlorine
→ Sodium chloride
SYMBOLS TO REMEMBER :
Representation of a chemical
reaction in terms of symbols and chemical formulae of the
reactants and products is known
as a chemical equation.
Zn(s) + dil.H2SO4(aq) → ZnSO4(aq) + H2(↑)
(Reactants) (Products)
• For solids, the symbol is
"(s)".
• For liquids, it is
"(l)".
• For gases, it is
"(g)".
• For aqueous solutions, it is
"(aq)".
• For gas produced in the
reaction, it is represented by "(↑)".
• For precipitate formed in
the reaction, it is represented by "(↓)".
BALANCING OF A CHEMICAL REACTION
:
CONSERVATION OF MASS
According to the law of
conservation of mass, no atoms can be created or destroyed in a
chemical reaction, so the
number of atoms for each element in the reactants side has to
balance the number of atoms
that are present in the products side.
In other words, the total mass
of the products formed in a chemical reaction is equal to the
total mass of the reactants
participated in a chemical reaction.
BALANCED CHEMICAL EQUATION
The chemical equation in which
the number of atoms of each element in the reactants side
is equal to that of the
products side is called a balanced chemical equation.
TYPES OF CHEMICAL REACTIONS :
Taking into consideration
different factors, chemical reactions are grouped into multiple
categories.
Few examples are:
·
Combination
·
Decomposition
·
Single Displacement
·
Double displacement
·
Redox
·
Endothermic
·
Exothermic
·
Precipitation
·
Neutralisation
COMBINATION REACTION
In a combination reaction, two
elements or one element and one compound or two
compounds combine to give one
single product.
H2 +
Cl2
→ 2HCl
element + element → compound
2CO +
O2
→ 2CO2
compound + element → compound
NH3 +
HCl →
NH4Cl
compound + compound → compound
DECOMPOSITION REACTION :
A single reactant decomposes
on the application of heat or light or electricity to give two or
more products.
Types of decomposition
reactions:
a. Decomposition reactions
which require heat - thermolytic decomposition or thermolysis.
Thermal decomposition of HgO
b. Decomposition reactions
which require light - photolytic decomposition or photolysis.
Photolytic decomposition of H2O2
c. Decomposition reactions
which require electricity - electrolytic decomposition or
electrolysis.
Electrolytic decomposition of H2O
DISPLACEMENT REACTION :
More reactive element
displaces a less reactive element from its compound or solution.
i) Zn(s)
+ CuSO4(aq)
→ ZnSO4(aq)
+ Cu(s)
ii) Cu(s)
+ 2AgNO3(aq)
→ Cu(NO3)2(aq) + 2Ag(s)
DOUBLE DISPLACEMENT REACTION :
An exchange of ions between
the reactants takes place to give new products.
For example, Al2(SO4)3(aq) + 3Ca(OH)2(aq) → 2Al(OH)3(aq) + 3CaSO4(s)
PRECIPITATION REACTION :
An insoluble compound called
precipitate forms when two solutions containing soluble salts
are combined.
For example, Pb(NO3)2(aq) + 2KI(aq) → 2KNO3(aq) + PbI2(↓)(s)(yellow)
REDOX REACTION :
Oxidation and reduction take
place simultaneously.
Oxidation: Substance loses electrons or gains oxygen or loses hydrogen.
Reduction: Substance gains electrons or loses oxygen or gains hydrogen.
Oxidising agent - a substance that oxidises another substance and self-gets
reduced.
Reducing agent - a substance that reduces another substance and self-gets
oxidised.
ENDOTHERMIC AND EXOTHERMIC
REACTION :
Exothermic reaction - heat is
evolved during a reaction. Most of the combination reactions
are exothermic.
CORROSION :
Gradual deterioration of a
material, usually a metal, by the action of moisture, air or
chemicals in the surrounding
environment.
Rusting:
RANCIDITY :
It refers to oxidation of fats
and oils in food that is kept for a long time. It gives foul smell
and bad taste to food. Rancid
food causes stomach infection on consumption.
Prevention:
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