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http://www.unit5.org/chemistry/Equations.html
CHEMICAL REACTIONS AND EQUATIONS
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Chemical Reactions are associated with chemical change.
Whenever a chemical change occurs chemical reaction is said to take place.
Chemical reactions are usually irreversible and a new product is formed.
CHEMICAL REACTIONS
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Signs of Chemical Reactions
There are five main signs that indicate a chemical reaction has taken place:
change in color Evolution of a gas Change in temperature Change in state
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Word Equations
A WORD EQUATION describes chemical change using the names of the reactants and products.
Write the word equation for the reaction of methane gas with oxygen gas to form carbon dioxide and water.
methane + oxygen
Reactant Product
CH4 O2 CO2 H2O+ + 22
carbon dioxide + water
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The simplest form of description of a chemical reaction in a shorter form is by writing it in the form of an equation. E.g.-
Mg + O2 MgO The reactants are written on the left
side and product is written on the right side.
An arrow is placed between them to show the direction of reaction.
CHEMICAL EQUATIONS
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The equation must represent known facts.
The equation must contain the correct formulas for the reactants and products.
The law of conservation of mass must be satisfied.
Characteristics Of Chemical Equations
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Showing Phases in Chemical Equations
Solid Phase – the substance is relatively rigid and has a definite volume and shape. NaCl(s)
Liquid Phase – the substance has a definite volume, but is able to change shape by flowing. H2O(l)
Gaseous Phase – the substance has no definite volume or shape, and it shows little response to gravity. Cl2(g)
H2O(s) H2O(l) H2O(g)
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Balanced Chemical Equations• According to the law of conservation of
mass, total mass must be equal on the both sides of the equation.
• This type of equation is known as a balanced chemical reaction. E.g.-
2Mg + O2 2MgO
• Here both sides have two atoms of Magnesium and two atoms of Oxygen.
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Cl
Cl
ClH
H
H
ClClCl
Cl HHH
H
H2 + Cl2 HCl H2 + Cl2 2 HCl
reactants products
H
Cl
reactants products
H
Cl
22
2 22 2
11
(unbalanced) (balanced)
Unbalanced and Balanced Equations
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CH4 + 2 O2 CO2 + 2 H2O
Reactants Products 1 C atom 1 C atom 4 H atoms 4 H atoms 4 O atoms 4 O atoms
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?
Visualizing a Chemical ReactionNa + Cl2 NaCl
___ mole Cl2 ___ mole NaCl___ mole Na
2
10 5 10
2
10 5 10
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TYPES OF CHEMICAL REACTIONS
1. Combination reaction
2. Decomposition reaction
3. Single-displacement reaction
4. Double-displacement reaction
A + B AB
AB A + B
A + BC AC + B
AB + CD AD + CB
5. Redox reaction
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These type of reactions occur when two reactants combine to form one or more products. E.g. –
1. CaO + H2O Ca(OH)2
2. C + O2 CO2
They are generally exothermic reactions which involve evolution of heat during reaction.
Combination Reactions
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Combination Reaction
General form: A + B AB
element or element or compoundcompound compound
Na
ClNa
Cl
2 Na + Cl2 2 NaCl
ClNa
Na
Cl
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Decomposition Reaction
• The types of reaction in which a single reactant breaks down to give simpler products are called decomposition reaction. E.g.-
• When a decomposition reaction is carried
out by heating, it is known as thermal decomposition.
2 H2O 2 H2 + O2
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H
H
H
H
Decomposition Reaction
H
H
H
HO
Decomposition reaction
2 H2O 2 H2 O2
General form: AB A B
+
+
+compound two or more elements
or compounds
OO
O
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H2O H2 + O2
electricity
Decomposition Reactions
H2O2 H2O + O2
NI3 N2 + I2
2 2
Hydrogen Peroxide
Electrolysis of water
2 2
Nitrogen triiodide
AB A + B
General Form
2 3
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Displacement Reactions
• The reaction in which a more reactive element displaces a less reactive element from its solution is known as displacement reaction.
• They are of two types:-1. Single Displacement Reaction2. Double Displacement Reaction
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SINGLE AND DOUBLE DISPLACEMENT REACTIONS
Double-replacement reaction
CaCO3 + 2 HCl CaCl2 + H2CO3
General form: AB + CD AD + CB
Single-replacement reaction
Mg + CuSO4 MgSO4 + Cu
General form: A + BC AC + B
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PbI2
K1+Pb2+
lead (II) chloride + potassium iodide
Cl2 I
Pb2+
Pb
K1+
K
Cl1- I1-Cl1- I1-
KCl
potassium chloride lead (II) iodide+
(aq) (ppt)+ +
Single Displacement Reaction
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Double Displacement Reaction
K2CO3 (aq)Potassium carbonate
BaCl2 (aq)Barium chloride
2 KCl (aq)Potassium chloride
BaCO3 (s)Barium carbonate
+ +
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Ca
Foiled again –Aluminum loses to Calcium
Element Reactivity
Reactivity Series
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Redox Reactions• If a substance gains oxygen during a
reaction, it is said to be oxidised.• If a substance loses oxygen during a
reaction, it is said to be reduced.• Reactions in which this type of change
occurs is known as Oxidation and Reduction reactions or Redox reactions. E.g. –
CuO + H2 Cu +H2O
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THE END
MADE BY – AAYUSH PRAKASHCLASS – XROLL NO. – 02