CBSE Class 10 Science Notes Chapter 1 Chemical Reactions and Equations
Chemical Reactions and Equations: Balanced and unbalanced chemical equations and balancing of chemical equations.
What is a chemical reaction Class 10?
Chemical Reaction: The transformation of chemical substance into another chemical substance is known as Chemical Reaction. For example: Rusting of iron, the setting of milk into curd, digestion of food, respiration, etc.
In a chemical reaction, a new substance is formed which is completely
different in properties from the original substance, so in a chemical
reaction, a chemical change takes place.
Only a rearrangement of atoms takes place in a chemical reaction.
- The substances which take part in a chemical reaction are called reactants.
- The new substances produced as a result of a chemical reaction are called products.
Example: The burning of magnesium in the air to form
magnesium oxide is an example of a chemical reaction.
2Mg(s) + O2(g) △→ 2MgO(s)
Before burning in air, the magnesium ribbon is cleaned by rubbing with
sandpaper.
This is done to remove the protective layer of basic magnesium carbonate from
the surface of the magnesium ribbon.
Reactant: Substances which take part in a chemical reaction
are called reactants.
Example: Mg and O2.
Product: New substance formed after a chemical reaction is
called a product.
Example: MgO.
Characteristics of Chemical Reactions :
(i) Evolution of gas: The chemical reaction between zinc and
dilute sulphuric acid is characterised by the evolution of hydrogen gas.
Zn(s) + H2SO4(aq) → ZnSO4(aq) +
H2(g) ↑
(ii) Change in Colour: The chemical reaction between citric
acid and purple coloured potassium permanganate solution is characterised by a
change in colour from purple to colourless.
The chemical reaction between sulphur dioxide gas and acidified potassium
dichromate solution is characterized by a change in colour from orange to
green.
(iii) Change in state of substance: The combustion reaction of candle wax is characterised by a change in state from solid to liquid and gas (because the wax is a solid, water formed by the combustion of wax is a liquid at room temperature whereas, carbon dioxide produced by the combustion of wax is a gas). There are some chemical reactions which can show more than one characteristics.
(iv) Change in temperature: The chemical reaction between
quick lime water to form slaked lime is characterized by a change in
temperature (which is a rise in temperature).
The chemical reaction between zinc granules and dilute sulphuric acid is also
characterised by a change in temperature (which is a rise in temperature).
(v) Formation of precipitate: The chemical reaction between
sulphuric acid and barium chloride solution is characterised by the formation
of a white precipitate of barium sulphate.
BaCl2(aq) + H2SO4(aq) → BaSO4(s)
(ppt) + 2HCl(aq)
What is a chemical Equation Class 10?
Chemical Equation: Representation of chemical reaction using
symbols and formulae of the substances is called Chemical Equation.
Example: A + B → C + D
In this equation, A and B are called reactants and C and D are called the
products. The arrow shows the direction of the chemical reaction. Condition,
if any, is written generally above the arrow.
When hydrogen reacts with oxygen, it gives water. This reaction can be
represented by the following chemical equation:
Hydrogen + Oxygen → Water
H2 + O2 → H2O
In the first equation, words are used and in second, symbols of substances are
used to write the chemical equation. For convenience, the symbol of substance
is used to represent chemical equations.
A chemical equation is a way to represent the chemical reaction in a concise
and informative way.
A chemical equation can be divided into two types: Balanced Chemical Equation
and Unbalanced Chemical Equation.
(a) Balanced Chemical Equation: A balanced chemical equation
has the number of atoms of each element equal on both sides.
Example: Zn + H2SO4 → ZnSO4 + H2
In this equation, numbers of zinc, hydrogen and sulphate are equal on both
sides, so it is a Balanced Chemical Equation.
According to the Law of Conservation of Mass, mass can neither be created nor
destroyed in a chemical reaction. To obey this law, the total mass of elements
present in reactants must be equal to the total mass of elements present in
products.
(b) Unbalanced Chemical Equation: If the number of atoms of
each element in reactants is not equal to the number of atoms of each element
present in the product, then the chemical equation is called Unbalanced
Chemical Equation.
Example: Fe + H2O → Fe3O4 + H2
In this example, a number of atoms of elements are not equal on two sides of
the reaction. For example; on the left-hand side only one iron atom is
present, while three iron atoms are present on the right-hand side. Therefore,
it is an unbalanced chemical equation.
Balancing a Chemical Equation: To balance the given or any chemical equation,
follow these steps:
Fe + H2O → Fe3O4 + H2
Write the number of atoms of elements present in reactants and in products in
a table as shown here.
Name of atom | No. of atoms in the reactant | No. of atoms in the product |
Iron | 1 | 3 |
Hydrogen | 2 | 2 |
Oxygen | 1 | 4 |
Balance the atom which is maximum in number on either side of a chemical
equation.
In this equation, the number of oxygen atom is the maximum on the RHS.
To balance the oxygen, one needs to multiply the oxygen on the LHS by 4, so
that, the number of oxygen atoms becomes equal on both sides.
Fe + 4 × H2O → Fe3O4 + H2
Now, the number of hydrogen atoms becomes 8 on the LHS, which is more than
that on the RHS. To balance it, one needs to multiply the hydrogen on the RHS
by 4.
Fe + 4 × H2O → Fe3O4 + 4 × H2
After that, the number of oxygen and hydrogen atoms becomes equal on both
sides. The number of iron is one on the LHS, while it is three on the RHS. To
balance it, multiply the iron on the LHS by 3.
3 × Fe + 4 × H2O → Fe3O4 + 4 × H2
Now the number of atoms of each element becomes equal on both sides. Thus,
this equation becomes a balanced equation.
Name of atom | No. of atoms in the reactant | No. of atoms in the product |
Iron | 3 | 3 |
Hydrogen | 8 | 8 |
Oxygen | 4 | 4 |
After balancing, the above equation can be written as follows:
3Fe + 4H2O → Fe3O4 + 4H2.
To Make Equations More Informative:
Writing the symbols of physical states of substances in a chemical
equation:
By writing the physical states of substances, a chemical equation becomes more
informative.
- Gaseous state is represented by symbol (g).
- Liquid state is represented by symbol (l).
- Solid state is written by symbol (s).
- Aqueous solution is written by symbol (aq).
- Writing the condition in which reaction takes place: The condition is generally written above and/or below the arrow of a chemical equation.
Thus, by writing the symbols of the physical state of substances and condition under which reaction takes place, a chemical equation can be made more informative.
What are the types of a chemical reaction Class 10?
Types of Chemical Reactions: Combination Reaction, Decomposition Reaction, Displacement Reaction, Double Displacement Reaction, Neutralization Reactions, Exothermic – Endothermic Reactions and Oxidation-Reduction Reactions.
Types of Chemical Reactions:
Chemical reactions can be classified in following types:
(i) Combination Reaction: Reactions in which two or more
reactants combine to form one product are called Combination Reactions.
A general combination reaction can be represented by the chemical equation
given here:
A + B → AB
Examples:
When magnesium is burnt in the air (oxygen), magnesium oxide is formed. In
this reaction, magnesium is combined with oxygen.
Mg(s) + O2(g) → 2MgO(s)
Magnesium + Oxygen → Magnesium Oxide
When carbon is burnt in oxygen (air), carbon dioxide is formed. In this
reaction, carbon is combined with oxygen.
C (s) + O2(g) → CO2(g)
Carbon + Oxygen → Carbon dioxide
(ii) Decomposition Reaction: Reactions in which one compound
decomposes in two or more compounds or elements are known as Decomposition
Reaction. A decomposition reaction is just the opposite of combination
reaction.
A general decomposition reaction can be represented as follows :
AB → A + B
Examples:
When calcium carbonate is heated, it decomposes into calcium oxide and carbon
dioxide.
CaCO3(s) △→ CaO(s) +
CO2(g)
Calcium carbonate → Calcium oxide + Carbon dioxide
When ferric hydroxide is heated, it decomposes into ferric oxide and water
2Fe(OH)3(s) △ → Fe2O3(s) + 3H2O(l)
Thermal Decomposition: The decomposition of a substance on
heating is known as Thermal Decomposition.
Example: 2Pb(NO3)2(s) heat→ 2PbO(s) + 4NO2(g) + O2(g)
Electrolytic Decomposition: Reactions in which compounds
decompose into simpler compounds because of passing of electricity, are known
as Electrolytic Decomposition. This is also known as Electrolysis.
Example: When electricity is passed in water, it decomposes into hydrogen and
oxygen.
2H2O(l) Electricity→ 2H2(g) + O2(g)
Photolysis or Photo Decomposition Reaction: Reactions in
which a compound decomposes because of sunlight are known as Photolysis or
Photo Decomposition Reaction.
Example: When silver chloride is put in sunlight, it decomposes into silver
metal and chlorine gas.
2AgCl(s) (white) Sunlight → 2Ag(s) (grey) +
Cl2(g)
Photographic paper has a coat of silver chloride, which turns into grey when exposed to sunlight. It happens because silver chloride is colourless while silver is a grey metal.
(iii) Displacement Reaction: The chemical reactions in which
a more reactive element displaces a less reactive element from a compound is
known as Displacement Reactions. Displacement reactions are also known as
Substitution Reaction or Single Displacement/ replacement reactions.
A general displacement reaction can be represented by using a chemical
equation as follows :
A + BC → AC + B
Displacement reaction takes place only when ‘A’ is more reactive than B. If
‘B’ is more reactive than ‘A’, then ‘A’ will not displace ‘C’ from
‘BC’ and reaction will not be taking place.
Examples:
When zinc reacts with hydrochloric acid, it gives hydrogen gas and zinc
chloride.
Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
When zinc reacts with copper sulphate, it forms zinc sulphate and copper
metal.
Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)
(iv) Double Displacement Reaction: Reactions in which ions
are exchanged between two reactants forming new compounds are called Double
Displacement Reactions.
AB + CD → AC + BD
Examples:
When the solution of barium chloride reacts with the solution of sodium
sulphate, white precipitate of barium sulphate is formed along with sodium
chloride.
BaCl2(aq) + Na2SO4(aq) → BaSO4(s)
(Precipitate) + 2NaCl(aq)
When sodium hydroxide (a base) reacts with hydrochloric acid, sodium chloride
and water are formed.
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
Note: Double Displacement Reaction, in which precipitate is formed, is also known as precipitation reaction. Neutralisation reactions are also examples of double displacement reaction.
Precipitation Reaction: The reaction in which precipitate is
formed by the mixing of the aqueous solution of two salts is called
Precipitation Reaction.
Example:
Neutralization Reaction: The reaction in which an acid reacts
with a base to form salt and water by an exchange of ions is called
Neutralization Reaction.
Example:
(v) Oxidation and Reduction Reactions:
Oxidation: Addition of oxygen or non-metallic element or
removal of hydrogen or metallic element from a compound is known as
Oxidation.
Elements or compounds in which oxygen or non-metallic element is added or
hydrogen or metallic element is removed are called to be Oxidized.
Reduction: Addition of hydrogen or metallic element or
removal of oxygen or non-metallic element from a compound is called
Reduction.
The compound or element which goes under reduction in called to be Reduced.
Oxidation and Reduction take place together.
Oxidizing agent:
- The substance which gives oxygen for oxidation is called an Oxidizing agent.
- The substance which removes hydrogen is also called an Oxidizing agent.
Reducing agent:
- The substance which gives hydrogen for reduction is called a Reducing agent.
- The substance which removes oxygen is also called a Reducing agent.
The reaction in which oxidation and reduction both take place simultaneously
is called Redox reaction.
When copper oxide is heated with hydrogen, then copper metal and hydrogen are
formed.
CuO + H2 → Cu + H2O
(i) In this reaction, CuO is changing into Cu. Oxygen is being removed from
copper oxide. Removal of oxygen from a substance is called Reduction, so
copper oxide is being reduced to copper.
(ii) In this reaction, H2 is changing to H2O. Oxygen is being added to hydrogen. Addition of oxygen to a substance is called Oxidation, so hydrogen is being oxidised to water.
- The substance which gets oxidised is the reducing agent.
- The substance which gets reduced is the oxidizing agent.
(vi) Exothermic and Endothermic Reactions:
Exothermic Reaction: Reaction which produces energy is called
Exothermic Reaction. Most of the decomposition reactions are exothermic.
Example:
Respiration is a decomposition reaction in which energy is released.
When quick lime (CaO) is added to water, it releases energy.
Endothermic Reaction: A chemical reaction in which heat
energy is absorbed is called Endothermic Reaction.
Example: Decomposition of calcium carbonate.
Effects of Oxidation Reactions in Everyday life: Corrosion
and Rancidity.
Corrosion: The process of slow conversion of metals into
their undesirable compounds due to their reaction with oxygen, water, acids,
gases etc. present in the atmosphere is called Corrosion.
Example: Rusting of iron.
Rusting: Iron when reacts with oxygen and moisture forms red
substance which is called Rust.
The rusting of iron is a redox reaction.
Corrosion (rusting) weakens the iron and steel objects and structures such as
railings, car bodies, bridges and ships etc. and cuts short their life.
Methods to Prevent Rusting
- By painting.
- By greasing and oiling.
- By galvanisation.
Corrosion of Copper: Copper objects lose their lustre and
shine after some time because the surface of these objects acquires a green
coating of basic copper carbonate, CuCO3.Cu(OH)2 when
exposed to air.
Corrosion of Silver Metal: The surface of silver metal gets
tarnished (becomes dull) on exposure to air, due to the formation of a coating
of black silver sulphide(Ag2S) on its surface by the action of
H2S gas present in the air.
Rancidity: The taste and odour of food materials containing fat and oil changes when they are left exposed to air for a long time. This is called Rancidity. It is caused due to the oxidation of fat and oil present in food materials.
Methods to prevent rancidity:
- By adding anti-oxidant.
- Vacuum packing.
- Replacing air by nitrogen.
- Refrigeration of foodstuff.
1. Chemical Reaction: During chemical reactions, the chemical composition of substances changes or new substances are formed.
2. Chemical Equation: Chemical reactions can be written in chemical equation form which should always be balanced.
3. Types of Chemical Reactions:
Combination reaction: A single product is formed from two or
more reactants.
2Mg + O2 → 2MgO
Decomposition reaction: A single reactant breaks down to yield two or more products.
- Thermal decomposition: 2Pb(NO2)2 → 2PbO + 4NO2 + O2
- Electrolysis: 2H20 → 2H2 + O2
- Photochemical reaction: 2AgBr → 2Ag + Br2
Displacement reaction: One element is displaced by another
element.
Zn + CuSO4 → ZnSO4 + Cu
Double displacement reaction: Exchange of ions between
reactants.
AgNO3 + NaCl → AgCl + NaNO3
Redox reaction: Both oxidation and reduction take place
simultaneously.
CuO + H2 → Cu + H2O
Exothermic reaction: A chemical reaction in which heat energy
is evolved.
C + O2 → CO2 (g) + heat
Endothermic reaction: A chemical reaction in which heat
energy is absorbed.
ZnCO3 + Heat → ZnO + CO2
Redox reaction: Chemical reaction in which both oxidation and reduction take place simultaneously.
4. Oxidation: Reaction that involves the gain of oxygen or loss of hydrogen.
5. Reduction: Reaction that shows the loss of oxygen or gain
of hydrogen.
ZnO + C → Zn + CO
ZnO is reduced to Zn—reduction. C is oxidized to CO—Oxidation.
6. Effects of Oxidation Reactions in Our Daily Life:
-
Corrosion: It is an undesirable change that occurs in
metals when they are attacked by moisture, air, acids and bases.
Example, Corrosion (rusting) of Iron: Fe2O3. nH2O (Hydrated iron oxide) -
Rancidity: Undesirable change that takes place in oil
containing food items due to the oxidation of fatty acids.
Preventive methods of rancidity: Adding antioxidants to the food materials, storing food in the airtight container, flushing out air with nitrogen gas and refrigeration.
NCERT Class 10 Science Revision Notes
Check below links of Class 10 Science Notes based on NCERT class 10 Science textbook.
- Chapter 1 Chemical Reactions and Equations Class 10 Notes
- Chapter 2 Acids Bases and Salts Class 10 Notes
- Chapter 3 Metals and Non-metals Class 10 Notes
- Chapter 4 Carbon and its Compounds Class 10 Notes
- Chapter 5 Periodic Classification of Elements Class 10 Notes
- Chapter 6 Life Processes Class 10 Notes
- Chapter 7 Control and Coordination Class 10 Notes
- Chapter 8 How do Organisms Reproduce Class 10 Notes
- Chapter 9 Heredity and Evolution Class 10 Notes
- Chapter 10 Light Reflection and Refraction Class 10 Notes
- Chapter 11 Human Eye and Colourful World Class 10 Notes
- Chapter 12 Electricity Class 10 Notes
- Chapter 13 Magnetic Effects of Electric Current Class 10 Notes
- Chapter 14 Sources of Energy Class 10 Notes
- Chapter 15 Our Environment Class 10 Notes
- Chapter 16 Management of Natural Resources Class 10 Notes
Y didn't u make these notes last year itself guyzs these are the bestest notes u need really
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