Chapter 1
Chemical reactions and equations :balanced and unbalanced chemical equations and balancing of chemical equations.
Chemical reaction:
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 take place.
Only 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:
(1) Evolution of gas : the chemical reaction between zinc and dilute sulphuric acid is characterized by the evolution of hydrogen gas.
Zn (s) + H2SO4 -----> ZnSO4(aq) +H2 (g)
(2) Change in colour : the chemical reaction between citric acid and purple coloured potassium permanganate solution is characterized 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.
(3) Change in state of substance : the combination reaction of candle wax is characterized bi 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.
(4) Change in temperature: the chemical reaction between quicklime water to form slaked lime is characterized by change in temperature ( which is a rise in temperature).
The chemical reaction between zinc granules sulfuric acid is also characterized by a change in temperature (which is rise in temperature).
(5) Formation of precipitate : the chemical reaction between sulphuric acid and barium chloride solution is characterized by the formation of white precipitate of barium sulphate.
BaCl2(aq)+H2SO4(aq) -----> BaSO4(S)(ppt) +2HCl(aq)
Chemical equation : representation of chemical reaction using symbols and formula 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 that shows the direction of the chemical reaction 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 equation.
chemical equation is a way to represent the chemical reaction in a concise and informative way.
The chemical equation can be divided into two types: balance chemical equation and unbalanced chemical equation
(a) Balanced chemical equation : A balance chemical equation has the number of atoms of each element equal on both sides
Example : Zn +H2SO4 ----> ZnSO4 + H2
in this equation number of zinc, hydrogen and sulphate are equal on both side 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 order this rule the total mass of element present in the reactant must be equal to the total mass of elements present in the products.
(b) Unbalanced chemical equation : if the number of atoms of each element ine 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 number of atoms of element 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 chemical equation: to balance the given for any chemical equation follow these steps :
Fe + H2O ------> Fe3O4+ H2
Balance the atom which is maximum in number on either sides 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, show that the number of oxygen atoms becomes equal on both sides.
Fe + 4 x H2O ------> Fe3O4+ H2
Now the number of hydrogen atoms become 8 on the LHS which is more than that on the RHS to balance that one needs to multiply the hydrogen on the RHS by 4.
Fe + 4 x H2O ------> Fe3O4+ 4 x 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 free on the RHS to balance it multiplied iron on the LHS by 3.
3 x Fe + 4 x H2O ------> Fe3O4+ 4 x H2
Now the number of atoms of each element becomes equal on both sides thus this equation becomes a balance equation.
After balancing the above equation can be written as follows
3Fe + 4H2O ------> Fe3O4+ 4H2
To make it questions more informative :
Writing the symbols of physical state of substance in a chemical equation.
By writing the physical state of substance a chemical equation becomes more informative.
Gaseous state is represented bi symbol (g).
Liquid state is represented by symbol ( l ).
Solid state is written by symbol (s).
Aqueous solution is written by symbol (aq).
Reading the condition in which reaction take 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.
Types of chemical reaction : combination reaction, decomposition reaction, displacement reaction, double displacement reaction, neutralization reaction, exothermic and endothermic reaction, and oxidation reduction reaction, redox reactions.
Chemical reactions can be classified in the following types:
1) Combination reaction : reactions in which two or more reactant combined to form one product are called the combination reactions.
a general combination reaction can be represented by the chemical equation given here.
A+B ----> AB
Example : When magnesium is burnt in the air (oxygen) magnesium oxide is formed in the 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
2) Decomposition reaction : reactions in which one compound decompose is in two or more compounds or elements are known as the composition reaction. a decomposition reaction is just the opposite of combination reaction.
General decomposition reaction can be represented as follows.
AB ----> A+B
Example : when calcium carbonate is heated it decomposes into calcium oxide and carbon dioxide.
CaCO3 (s) heat -----> CaO(s) + CO2(g)
Calcium carbonate ----> calcium oxide + carbon dioxide
When Fariq hydroxide is heated it decomposes into ferric oxide and water.
2Fe(OH)3(s) heat -----> 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 decomposed into simpler compounds because of passing of electricity are known as electrolytic decomposition this is known as electrolysis.
Example - when electricity is passed in water it decomposes into hydrogen and oxygen.
2H2O(l) -----> 2 H2(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 ------> 2Sg(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.
(3) Displacement reaction : the chemical reactions in which more reactive element displace a Less reactive element from a compound is known as displacement reactions.displacement reaction are also known as substitution reaction for single displacement/ replacement reaction.
General displacement reaction can be represented by using the 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)
(4) Double displacement reaction : reactions in which ions are exchanged between two reactants forming new compounds are called double displacement reaction.
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, neutralization reaction are also example of double displacement reaction.
(5) Precipitation reaction : the reaction in which precipitate is formed by the mixing of the aqueous solution of two salts is called precipitation reaction.
Example - AgNo3(aq) + NaCl(aq) -----> AgCl(s) NaNO3(aq)
Silver nitrate + sodium chloride -----> silver chloride (precipitate) + sodium nitrate
(6) 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
NaOH(aq) +HCl(aq) NaCl(aq) +H2O(l)
Sodium hydroxide + hydrochloric acid ------> sodium chloride + water
(7) Oxidation reaction : addition of oxygen or non metallic element or removal of hydrogen or metallic element from a compound is known as oxidation.
element or compound in which oxygen or non metallic element is added or hydrogen or metallic element is removed are called to be oxidized.
Example - 2Mg(s) + O2 (g) ----> 2mgO (s)
(8) Reduction reaction : 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 is called to be reduced.
Example MgO(s) + C(s) -----> Mg(s) + CO(g)
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 production is called a reducing agent
The substance which remove the oxygen is also called a reducing agent.
(9) Redox reaction: 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 hydrogen is changing into water. Oxygen is being added to hydrogen. addition of oxygen to a substance is called oxidation so hydrogen is being oxidized to water.
substance which gets oxidized is the reducing agent.
substance which gets reduced is the oxidizing agent.
(10) Exothermic and Endothermic reactions :
Exothermic reaction - reaction which produced energy is called exothermic reaction most of the decomposition reaction are endothermic.
Example :
Respiration is a decomposition reaction in which energy is released.
C6H12O6(aq) + 6O2(g) ------> 6CO2(g) + 6H2O(l) +Energy
Glucose + oxygen -----> carbon dioxide + water + energy
When quicklime (CaO) is added to water, it releases energy.
Cao(s) +H2O(l) -----> Ca(OH)2(aq) + Energy
Quicklime + water -----> calcium hydroxide slaked lime + energy
Endothermic reaction : a chemical reaction in which heat energy is absorbed is called endothermic reaction.
Example : decomposition of calcium carbonate
CaCO3(s) heat ----> CaO (s) + CO2(g)
Calcium carbonate ------> calcium oxide + carbon dioxide
Effect of oxidation reaction in everyday life:
Corrosion : the process of slow conservation 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.
4Fe (s) + 3O2(g) +H2O(l) ------> 2Fe2O3.xH2O(s)
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 cut short their life.
Methods of preventing rusting
By paintings
By greasing and oiling
By galvanization
Corrosion of copper : Copper objects lost their lustre and shine after sometimes because the surface of these objects acquires a green coating of basic copper carbonate CuCO3 , Ca(OH)2
when exposed to air
2Cu(s) + CO2(g) + O2(g) + H2O (l) -----> CuCO3.Cu(OH)2
Copper + moist air -----> basic copper carbonate (green)
Corrosion of silver metal :
The surface of silver metal gets tarnished (become 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.
2Ag (s) +H2S(g) ------> Ag2S(g) +H2(g)
Silver + hydrogen sulphide -----> silver sulphide (black) + hydrogen
Rancidity : the taste and odour of food material 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 the food materials.
Methods to prevent rancidity :
By adding antioxidants
Vacuum packing
Replacing air by nitrogen
Refrigeration of food stuff
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 equations form which should always be balanced.
3) Types of chemical reactions
i) Combination reaction : a single product is formed from two or more reactants
2Mg(s) + O2 (g) ----> 2mgO (s)
ii) Decomposition reaction : Single reactant breaks down to yield two or more products.
CaCO3 (s) heat -----> CaO(s) + CO2(g)
Thermal decomposition
Electrolysis
Photochemical reaction
iii) Displacement reaction : one element is displaced by another element.
Zn (s) +2HCl (aq) -----> ZnCl2(aq) +H2(g)
iv) Double displacement reaction : exchange of points between reactants.
BaCl2(aq) +Na2SO4(aq) --------> BaSO4(s) (precipitate) + 2NaCl (aq)
V) Redox reaction : both oxidation and reduction take place simultaneously.
CuO+H2 ----> Cu + H2O
vi) Exothermic reaction : Chemical reaction in which heat energy is evolved.
C6H12O6(aq) + 6O2(g) ------> 6CO2(g) + 6H2O(l) +Energy
vi) Endothermic reaction : Chemical reactions in which heat energy is absorbed.
CaCO3(s) heat ----> CaO (s) + CO2(g)
vii) Oxidation : reaction that involves the gain of oxygen or loss of hydrogen
2Mg(s) + O2 (g) ----> 2mgO (s)
viii) Reduction reaction : reaction that shows the loss of oxygen or gain of hydrogen.
MgO(s) + C(s) -----> Mg(s) + CO(g)
4) Effect of oxidation reaction in our daily life
Corrosion : it is an undesirable change that occurs in metals when they are attacked by moistures,air, acids and bases.
Example : corrosion (rusting) of iron
4Fe (s) + 3O2(g) +H2O(l) ------> 2Fe2O3.xH2O(s)
Rancidity : undesirable change that takes place in oil containing food items due to the oxidation of fatty acids.
Preventive methods of rancidity :
Adding antioxidant to the food material ,storing food in the air tight container, flushing out air with nitrogen gas and refrigeration.
We hope the given CBSE / MP Board Class 10th Science Chapter 1 "Chemical Reactions & Equations" will help you.
Written By - Himanshu Sharma