Chemistry class-10 chapter 1 Notes
So hey! guys today i bring you the notes of chapter-1 of chemistry (chemical reaction and equation) .Learn more about chemical reactions and equations by exploring CBSE Notes for Class 10 Science Chapter 1 These notes are very important for cbse exam preparation.
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In our daily life, we came across a variety of changes which may be physical or chemical changes.
A Physical change can be easily reversed but a chemical change cannot be reversed easily.Evaporation,melting of wax,freezing of water etc. are physical changes whereas conversion of milk to curd,rusting of iron,digestion of food etc are chemical changes.
All chemical changes are accompained by the chemical reactions and these are represented with the help of chemical equations.
In this chapter,we will study about the various types of chemical reactions and tthe chemical equations which represent chemical changes.
Chemical Reactions
A chemical reaction is a change in which one or more substance(s) or reactant(s) react(s) to form new substance(s)with different properties.
The reacting speacies(molecule,atom,ion) are known as reactants(the substance that undergo chemical change in the chemical reaction)and the new speacies formed as a result of the reaction are called products(the new substance formed during reaction).
2Na(s)+2H2o⟶2NaOH(aq)+H2(g)
In the above chemical reaction,sodium hyrdoixde and hydrogen are the products and sodium and water are the reactants.
Identification of Chemical Reaction
A chemical reaction can be identified by either of the following observations-
- Change in state
- Evolution of a gas
- Formation of precipitate
- Change in colour
- Change in Temperature
Chemical Equations
A chemical equation is the symbolic representation of a chemical reaction.Symbols and formulae of the reactants and products are used for the same
Example-the reaction of burning of methane gas can be written in words
methane+oxygen→carbon dioxide+water
This equation is called word equation.The word equation can be changed into a chemical equation by writing symbol and formulae of the substance in the place of their name.
CH4(g)+2O2(g)→CO2(g)+2H2O(l)
Writing a Chemical Equation
A chemical equation shows a change of reatants to products through and arrow (→) placed between them.
On the left hand side(LHS) of the arrow,reactants are written with a plus sign(+)between them.Similarly on the right hand isde(RHS)products are written with a plus sign(+)between them.
The arrow head points towards the products and shows the direction of the reactants
Example-The reaction between magnesium(Mg) and oxygen(O2) resulting itne the formation of magnesium oxide can be written as-
Mg+O2→MgO
In the above equation,number of magnesium and oxygen atoms are not same on the both sides of an equation.Such an unbalanced equation is called Skeletal chemical equation.
Balanced Chemical Equations
A balanced chemical equation is that in which the total number of atoms of each element are equal on both sides of the equation.
The balancing of a chemical equation is based on law of conservation of mass.Acoording to the law of conservation of mass'mass cam neither be created nor be destroyed during a chemical reaction'.In the same way,the number of atoms of each element remains the same,before and after chemical reaction.
The method used for balancing chemical equations is called hit and trial method
as we make trials to balace the equation by using the smallest whole number coefficient.In this method,the numberof atoms of each element remains the same,before and after a chemical reaction.
Balancing of a Chemical Equation
Several steps are involved in balancing a chemical equation
These steps are as follows-
Step1-Writing unbalanced equation and enclosing the formula in brackets.
Step2-Making list of number of atoms of different elements present in unbalanced equation.
Step3-Balancing First element,the hydrogen atoms,which are unbalanced.
Step4-Balancing second element
We examine the obtained equation and select another element which is still unbalanced.
Step5-Balancing other elements.
Check the other elements now try to balance them.
Making a chemical equation more informative
The following facts remains unexplained in the chemical equation-
1)Physical states of substances
2)Reaction conditions
3)Evolution/absorption of energy.
Some of these limitations of a chemical equation can be overcome by adding the following symbols or information as discussed below-
1)The physical states of the reactants and products can be represented by using the symbols(s),for solid,(l) for liquid,(g) for gas and (aq) for aqueous solution,alongwith their respective formulae.The word aqueous (aq) is written if the reactant and product is present as a solution in water.
2Na(s)+2H2O(l)→2NaOH(aq)+H2(g)
Precipitate can also be represented by using an arrow pointing downwards(↓)instead of using symbol(s).
In the same way,the gaseous state of an evolved gas can be represented by using an arrow pointing upward direction(↑) instead of using symbol (g),
Example-magnesium reacting with dilute sulphuric acid is represented by th chemical equation.
Mg(s)+H2SO4(aq)→MgSO4(aq)+H2↑
Types of Chemical Reactions
1)Combination Reaction
A reaction in which two or more reactants combine to form a singl product,is called combination reaction.
Example-Calcium oxide(quick lime)reacts vigorously with water to form calcium hydroxide(slaked lime).The reaction is higly exothermic,as a lot of heat is produced during the reaction.
CaO(s)+H2O(l)→Ca(OH)2(aq)+Heat
2)Burning of Coal
C(s)+O2(g)→CO2(g)
2)Decomposition Reaction
A reaction in which a single reactant breaks down to form two or more products,is known as Decomposition Reaction.This reaction is opposite to combination reaction.On the basis of the form of energy required for the reaction,these reactions are of three types-
Thermal decomposition-These reactions use the nergy in the form of heat for decomposition of the reactant
Example-(a)Calcium Carbonate on heating,decomposs to give calcium oxide and carbon dioxide.Calcium oxide is used for manufacturing of cement.
CaCO3(s)→CaO(s)+CO2(g)
2)Ferrous sulphate,the green colour crystals FeSO4*7H2O on heating lose water of crystallisation and forms dehydrated FeSO4 which on decomposition givs ferric oxide,sulphur dioxide SO2 and sulphur trioxide SO3,Ferric oxide is solid,whileSO2 and SO3 are gases.
2FSO4(s)→Fe2O3(s)+SO2(g)+SO3(g)
3)On heating lad nitrate,it decomposes to give yellow lead monocide,nitrogen dioxide and oxygen gas.
2Pb(NO3)2(s)→2PbO(s)+4NO2(g)+O2(g)
Electrolysis
These reactions involvethe use of electrical energy for the decomposition of the reactant molecules
example-(a)When electric current is passed through water,it decomposes to give oxygen and hydrogen.
2H2O(l)→2H2(g)+O2(g)
(b)When electric current is passed through molten sodium chloride,it decomposes to give sodium metal and chlorine gas.
2NaCl(l)→2Na(s)+Cl2(g)
Photolysis or photochemical decomposition
These reactions involve the use of light energy for the purpose of decompostion
Example-
(a)When silver chloride is exposd to sun light,it decomposes to give silver metal and chlorine gas
2AgBr(s)→2Ag(s)+Cl2(g)
(b)Similarly,silver bromide gives silver metal and bromine gas in the presence of sunlight.
2AgBr(s)→2Ag(s)+Br2(g)
Exothermic and Endothermic Reactions
Depending upon wheather heat is evoloved and absorbed during a reaction,the reaction can be exothermic or endothermic.
Exothermic Reactions-The reactions which are accompanied by the evolution of heat,are called exothermic reactions(combustion reaction)or the reactions in which heat is releasd along with the formation of products are called exothermic reactions.
(b)Respiration is an exothermic process.The decomposition of vegetable matter into compost is also an example of exothermic reaction.
Endothermic reactions-The reactions which occur by the absorption of heat/energy(either in the form of light or electricity),are called endothermic reactions.
Photosynthesis is an endothermic process.All decomposition reactions are endothermic ractions as such reactions requires energy either in the form of heat,light or electricity for breaking down the reactants.
Displacement Reaction
When a more reactive element displaces less reactive element from its compound,it is called Displacement reaction.
This reaction is of two types:
1) Single displacement reaction-It is a type of chemical reaction where an element reacts with a compound and takes the place of another elementin that compound is called Single displacement.
a)Zinc being more reactive than Cu,displaces Cu from CuSo4 solution and forms new product,zinc sulphate and Cu metal
Zn(s)+CuSO4(aq)→ZnSO4(aq)+Cu(s)
2)Double Displacement reaction-The reaction in which two different ions or group of atoms in the reactant molecules are displaced by each other is called Double displacement Reaction.
It is also called precipitation reaction as precipitate is produced in such reactions.
Example-(A)On adding sodium sulphate to barium chloride,a curdy white precipitate of barium sulphate and a solution of sodium chloride are formed.
Na2SO4(aq)+BaCl2(aq)→BaSO4+2NaCl(aq)
(B)On adding silver nitrate solution to sodium bromide,a yellow precipitate of silver bromide and solution of sodium nitrate are formed.
AgNO3(aq)+NaBr(aq)→AgBr↓+NaNO3(aq)
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