What are chemical reactions?
Chemical reactions are the processes in which a rearrangement of atoms takes place between the reacting substances to form new substances having entirely different properties. During a chemical reaction breaking down of old bonds and formation of new bonds takes place. In a chemical reaction atoms of one element do not change into those of another element, only a rearrangement of atoms takes place. A few examples are rusting of iron. spoiling of milk in summer, cooling of food, digestion of food etc.
What are reactants and products?
Reactants: The substances which take part in chemical reaction are called as reactants. Products: The new substances formed as a result of a chemical reaction are called products .For example: in the burning of coke in 'air, carbon and oxygen are the reactants while carbon dioxide formed is the product.
C+02------------------>CO2
(coke)
Reactants - Product
Similarly, in the burning of magnesium in air, magnesium and oxygen are the reactants. While magnesium oxide formed is the product.
2Mg +02------------------------> 2Mgo
Reactants Products
State the characteristics of chemical reactions?
The easily observable changes that take place in a chemical reaction are called the characteristics of the chemical reaction. Some important characteristics of chemical reactions are given below.
(1) Formation of precipitate: Precipitate is a solid substance formed on mixing of two
solutions. Some chemical reactions are accompanied by the formation of a precipitate
between the reactants. e.g.
AgNO. (ar). + NaCl (aq)----> AgCl (1) ppt - NaNO, (aq)
(2)Evolution of a gas: In some chemical reactions evolution of a gas takes place.
In the reaction between zinc and dilute sulphuric acid (or HCl) hydrogen gas is evolved.
Zn+H2SO4------------->ZnSO4 (aq) + H2
(3) Change of colour: In some chemical reactions change in colour takes place as a result of chemical reaction. c.g. Lead nitrate and potassium iodide solutions are colourless, but on mixing the two yellow coloured ppt of lead iodide is formed.
PbNO3 (aq) +KI (aq)------------->Pb 1+KNO: (aq)
colourless) (colour less) (yellow colour ppt)
(4) Change in state: in some chemical reactions change in state takes place as a result of chemical reaction .e.g. solid wax in the form of candle) burns to form water vapours and carbon dioxide which are gaseous.
Wax + air------------------->.CO2 + H2O
(sol.) (gas) (g) (g)
(5) Change in temperature: In some chemical reactions, rise or fall in temperature takes place as a result of chemical reaction, e.g. when water is added to quick lime (Cao), it results in the formation of slaked lime Ca (OH)2. During this reaction a large amount of energy is evolved.
Ca0 +H2O--------------->Ca (OH)2 + heat
Similarly, when barium hydroxide Ba(OH)2 is added to ammonium chloride NH4CI a ppt of
BaCl2, is formed. Only when heat energy is absorbed from the surroundings.
Ba(OH)2 + 2NH4Cl+heat------------------->BaCl2 +2NH4OH
BALANCING OF CHEMICAL EQUATIONS
The method by which the number of atoms of each element on both sides of the arrow (-->) in a chemical reaction are made equal, is called balancing of chemical equation.,
WHY IS IT NECESSARY TO BALANCE A CHEMICAL EQUATION?
In a balanced chemical equation, the number of atoms of each element on both the sides should be equal. This is because; no matter is lost or gained during a chemical reaction, (law of conservation of matter). Therefore, balancing of a chemical equation is necessary because no matter. Hence, no atom is lost or gained during a chemical reaction.
HOW ARE CHEMICAL EQUATIONS BALANCED?
Chemical equations are balanced by adjusting the coefficients placed before the symbols or formulae of the reactants and products. There are a few methods which can be used for the balancing of chemical equations.
( i) Hit and trial method
( ii) Partial equation method.
( i) HIT AND TRIAL METHOD This method is also called trial and error method, or inspection method. In this method, coefficients before the formulae or symbols of the reactants and products are adjusted in such a way that the total number of atoms of each element on both sides become equal .This is called material balance, or mass balance. In this method first of all, atoms of the element which appears least in the chemical equation should be balanced. Then, the next one, and so on. Example: Balance the equation,
Mg + HCl MgCl2 + H2
Solution: (i) Each type of atom is counted on each side of the equation. Then we decide which atoms are unbalanced.
Left side Right side
Mg 1 1
Cl 1 2
H 1 2
We see that Cl and H atoms are unbalanced. In case all the atoms are balanced, there is no need to proceed further, as we already have a balanced equation.
(ii) The most complicated formula of the equation is used to balance atoms other than H and O. In this equation, MgCl2 is the most complicated formula. The equation is already balanced with respect to Mg. So, we can balanced Cl by setting 2 just before HCl.
Mg + 2HCl MgCl2 + H2
By doing so hydrogen is also balanced
(iii) Each type of atom is now counted on both sides of the arrow to check whether or not the equation is balanced.
Left side Right side
Mg 1 1
Cl 2 2
H 2 2
Since all types of atoms are equal in number, the equation is balanced. The balanced equation is Mg + 2HCl MgCl2 + H2
What is a chemical equation. What are the steps involved in writing a chemical equation
Ans) A shorthand representation of a chemical reaction in terms of symbols and formula of declarant reactants and products is called a chemical equation while writing a chemical• equal in the following steps are involved
. Step 1 : Identify the reactants and the products of the chemical reaction.
Step 2 : Write down the formulae or symbols of the reactants on the left hand side with a sign of plus (+) between them. The formulae or symbols of the products formed in the reaction are written on the right hand side with a sign of plus (+) between them. Such a chemical equation is called the skeleton equation.
Step 3 : Count the number of atoms of each element on both the sides. If the numbers of atoms of each element on both the sides are equal, then the equation is called a balanced chemical equation. If the number of atoms of any one or more of the elements on both the sides are not equal, then these are made equal by adjusting the coefficients before the symbols and formulae of the reactants and element on both sides are sides are made equal, is called balancing of chemical equation.
Step 4 : In the end, the chemical equation is made molecular, if required.
How is to make chemical equations more informative?
Ans) The chemical equations can be made more informative in following ways:
(1) By indicating the physical states" of the reactants and products,
(2) By indicating the heat changes" taking place in the reaction.'
(3) By indicating the condition" under which the reaction takes place.
(1) By indicating the "physical states” of the reactants and products:
The reactants and products of a chemical reaction could be in any of following four states:
Solid state which is represented by "S".
Liquid state which is indicated by "L"
Aqueous solution which is indicated by "aq”.
Gaseous state which is represented by "g".
These stated symbols of the reactants and products are written just after their formula in an equation, e.g. consider the following reaction:
Ca (OH)2 (aq)+Co2(g)----------------------> CaCO3 (s) + H2O (l)
Calcium hydroxide calcium carbonate (Lime) water
( white ppt)
Further, if a reaction involve an acid as reactant. The acid may be dilute or concentrated. For dilute acids symbol "dil." is used and for concentrated acids symbol "conc." is used.
Zn (s) + dil. H2SO4------------------>ZnSO4 (aq) + H2(g)
Zin sulphuric acid zinc sulphate hydrogen
(2) To indicate the heat changes in an equation: There are two types of reactions on the basis of heat changes involved.
(i) Exothermic reactions
(ii) Endothermic reaction
(i) Exothermic reactions: The chemical reactions in which heat energy is given out
(evolved) are called exothermic reactions, e.g.
C(s) + O2(g)---------------------->CO2 (g) + heat
In exothermic reaction heat evolved is indicated by writing” + heat" on product side.
(ii) Endothermic reactions:- The chemical reactions in which heat energy is absorbed are called endothermic reactions
CaCo3+heat------------------------>CaO(s) + CO2(g)
calcium carbonate Calcium oxide carbon dioxide
In endothermic reaction heat is absorbed indicated by writing "+heat" on reactant side or heat on product side or the sign delia (∆) is put over the arrow of the equation.
2KCIO(S)------------------------------->2Kcl(s) O2(g)
Polarssium MnO2 Potassium Chloridechlorate
To indicate the conditions under which the reaction takes place.:-
If the reaction takes place in presence of a catalyst then the symbol or the formula of the catalyst is also written above or below the arrow sign in the equation, e.g.
CO(g) + 2H2(g) --------------------> CH3OH(l)
Carbon monoxide Hydrogen ZnO+CrO3 methanol
Name and discuss the various types of chemical reactions? (V.V.Imp)
Depending upon the types of chemical change the chemical reactions have beenclassified into the following types:
(i) Combination reactions
(ii)Decomposition reactions
(III) Displacement reactions
(iv) Double displacement reactions
(v) Precipitation reactions
(vi) Neutralization reactions
(vii) Reduction-oxidation (redox) reactions
(I)Combination reactions: Combination reactions are those reactions in which two
or more compounds react to form single product. Combination reactions are also
called as synthesis reactions.
2H2 + 02-------------------------------->2H2O
Hydrogen oxygen water
2Na + Cl2 ------------------------------> 2Naci
sodium chlorine sodium chloride
(ii) Decomposition reactions: Decomposition means the breaking down of a compound into two or more simpler compounds or elements. The reactions in which a compound breaks up into two or more simpler compounds are called decomposition reactions. Most of the decomposition reactions are endothermic in nature and require energy in the form of heat, light or electricity. Hence, decomposition reactions are of three types.
(a) Thermal decompositions. It is a decomposition reaction brought about by heat, e.g.
CaCO(S)--------------------------->CaO(s)+CO2(g)
(b) Photolysis: It is a decomposition reaction brought about by light, e.g.
2AgCl(s) --------------sunlight-----------> 2Ag + Cl2
(C) Electrolysis: It is a decomposition reaction brought about by electricity.
2NaCl------electric current---------------------2Na(s) + Cl2(g)
(III) Displacement reactions: Displacement reactions are also called as substitutionreactions. The reactions in which an atom or group of atoms in a molecule isdisplaced by another atom or group of atoms are called as displacement reactions.
Zn(s) + CuSO4(aq)------------->ZnSO4(aq) + Cu(s)
2Cv + 2AgNO3------------------>Cu(NO3)2 + 2Ag
Cl2(g) + 2NaBr(aq)-------------->2NaCl + Br2(I)
These reactions are accompanied by the displacement of a less reactive element by a
more reactive element from its compound.
(iv)Double displacement reaction: A chemical reaction in which there is an exchange of atoms or group of atoms between the reactants to form new substances is called a double displacement reaction. The reactions generally occurs in aqueous medium and such double displacement reactions take place with the:
(a) Formation of precipitate
(b) Evolution of a gas
(c) Change in colour
Examples of formation of Ppt
BaCl2 (aq) + Na2SO4 (aq)------>BaSO4 + 2NaCl (aq)
Barium chloride sodium sulphate Barium sulphate (ppt) sodium chloride
Example of Evolution of gas
FeS(s) + 'dil. H2SO4---------------------->Fe$O4(aq) + A2S(g)
Iron sulphide sulpheric acid iron sulphate hydrogen sulphate
Example of change in colour
CuSO4(aq + H2S(g)---->CuS + H2SO4(aq)
Copper sulphate hydrogen sulphate copper sulphide sulpheric acid
(blue in colour) (black ppt)
(iv) Precipitation reaction: A ppt. is a solid substance formed on mixing two solutions. Reactions which are accompanied by the formation of ppt are known as precipitation reactions, e.g.
NaCl(aq) + AgNO3(aq) ---------------------> AgCl(s) + NaNO3(aq)
Sodium chloride Silver nitrate Silver chloride sodium nitrate
(white ppt)
BaCl2(aq) + Na2SO4(aq)-----------------------> BaSO4(s) + 2NaCl(aq)
Barium chloride sodium sulphate Barium sulphate Sodium chloride
(V) Neutralization reactions: A reaction in which an acid reacts with a base to form salt and water is called neutralization reaction.
Acid + base--------------------->salt + water
e.g. HCl(aq) + NaOH(aq)---------------NaCl + H2O
NH4OH + HCl---------------------------> NH4Cl + H2O
(vi) Oxidation-reduction reactions:
(a) Oxidation: Oxidation may be defined as a chemical process in which a substance gains oxygen or loses hydrogen, e.g.
4Na,+O2------------------------------2Na2O
sodium oxygen sodium oxide
According to modern electronic concept, oxidation is a process in which an atom or an ion loses one or more electrons
Ña------------------------------------------Na+ +e– (oxidation)
2N-------------------------------------------->2N2+ +2e– (oxidation)
(b) Reduction: Reduction is a process in which a substance gains hydrogen or loses oxygen.
e.g.
Cu O + H2------------------------------>Cu + H2O
Copper oxide copper
According to modern electronic concept reduction is a process in which an atom or ion gains one or more electrons, e.g.
Cu2+ + -------------------------> 2e– (reduction)
Zn + Cu2+------------------------------------> H2'(reduction)
(Reduction reactions)
Oxidation and reduction take place simultaneously in a reaction and such a reaction is called redox reaction.
State the effect of oxidation reaction in everyday life?
The commonly observed effects of oxidation are as follows:
(a) Corrosion: The process of slowly, eating up of the metals by the action of atmospheric gases such as oxygen, carbon dioxide, hydrogen sulphide and moisture is known as corrosion. During corrosion metals changes into their oxides, carbonates, sulphides etc. Some examples of corrosion are:
(i) Formation of oxide layer on the surface of aluminium.
ii) Tarnishing of silver metal on exposure to hydrogen sulphide.
(iii) The formation of reddish brown layer on the surface of iron articles etc.
• Corrosion in case of iron is called rusting. Chemically rust is hydrated ferric oxide,
i.e. Fe2o3 x H2O
(b) Rancidity: When oils and fats or foods containing oils and fats are exposed to air oroxygen, they get oxidized due to which the food becomes stale and its colour and smellchanges. It is called rancidity.
0 Comments