Chemical Reaction
Definition
A chemical reaction involves making new chemical bonds and breaking old chemical bonds.
Chemical reactions are chemical changes in which one or more chemical substances are formed from one or more the original substances by the conversion of reactants into products.
OR
A chemical reaction is the transformation of one chemical or collection of chemicals into another chemical or collection of chemicals.
The transformation of chemical substance into a new chemical substance by making and breaking of bonds between different atoms is known as Chemical Reaction.
Reactants are the starting substances that undergo chemical change while products are new substances formed as a result of chemical change.
Burning of wood, rusting of iron, the digestion of food, egg cooking, nail rusting, combustion of Sui gas, respiration etc. are all typical examples of chemical reactions.
Examples
Indicators
of chemical change or reaction
In a chemical reaction, at least one of the following will occur:
1. Change in state
2. Change in colour and odour
3. Change in temperature/energy change (heat, light)
4. Evolution of a gas
5. Formation of a precipitate (solid product)
Chemical Equation
A chemical equation is the symbolic representation of a chemical reaction in the form of chemical formulae, symbols, signs and directions in which the reactant entities are given on the left hand side and the product entities on the right-hand side.
A chemical equation is a precise and brief way of describing (or expressing or representing) a chemical reaction in short hand manner with the help of symbols and formulae of the substances involved.
The representation of chemical reaction by means of symbols of substances in the form of formulae is called chemical equation. E.g. - 2H2 + O2 ⇒ 2H2O
It is the short hand and symbolic representation of chemical change (reaction) in terms of symbols and formulae.
Total number of atoms on the LHS = Total number of atoms on the RHS
Examples
1. zinc metal reacts with dilute sulphuric acid forming zinc sulphate and hydrogen gas according to following equation.
2. Methane burns in air to form Carbon dioxide and Water vapours.
Balanced Chemical Equation
Definition
An equation in which the number of atoms of each element is same on both sides i.e. reactant and the product sides of the equation is called Balanced Chemical Equation (BCE). All chemical equations must be balanced in order to comply with the law of conservation of mass.
Chemical equations are balanced by using simple numbers in front of symbols and formulae which are called co-efficients or balancing numbers.
A balanced chemical equation has number atoms of each element equal on both left and right sides of the reaction.
*Note- According to 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.
For example
Coefficient
Chemical equations are balanced by using simple whole numbers in front of symbols and formulae called co-efficients. A co-efficient is a small simple whole number that place in front of symbols and formulae of elements or compounds involved in the reaction to equalize the number of atoms of each element on each side of an arrow. When no co-efficient is written, it is assumed (or understood) to be 1.
Balancing the Equation
The process of adjusting co-efficients in front of symbols and formulae of substances involved so that reactants and products have the same number of each kind of atoms on both sides of the equation is called balancing the chemical equations.
Generally balancing of equation is done by Trial and Error Method in which various co-efficients are tried keeping a count of each kind of atoms.
Characteristics of a Chemical Equation
1. A chemical equation must be a representative of chemical reaction.
2. It should represent molar quantities.
3. It should be balanced in terms of atoms or molecules of reactant and products.
Information obtained from Balanced Equation
1. It gives names of various reactants and products.
2. It gives formulae of reactants and products.
3. It gives number of molecules of each substance
4. It gives number of atoms of each substance.
5. It tells the number of moles of each substance.
6. It shows relative number of moles (molar quantities) of reactants and products.
7. It provides relative masses of the reactants and products.
8. It gives relative volumes of gaseous reactants and products.
Steps involved in balancing a chemical equation
1. Write the correct skeleton equation.
2. Start with the compound that has maximum kinds of atoms and they are balanced first.
3. Elementary substances are balanced last of all.
4. If required the whole equation is multiplied by 2 or some suitable number in order to make all the coefficients whole numbers.
Balancing of Ionic equation
A balanced ionic equation must satisfy mass as well as charge balance.
Limitations of a chemical equation
1. It does mention the state of the substance. Accordingly, the following symbols should be added to make it informative:
(s) for solids, (l) for liquid, (g) for gas, (vap) for vapour and (aq) for reactants or products present as an aqueous solution.
2. An equation does not reveal whether the reaction is complete or not.
3. It does not give any information regarding the speed of the reaction.
4. It does not give the concentration of the substances. In some cases, terms like diluted (dil) and concentrated (conc.) may be added.
5. It does not give the conditions of temperature, pressure, catalyst etc. This is overcome by mentioning these above or below the arrow.
Types of Chemical Reactions According to Composition
On the
basis of nature of reactants and products, chemical reactions have been divided
into five common types:
1. Combustion
2. Decomposition Reactions or Thermal Decomposition
3. Addition Reactions or Synthesis
4. Single Displacement Reactions or Substitution Reactions
5. Double displacement Reactions OR Double decomposition Reactions OR
Metathesis
(a) Neutralization
(b) Hydrolysis
(c) Precipitation
(d) Acid displacement reaction
(e) Base displacement reaction
Summary
of Types of Chemical Reactions-
I. Combustion-
The burning of a substance usually organic compound with oxygen to form carbon
dioxide and water and heat.
II. Combination-
When two elements or one element and one compound or two compounds combines to
give one single product.
III. Decomposition-
Splitting of a compound into two or more simple products.
IV. Displacement-
It takes place when a more reactive metal or non-metal displaces a less
reactive metal or non-metal.
V. Double
displacement- Reactions in which ions are exchanged between two reactants
forming new compounds.
VI. Neutralization-It
is a type of DDR in which acid and base react to form salt and water.
VII. Hydrolysis-It
is a type of DDR in which salt and water react to form acid and base.
VIII. Precipitation-
It is a type of DDR in which two reactants in solution react to form insoluble
compound called precipitate.
IX. Exothermic-
Reactions which produce energy are called exothermic reaction. Most of the
decomposition reactions are exothermic.
X. Endothermic-
Reactions which absorb energy are called endothermic reaction. Most of the
combination reactions are endothermic.
XI. Oxidation:
Gain of oxygen or removal of hydrogen or metallic element from a compound is
known as oxidation.
XII. Reduction:
Addition of hydrogen or removal of oxygen from a compound is called reduction.
XIII. Redox-
A chemical reactions where oxidation and reduction both take place
simultaneously are also known as redox reaction.
XIV. Rusting-
When iron reacts with oxygen and moisture forms a red substance called rust.
XV. Rancidity-
Oils and fats when get oxidized on exposure to air show a change in taste and
smell.
XVI. Corrosion-
Metals when attacked by oxygen, water, acids, gases, present in air changes its
surface which is called corrosion.
1. Combustion Reactions
Definition
The rapid reaction of a chemical substance on ignition
with either
free oxygen or oxygen of the air to give CO2
and water vapours,
with the rapid evolution (release) of heat and flame
is called
Combustion Reactions. Combustion is the characteristic reaction of
organic compounds especially of hydrocarbons.
Nature
Combustion is an exothermic process and evolves large amount of heat called heat of combustion and it is the fuel value of compounds.
Examples
1. Carbon (coal) burns in air with smoky flame to produce CO2 gas and heat.
2. Methane (sui gas or marsh gas), ethene and ethyne burn in air with non-luminous or luminous flame forming CO2 gas and water vapours releasing large amount of heat called heat of combustion.
2. Decomposition Reactions
Definition
A chemical reaction in which a single compound breaks down (or splits up) into two or more simpler substances by the application of heat (energy), light or electricity is called Simple Decomposition Reactions. In decomposition reactions, a single reactant breaks up into several simple products.
AB ¾¾¾® A + B DH = + kJ/mole
It is reverse or opposite of synthesis reactions, therefore, it is also known as Desynthesis Reactions.
Nature
These reactions are always endothermic i.e. heat energy is required to bring about decomposition of compounds, so these reactions more correctly called Thermal Decomposition. [Only compounds may decompose, elements cannot].
Examples
1. The industrial preparation of carbon dioxide involves the decomposition of lime stone (calcium carbonate) at high temperature. Calcium oxide is also formed.
2. In the laboratory preparation of oxygen gas, potassium chlorate is decomposed on heating into potassium chloride and oxygen gas.
3. Fermentation, the biochemical degradation of glucose by yeast into ethyl alcohol and carbon dioxide is another example of decomposition.
4. Ferrous sulphate on heating decomposes into ferric oxide releasing sulphur dioxide and sulphur trioxide gases.
Types of Decomposition
1. Thermal decomposition (decomposition of a compound by heat)
2. Photolytic decomposition/photolysis (decomposition of a compound by light)
3. Electrolytic decomposition (decomposition of a compound by electricity)
4. Cracking or pyrolysis
1.Thermal decomposition (decomposition of a compound by heat)
Mercury oxide, when heat, undergoes thermal decomposition to give silvery liquid mercury and oxygen
Similarly, if blue crystals of copper nitrate are heated, they thermally decomposes to give black coloured copper oxide, reddish brown fumes of nitrogen dioxide and a colourless gas of oxygen.
2. Photolytic decomposition/photolysis (decomposition of a compound by light)
3. Electrolytic decomposition (decomposition of a compound by electricity)
When water taken in an electrolytic cell, is acidified with a small quantity of sulphuric acid and a direct current passed through it, it undergoes electrolytic decomposition to yield hydrogen and oxygen gas.
If an electric current is passed through molten lead bromide, it electrolytically decomposes to give lead and bromine.
1. Decomposition of Carbonates Salts into metal oxide and carbon dioxide gas
2. Decomposition of Bicarbonates Salts into carbonate salt, water and carbon dioxide
3. Decomposition of Group II Nitrates Salts into metal oxide, NO2 gas and O2 gas
4. Decomposition of Group IA Nitrates Salts into metal nitrite and O2 gas
3. Synthesis or Addition Reaction
A chemical reaction in which two or more substances (element or compound) combine to form a single compound is called an Addition Reaction or Synthesis Reaction or Combination Reaction or Composition Reaction or Formation Reaction”. In combination reaction, two or more reactants combine to form one single product.
A + B ¾¾¾® AB DH = - kJ/mol
These reactions are reverse of decomposition reactions.
In organic chemistry, addition reactions are characteristic of unsaturated organic compounds, which are of three types namely electrophilic addition, nucleophilic addition and polymerization.
Nature
Addition reactions are usually exothermic. Very few combination reactions involving molecules of reactants (molecular addition reactions) are endothermic e.g. formation of NO2 from nitrogen and oxygen.
Types
Addition reactions may take place between metal and a non-metal, between two non-metals, between a compound and an element (non-metal) or between two compounds. Thus addition reactions are of three types:
1. Combination of two elements metal and a non-metal, between two non-metals to form a compound
2. Combination reactions involving an element and a compound
3. Combination reactions involving two compounds
Examples
4. Single Displacement Reaction/Substitution Reactions
A chemical reaction in which an atom or group of atoms of a molecule of a compound is displaced by another atom or group of atoms is called Single Displacement Reaction or Simple Displacement or Substitution Reaction. In this reaction atoms of one element replace the atoms of another element from the molecule of a compound.
In displacement reactions, a more reactive metal or non-metal can displace a less reactive metal or non-metal from their compounds in aqueous solutions. (However, a less reactive metal or non-metal cannot displace a more reactive metal or non-metal). In these reactions, elements which are higher in the activity series displace those elements which are placed below them i.e. more electropositive elements displace lesser electropositive elements. Conversely, more electronegative elements will displace lesser electronegative elements.
The tendency of an atom or radical to displace another depends upon its electropositive or electronegative nature. The general principle of displacement is that a more electropositive element can displace a less electropositive element form its compound (i.e. a more reactive metal can displace a less reactive metal from its compound). Similarly a more electronegative element can replace a weak or less electronegative element form its compound.
The activity series of metals helps in guiding the displacement of an electropositive element form its compound by another electropositive element. The general rule for metal displacement is that a metal lying above in activity series can displace a metal lying below it.
Examples
The displacement power of a non-metal depends upon its reactivity which in turn depends upon electronegativity (E.N). A more electronegative element can displace a less electropositive element from its compounds. The most electronegative element is F (E.N. = 4), oxygen is the 2nd most electronegative element (3.5) and Cl is the 3rd most electronegative element (3). The electronegativity of Br is 2.8 and that of I is 2.5. The electronegativity of S is 2.8 and that of H is 2.1.
1. Chlorine replaces bromine or iodine from bromide or iodide salts to form respective chloride and bromine vapours or iodine vapours.
The displacement power of a metal depend upon its reactivity which in turn depend upon its position in the reactivity series which is a convenient summary of the results of many possible metal or hydrogen displacement reactions. This series is based on metal-acid reaction and metal compound-metal reaction. According to this series, any metal above hydrogen will displace it from water or from an acid, but metals below hydrogen will not react with either water or an acid. [In fact, any species listed in the series will react with a compound containing any species listed below it]. A list of metals arranged in order of decreasing ease of oxidation is called an Activity Series. It ranks the elements in order of their reducing ability in aqueous solution. The metals at the top are most readily oxidized and are stronger reducing agents whereas the metals at the bottom of the series are less readily oxidized and are weaker reducing agents.
1. Some metals displace hydrogen from dilute acids, bases or even water or alcohol to produce hydrogen gas along with respective metal salts.
2. Magnesium replaces copper from copper sulphate solution to form magnesium sulphate and copper.
5. Double Displacement Reactions/metathesis reactions
Definition
A chemical reaction in which two compounds (reactants) are decomposed to form two new compounds by exchanging (interchanging) their ions or radicals is called DoubleDecomposition/Double Displacement Reaction (DDR).
Owing to formation of two new compounds, such reactions are also called metathesis reactions.
These reactions involve the exchange of partners of two compounds, which is usually brought about by an exchange of their ionic radicals i.e. exchange of ions occurs between two compounds. That is why; these reactions are also referred as Exchange Partners Reactions.
Examples
Types of DDR
They are of following four types:
1. When the aqueous solution of two compounds react by exchanging their respective ions, such that one of the products formed is insoluble salt and appears in the form of a precipitate, then the reaction is said to be precipitation reaction.
2. When the aqueous solution of two compounds acid and base reacts by exchanging their respective ions, such that only salt and water are formed, then the reaction is called neutralization reaction.
3. When the aqueous solution of two compounds react by exchanging their respective ions in such a manner that one of the product formed either decomposes into gaseous compounds or is formed in the gaseous state, then the reaction is called gas-forming reaction.
5.A Precipitation Reactions
Definition
A double decomposition reaction in which aqueous solution of two compounds (reactants) are decomposed to form two new compounds by exchanging (interchanging) their radicals one of which is in the form of solid particles called precipitate is called precipitation reaction. The common precipitating agents are NaOH, dilute H2SO4, dilute HCl, H2S, NH4OH, (NH4)2CO3, Na2HPO4, AgNO3, Na3PO4, Na2CO3 etc.
In precipitation reactions, a white or a coloured insoluble compound is formed which is known as precipitate denoted as ppt. A reaction that produces a ppt of a compound is called a precipitation reaction.
Examples
Precipitation by using sodium carbonate
The mixing of aqueous solutions of calcium chloride with sodium carbonate, gives two new different compounds (sodium chloride and white ppt of calcium carbonate) by the exchange of their radicals.
Precipitation by using silver nitrate
Silver nitrate is used to precipitate chlorides, bromides, iodides and sulphates radicals from their compounds. The respective ppt of silver chloride, silver bromide, silver iodide and silver sulphate are white, light yellow, deep yellow and white in colour respectively.
5.B Hydrolysis
Definition
The type of double decomposition reaction in which two reactants are salt and water which react to form acid and base resulting in acidic or basic solution is referred to as Hydrolysis and the salt is said to be hydrolyzed. It is reverse of neutralization.
Examples
1. Hydrolysis of Salts of Weak Acids and Strong Bases
Salts of weak acids with strong bases undergo hydrolysis with water producing basic solution due to formation of strong base. Salts of weak acids and strong bases (such as CH3COONa, Na2CO3, KCN, NaCN etc) hydrolyze in water to produce weak acid and strong base. Thus due to formation of strong base, concentration of OH– ions become greater which changes the pH of solution towards basic (pH > 7) and turn red litmus blue. Therefore, solution becomes basis. e.g.
Aqueous Solution of CH3COONa is basic due to hydrolysis
2. Hydrolysis of salts of strong acids and weak bases
Salts of strong acids with weak bases undergo hydrolysis with water producing acidic solution due to formation of strong acid. Salts of strong acids and weak bases (such as CuSO4, NH4Cl, ZnCl2, AlCl3, FeCl3 etc.) hydrolyze in water to produce strong acid and weak base. Thus due to formation of strong acid, concentration of H+ ions become greater which changes the pH of solution towards acidic (pH < 7) and it turns blue litmus red. Therefore, solution becomes acidic. e.g.
Aqueous Solution of CuSO4 is acidic due to hydrolysis
3. Hydrolysis of salts of strong acids and strong bases
Salts of strong acids and strong bases (such as NaCl, KCl, Na2SO4, NaNO3, KNO3) do not hydrolyze in water. Therefore pH of solution remains same and thus solution becomes neutral. e.g. Aqueous NaCl solution is neutral.
5.D. Neutralization
Definition
Acids are
the substances which give H+
ion in aqueous solution e.g. HCl, HNO3 H2SO4 H2CO3
and CH3COOH etc. Bases are compounds that yield OH– ions
in water. The water-soluble bases are
called Alkalis. e.g. Caustic Soda
(NaOH), Caustic Potash (KOH),NH4OH,Ca(OH)2, Ba(OH)2. Salt is an ionic compound that is the
neutralization product (other than water) of an acid and base, which is the
aggregation of cation and anions. e.g. Na+Cl–.
If we add an acid solution to a base solution drop by drop, the acidic
character of the acid decreases gradually. A stage will reach at which the
resultant solution becomes neutral to litmus. This stage is called
neutralization.
The type of double
decomposition reaction in which equivalent quantities of two reactants acid and
base react to form salt and water is called neutralization. i.e.
Since
naturalization reaction always involves the reaction between an H+
ion of an acid and a OH¯ ion of a base, therefore, water is the resultant
product. During neutralization, cation of base and anion of acid remains in
solution and do not react. These ions are called Spectator Ions. Thus
neutralization may also be defined as:
“The mutual chemical combination of
hydrogen ion or proton (H+) of an acid and hydroxide (OH–) ion of a base to
form neutral water molecule is called Neutralization”. Thus neutralization
reactions can be denoted by a single net ionic equation:
Examples
Nature
The neutralization is an exothermic reaction
releasing heat called heat of neutralization which is the amount of heat
evolved during a neutralization in which 1 mole of water is formed. In other words, heat of neutralization is the amount of heat evolved
when 1mole of H+ ions of an acid reacts with 1mole of OH– ions of a base to form salt and 1 mole of
water. In
fact, heat of neutralization is the heat of formation of water from H+ and
OH– ions.
The heat of neutralization for any strong acid
against any strong base is approximately same i.e. - 13700 cal/mol or -57.3kJ/mol.
The heat of neutralization for weak acid
against weak base or strong acid against weak base (i.e. in case where either
acid or base is not completely ionized and the neutralization reaction may not
go to completion) may be less than 13700 cal e.g. when
strong acid HCl reacts with a weak base Ca(OH)2, the heat of
neutralization is 24700 cal or 12350 cal/mol.
Types of Neutralization
Neutralization is of following two types:
a) Complete Neutralization
A neutralization in which all H+
ions of the acid are neutralized by OH– ions of base or vice versa is known as Complete Neutralization.
Salt obtained by Complete Neutralization do
not have replaceable hydrogen atoms or hydroxide ions and normal (neutral) e.g.
NaCl, NaNO3, K2SO4, KNO3, KCl etc.
b) Partial Neutralization
A neutralization in which all H+
ions of the acid are not neutralized by OH– ions of base or vice versa (i.e. all OH– ions of a base are not neutralized by the acid) is known as Partial
Neutralization.
Salt obtained by partial Neutralization have
either replaceable hydrogen ions or hydroxide ions and are either acidic or
basic.
Salts formed by the partial neutralization of
an acid by base containing replaceable hydrogen ion are acidic. They further
react with bases to form normal salts. e.g. NaHSO4, KHSO4, NaHCO3,
KHCO3 Na2HPO4, K2HpO4,
NaH2pO4, KH2PO4.
Salts formed by the partial neutralization of
a base by an acid containing replaceable hydroxyl group are basic. They further
react with acids to form normal salts. e.g. Ca(OH)Cl, Mg(OH)Cl, Zn(OH)Cl etc.
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