RATE OF RATE OF CHEMICAL CHEMICAL

REACTION AND REACTION AND CHEMICAL CHEMICAL

EQUILIBRIUMEQUILIBRIUM

CHEMICAL REACTIONSCHEMICAL REACTIONS

A chemical change is called A chemical change is called a chemical reaction.a chemical reaction.

Iron rusts when kept exposed Iron rusts when kept exposed to humid air for a to humid air for a considerable time.considerable time.

Change of milk into curd.Change of milk into curd. Burning of a paper.Burning of a paper. Combination of Nitrogen and Combination of Nitrogen and

Hydrogen to form AmmoniaHydrogen to form Ammonia

SLOW CHEMICAL SLOW CHEMICAL REACTIONSREACTIONS The reactions which take place in a few The reactions which take place in a few

minutes or more reactants are called minutes or more reactants are called slow reactions.slow reactions.

When Rusting of ironWhen Rusting of iron Weathering of rocksWeathering of rocks Generally the reactions between Generally the reactions between

covalent compounds are slowcovalent compounds are slow When ethyl alcohol and acetic acid are When ethyl alcohol and acetic acid are

heated in the presence of a little heated in the presence of a little concentrated sulphuric acid(catalyst), concentrated sulphuric acid(catalyst), the reaction takes place in a few minutes the reaction takes place in a few minutes and an ester called ethyl acetate is and an ester called ethyl acetate is formed. This is a slow reaction.formed. This is a slow reaction.

FAST CHEMICAL FAST CHEMICAL REACTIONSREACTIONS

The reaction which takes place The reaction which takes place immediately on mixing the immediately on mixing the reactants are called fast reactants are called fast reactions.reactions.

Reaction between an Acid and a BaseReaction between an Acid and a Base HCl(aq) + NaOH(aq) HCl(aq) + NaOH(aq) NaCl(aq) + NaCl(aq) +

HH22O(l)O(l)

2Mg(s) + O2Mg(s) + O22(g) (g) 2 MgO(s) 2 MgO(s)

RATE OF A CHEMICAL RATE OF A CHEMICAL REACTIONREACTION

The rate of a chemical reaction The rate of a chemical reaction means how fast the reaction is means how fast the reaction is taking place by indicating how much taking place by indicating how much of a reactant is consumed, or how of a reactant is consumed, or how much of a product is formed in a much of a product is formed in a given time.given time.

Therefore Reaction rate = Therefore Reaction rate =

Change in concentration of a Change in concentration of a reactant or product/ time takenreactant or product/ time taken

UNIT OF RATE OF UNIT OF RATE OF REACTIONREACTION

The concentration is usually The concentration is usually expressed in molarity M expressed in molarity M

( the number of moles of ( the number of moles of substance dissolved per litre of substance dissolved per litre of the solution) and time in secondsthe solution) and time in seconds

Then unit of the rate of reaction Then unit of the rate of reaction is is moleLis is moleL-1-1/s or molesL/s or molesL-1-1ss-1-1

REACTION RATEREACTION RATEWhen acidified hydrogen peroxide (HWhen acidified hydrogen peroxide (H22OO22) is ) is

added to a solution of potassium iodide added to a solution of potassium iodide (KI) , iodine is liberated(KI) , iodine is liberated

HH22OO22(aq)+2 KI (aq) + H(aq)+2 KI (aq) + H22SOSO44(aq) (aq) 2H2H22O(I)+ O(I)+ II22(aq)+ K(aq)+ K22SOSO44(aq)(aq)

Here the concentration of iodine is zero Here the concentration of iodine is zero initially. With the passage of time, it initially. With the passage of time, it increases and the reaction solution increases and the reaction solution becomes brownish. Concentration of iodine becomes brownish. Concentration of iodine can be measured at different intervals of can be measured at different intervals of time by titration against sodium time by titration against sodium thiosulphate.thiosulphate.

REACTION RATEREACTION RATE

If the concentration of iodine rises from If the concentration of iodine rises from 0 to 100 to 10-5-5mol Lmol L-1-1 in 10 seconds, we write in 10 seconds, we write

∆ ∆ [ I[ I22] 10] 10-5-5mol Lmol L-1-1

Reaction rate = ----------- = --------------Reaction rate = ----------- = --------------

∆ ∆ t t 10s10s

= 10= 10-6-6 mol L mol L-1-1ss-1-1

Here, symbol ∆ represents a change Here, symbol ∆ represents a change and [Iand [I22], molar concentration of iodine.], molar concentration of iodine.

AVERAGE RATEAVERAGE RATE The rate of a reaction which is obtained The rate of a reaction which is obtained

by measuring the change in by measuring the change in concentration of a reactant or product concentration of a reactant or product over a considerable period of time then over a considerable period of time then it is called average reaction rate.it is called average reaction rate.

Suppose iodine( ISuppose iodine( I22) is being evolved in a ) is being evolved in a reaction as a product and we measure reaction as a product and we measure the change in its concentration over a the change in its concentration over a time, say 10 seconds, so in this case time, say 10 seconds, so in this case

∆∆ [I[I22] / ] / ∆∆ t will give us an average t will give us an average reaction rate.reaction rate.

So Average rate of reaction = So Average rate of reaction = ∆∆ [I [I22]/ ]/ ∆∆ t t

INSTANTANEOUS RATEINSTANTANEOUS RATE The rate of a reaction at a particular The rate of a reaction at a particular

instant of time is called instantaneous instant of time is called instantaneous reaction rate.reaction rate.

It is the accurate estimate of reaction rate at It is the accurate estimate of reaction rate at any moment.any moment.

In order to determine the rate at a particular In order to determine the rate at a particular instant, one should make the time interval instant, one should make the time interval almost zero. i.ealmost zero. i.e

d[Id[I22]] Instantaneous rate = -------------- Instantaneous rate = --------------

dt dt Here d[IHere d[I22] denotes very small change in the conc. ] denotes very small change in the conc.

of Iof I22 and dt denotes very small change in time and dt denotes very small change in time

RATE OF REACTIONRATE OF REACTION One can think of expressing the rate in terms of One can think of expressing the rate in terms of

change in concentration of Hchange in concentration of H22OO22 and KI is and KI is

expressed as,expressed as, ∆∆[H[H22OO22] 1 ] 1

∆∆[KI][KI] Rate of reaction = ------------------ = - --- ---------Rate of reaction = ------------------ = - --- ---------

∆ ∆ t 2 t 2 ∆∆ t t Here negative sign indicates that concentration Here negative sign indicates that concentration

of Hof H22OO22 and KI will decrease with time. Here the and KI will decrease with time. Here the rate of consumption of KI is twice the rate of rate of consumption of KI is twice the rate of consumption of Hconsumption of H22OO22 therefore, in order to make therefore, in order to make the two rates equal we divide it by two the two rates equal we divide it by two (stoichiometric coefficient) and write (stoichiometric coefficient) and write ½( ½( ∆∆[[KI]/ KI]/ ∆∆ t) t)

RATE OF REACTIONRATE OF REACTION Cisplatin, Pt(NHCisplatin, Pt(NH33))22ClCl22 is used in is used in

chemotherapy of cancer. It reacts chemotherapy of cancer. It reacts with water and releases chloride ion with water and releases chloride ion bound to central platinum metal. bound to central platinum metal. The reaction is represented as The reaction is represented as Pt(NHPt(NH33))22ClCl22 + H + H22O O Pt(NH Pt(NH33))22ClCl++ + + ClCl--

Here the conc. of cisplatin Here the conc. of cisplatin decreases with lapse of time but decreases with lapse of time but conc. of Cl increases. conc. of Cl increases.

REACTION RATE REACTION RATE CURVESCURVES

Reaction Rate Graphs -Reaction Rate Graphs -

Here conc. of reactant changes with time

Here the conc. Of Product changes with time

Time (s) Time (s)

Conc. of reactants(mol L-1)

Conc of products (mol L-1)

QUESTIONSQUESTIONS Give an example of slow reaction.Give an example of slow reaction. Give an example of fast reaction.Give an example of fast reaction. Which reactions are slow and which are fastWhich reactions are slow and which are fast Rusting of iron, Burning of Magnesium wire, Rusting of iron, Burning of Magnesium wire,

setting of cement, Formation of coal, formation setting of cement, Formation of coal, formation of sodium oxideof sodium oxide

Fill in the blanksFill in the blanks Reactions between covalent compounds are Reactions between covalent compounds are

usually -----usually ----- Reactions between ionic compounds are very Reactions between ionic compounds are very

------------

QUESTIONSQUESTIONS Define the rate of a chemical reaction ( in term products and reactants)Define the rate of a chemical reaction ( in term products and reactants) What is meant by average rate of reaction.What is meant by average rate of reaction. What is meant by instantaneous reaction rate.What is meant by instantaneous reaction rate. Why do we put a minus sign in the formula for the rate of a reaction in Why do we put a minus sign in the formula for the rate of a reaction in

terms of the change in conc. of a reactant but no such sign in case of terms of the change in conc. of a reactant but no such sign in case of product.product.

Write the unit for expressing i) the concentration of an aqueous Write the unit for expressing i) the concentration of an aqueous solution ii) rate of a chemical reactionsolution ii) rate of a chemical reaction

Define molarity ?Define molarity ? Draw a graph to show the variation of conc. Of i) a reactant with time Draw a graph to show the variation of conc. Of i) a reactant with time

ii) a product with time.ii) a product with time. Write the chemical name and formula of Cisplatin. For what purpose is Write the chemical name and formula of Cisplatin. For what purpose is

it used.it used. A solution contains 0.021 mole of a substance in 3.0 L of the solution. A solution contains 0.021 mole of a substance in 3.0 L of the solution.

What is its molarity ?What is its molarity ? T he formula of H2O2 has been put within a square bracket as [H2O2]. T he formula of H2O2 has been put within a square bracket as [H2O2].

What does it represent.What does it represent. Consider the reactionConsider the reaction 2K2K

ENERGY CHANGES IN A ENERGY CHANGES IN A CHEMICAL REACTIONCHEMICAL REACTION

Most of the chemical reactions are Most of the chemical reactions are accompanied by energy changes. accompanied by energy changes. The energy is either released or The energy is either released or absorbed during a chemical absorbed during a chemical reaction. In some reactions, energy reaction. In some reactions, energy is given in form of heat and in some is given in form of heat and in some cases, in form of light.cases, in form of light.

EXOTHERMIC EXOTHERMIC REACTIONREACTION

Exothemic reaction – reactions which Exothemic reaction – reactions which release heat release heat

Ex- all combustion reactions.Ex- all combustion reactions. CHCH44(g) + 2O(g) + 2O22(g) CO(g) CO22(g) +2H(g) +2H22O(l) O(l)

+energy(890.4 kj mol-1)+energy(890.4 kj mol-1) 2AL(s) + Fe2AL(s) + Fe22OO33(s) Al(s) Al22OO33(s) + 2 Fe(l)+ (s) + 2 Fe(l)+

heatheat

N2 (g) + 3H2 (g) 2NH3 (g) + heatN2 (g) + 3H2 (g) 2NH3 (g) + heat

Fe

ENDOTHERMIC ENDOTHERMIC REACTIONREACTION

endothermic reactions- reactions endothermic reactions- reactions which absorb heat.which absorb heat.

Ba(OH)Ba(OH)22.8H.8H22O(s) + 2NHO(s) + 2NH44SCN(s)+ SCN(s)+ heat Ba(SCN)heat Ba(SCN)22(aq) + 2NH(aq) + 2NH33(aq) + (aq) + 10H10H22O(l)O(l)

NN22 (g) + O (g) + O22(g) + heat -- 2 NO ( g ) (g) + heat -- 2 NO ( g )

PHOTOCHEMICAL PHOTOCHEMICAL REACTIONREACTION

Photochemical reactions – Reactions that Photochemical reactions – Reactions that take place in presence of light. For example, take place in presence of light. For example, photosynthesis of plants. Photographic films photosynthesis of plants. Photographic films which are coated with silver bromide (very which are coated with silver bromide (very small amount of silver iodide) undergo small amount of silver iodide) undergo chemical reaction when exposed to sunlight.chemical reaction when exposed to sunlight.

CHCH44 (g) + Cl (g) + Cl22 (g) (g) CH CH33Cl (g) + HCl(g)Cl (g) + HCl(g)

6CO6CO22(g) + 6H(g) + 6H22O (l) O (l) C C66HH1212OO66(aq) + (aq) + 6O6O22(g)(g)

2AgBr (s) 2AgBr (s) 2Ag (s) + Br 2Ag (s) + Br22(l)(l)

FACTORS AFFECTING THE FACTORS AFFECTING THE RATE OF REACTIONRATE OF REACTION

The following factors affect the rate of The following factors affect the rate of reaction. These arereaction. These are

1)1) Effect of ConcentrationEffect of Concentration

2)2) Effect of TemperatureEffect of Temperature

3)3) Effect of CatalystEffect of Catalyst

4)4) Effect of lightEffect of light

EFFECT OF EFFECT OF CONCENTRATIONCONCENTRATION

Increase in the concentration of reactants Increase in the concentration of reactants increases the number of molecules of increases the number of molecules of reactants. There will be more collisions reactants. There will be more collisions between the reactant molecules and the rate between the reactant molecules and the rate of reaction will increase.of reaction will increase.

When nitrogen oxide (NO) reacts with When nitrogen oxide (NO) reacts with oxygen ( Which is a homogenous system) oxygen ( Which is a homogenous system)

2NO (g) + O2NO (g) + O22 (g) (g) 2 NO 2 NO22 (g) , the reaction (g) , the reaction rate doubles when oxygen concentration rate doubles when oxygen concentration doubles. But rate quadruples when the doubles. But rate quadruples when the concentration of nitrogen monoxide is concentration of nitrogen monoxide is doubled.doubled.

EFFECT OF TEMPERATUREEFFECT OF TEMPERATURE

On increasing the temperature of reactants, On increasing the temperature of reactants, the energy of the molecules increases. Due the energy of the molecules increases. Due to increased energy the frequency of to increased energy the frequency of collision increases and finally the rate of collision increases and finally the rate of reaction increases.reaction increases.

CaCOCaCO33 (s) + 2 HCl (aq) (s) + 2 HCl (aq) CaCl CaCl22 (aq) + CO (aq) + CO22 (g) + H(g) + H22O (l) . In this reaction the evolution O (l) . In this reaction the evolution of COof CO22 is slow but if we increase the is slow but if we increase the temperature the evolution of gas is more temperature the evolution of gas is more rapid indicating that the rate of reaction rapid indicating that the rate of reaction increases on increasing the temperature.increases on increasing the temperature.

EFFECT OF CATALYSTEFFECT OF CATALYST A catalyst is a substance which increases the rate A catalyst is a substance which increases the rate

of a chemical reaction but there is no change in of a chemical reaction but there is no change in the catalyst itself. In the presence of a catalyst a the catalyst itself. In the presence of a catalyst a reaction takes place at a faster rate and at lower reaction takes place at a faster rate and at lower temperature.temperature.

Iron is used as a catalyst in the manufacture of Iron is used as a catalyst in the manufacture of ammonia. Iron catalyst increases the rate of ammonia. Iron catalyst increases the rate of reaction between nitrogen and hydrogen to form reaction between nitrogen and hydrogen to form ammonia.ammonia.

NN22 (g) + 3H (g) + 3H22(g) (g) 2 NH2 NH33 (g) + heat (g) + heat

Similarly , the reaction of SOSimilarly , the reaction of SO22 and O and O22 gives SO3 in gives SO3 in presence of nitrogen monoxide (NO), which acts presence of nitrogen monoxide (NO), which acts as a catalyst. 2 SOas a catalyst. 2 SO22 (g) + O (g) + O22 (g) 2SO (g) 2SO33 ( g) ( g)

NO (g)

EFFECT OF LIGHTEFFECT OF LIGHT

Photosynthesis and photography both Photosynthesis and photography both involve light sensitive reactions.involve light sensitive reactions.

In our daily life, a coloured cloth In our daily life, a coloured cloth gradually fades when dried in gradually fades when dried in sunlight. sunlight.

Dyes of the clothes undergo chemical Dyes of the clothes undergo chemical change and fade gradually.change and fade gradually.

Plants prepare food in presence of Plants prepare food in presence of sunlight.sunlight.

REVERSIBLE REVERSIBLE REACTIONSREACTIONS

The reaction in which the product can The reaction in which the product can react under any conditions to give back react under any conditions to give back reactants are called reversible reactions reactants are called reversible reactions or a reaction which takes place in a or a reaction which takes place in a forward as well as reverse direction forward as well as reverse direction both, is called reversible reaction. It is both, is called reversible reaction. It is represented by putting a double arrow represented by putting a double arrow between the reactants and products. between the reactants and products. One arrow pointing towards the One arrow pointing towards the products and other towards reactants.products and other towards reactants.

Examples of reversible Examples of reversible reactionsreactions

2H2H22 + O + O22 2H2H22OO

NN22 + O + O22 2NO2NO

CaCOCaCO33 CaO + COCaO + CO22

NN22 + 3H + 3H22 2NH2NH33

IRREVERSIBLE IRREVERSIBLE REACTIONSREACTIONS

Those chemical reactions in which the Those chemical reactions in which the products cannot recombine to give products cannot recombine to give back reactants are called irreversible back reactants are called irreversible reactions.reactions.

2 Na(s) + 2 H2 Na(s) + 2 H22O (l) O (l) 2 NaOH ( aq) + 2 NaOH ( aq) + HH22 (g) (g)

Here the products, sodium hydroxide Here the products, sodium hydroxide and hydrogen can’t recombine to give and hydrogen can’t recombine to give back the reactants sodium and water.back the reactants sodium and water.

REVERSIBLE PROCESS REVERSIBLE PROCESS AND CHEMICAL AND CHEMICAL EQUILIBRIUMEQUILIBRIUM

The formation of ammonia isa reversible process. we say that nitrogen and hydrogen react under appropriate conditions and give ammonia,then this statement is expressed in the form ofan equation as

N2 (g) + 3 H2 (g) → 2 NH3 (g) The reaction is taking place in forward

direction and we, therefore, say it is a ‘forward reaction’.

We can also write decomposition of NH3 as,2NH3 (g) → N2 (g) + 3H2 (g)which is, in fact, a reverse of the forward reactionand takes place in opposite direction, and therefore, we call this reaction as ‘reverse reaction’. We represent forward and reverse reactions together asN2 (g) + 3H2 (g) 2 NH3 (g)

Such reactions never go to completion if performed in a close container.

Chemical Equilibrium

Rate

of

React

ion

Time increases

N2 (g) + 3 H2 (g) 2 NH3 (g)

(Rate of formation of ammonia decreases with time)

Rate of decomposition of ammonia increases with time )

2NH3 (g) N2 (g) + 3H2( g )

O

For a reversible chemical reaction, an equilibrium state is attained when the rate at which a chemical reaction isproceeding in forward direction equals the rate at which the reverse reaction is proceeding.

At equilibrium, Rate of forward reaction= Rate of reverse

reaction In case of ammonia, when reaction is

performed at high pressure and temperature in a close container, at equilibrium,

Rate of formation of ammonia = Rate of decomposition of ammonia

DYNAMIC EQUILIBRIUMDYNAMIC EQUILIBRIUM

Since the rate of decomposition and rate ofSince the rate of decomposition and rate of

formation is the same at equilibrium, theformation is the same at equilibrium, the

concentrations of ammonia, hydrogen andconcentrations of ammonia, hydrogen and

nitrogen remain constant. nitrogen remain constant. This equilibrium is dynamic in nature and This equilibrium is dynamic in nature and

is therefore, called dynamic equilibrium.is therefore, called dynamic equilibrium. In a close container containing water, rate of In a close container containing water, rate of

evaporation and rate of condensation of water evaporation and rate of condensation of water molecules become equal at equilibrium. molecules become equal at equilibrium.

CHARACTERISTIC CHARACTERISTIC FEATURES OF DYNAMIC FEATURES OF DYNAMIC

EQUILIBRIUMEQUILIBRIUM

(i) At equilibrium, macroscopic (observable) (i) At equilibrium, macroscopic (observable) properties such as concentration, density, properties such as concentration, density, colour, etc., are constant under the given colour, etc., are constant under the given condition of temperature, pressure and initial condition of temperature, pressure and initial amount of the substances.amount of the substances.

(ii) At equilibrium, microscopic (at molecular (ii) At equilibrium, microscopic (at molecular level) processes continue, but they are in level) processes continue, but they are in balance. This means that no overall large scale balance. This means that no overall large scale (macroscopic) changes occur. Rate of forward (macroscopic) changes occur. Rate of forward process (reaction) is equal to the rate of process (reaction) is equal to the rate of reverse process, and as a consequence of this, reverse process, and as a consequence of this, no net change results.no net change results.

(iii) The equilibrium can be attained from (iii) The equilibrium can be attained from either direction, beginning with only the either direction, beginning with only the materials on one side of the change. materials on one side of the change. Such changes are described as Such changes are described as reversible.reversible.

(iv) Equilibrium can be achieved in a (iv) Equilibrium can be achieved in a closed system. In a closed system. In a closed systemclosed system, there , there is no loss or gain of matter to or from the is no loss or gain of matter to or from the surroundings. surroundings.

An An open system open system may allow matter to may allow matter to escape or to enter. Therefore, in an open escape or to enter. Therefore, in an open system, the equilibrium cannot be system, the equilibrium cannot be attained.attained.

EQUILIBRIUM EQUILIBRIUM CONSTANTCONSTANT

It has been experimentally found that at a It has been experimentally found that at a particular temperature, when equilibrium is particular temperature, when equilibrium is attained, the ratio between concentration of attained, the ratio between concentration of reactants and products becomes constant. For reactants and products becomes constant. For reactionreaction

N2 (g) + 3 H2 (g) 2 NH3 (g) + 92 kJN2 (g) + 3 H2 (g) 2 NH3 (g) + 92 kJ At equilibrium,At equilibrium,

Here, concentration of product (ammonia) occurs Here, concentration of product (ammonia) occurs in numerator, and those of reactants (hydrogen in numerator, and those of reactants (hydrogen and nitrogen) occur in denominator. Each and nitrogen) occur in denominator. Each concentration term, [NH3], or [N2], or [H2], is concentration term, [NH3], or [N2], or [H2], is raised to a power equal to stoichiometric*raised to a power equal to stoichiometric*coefficient in the balanced equation. coefficient in the balanced equation. K is called K is called equilibrium constantequilibrium constant. .

For a general reaction,For a general reaction, a A + b B c C + d Da A + b B c C + d D at equilibrium, equilibrium constant can beat equilibrium, equilibrium constant can be written as,written as, In the above expression, square bracketIn the above expression, square bracket denotes molar concentration, i.e. denotes molar concentration, i.e.

concentrationconcentration in mol/L. The expressionin mol/L. The expression (1.2) is also known as (1.2) is also known as Law of ChemicalLaw of Chemical Equilibrium. Equilibrium. At a particular temperature,At a particular temperature, equilibrium constant has a definite value. equilibrium constant has a definite value.

WhenWhen we express concentration in mol Lwe express concentration in mol L-1-1, ,

equilibriumequilibrium constant is denoted by constant is denoted by KKc.c.

Magnitude of equilibrium constant, Magnitude of equilibrium constant, KK, , indicates indicates how far how far a chemical reaction can go. a chemical reaction can go. In other words, it is an indicator of the In other words, it is an indicator of the extent of a chemical reactionextent of a chemical reaction. .

Larger the value of Larger the value of KK, higher will be the , higher will be the equilibrium concentration of products on the equilibrium concentration of products on the right hand side of the reaction. Smaller value right hand side of the reaction. Smaller value of of K K indicates lower concentration of the indicates lower concentration of the products at equilibrium.products at equilibrium.

For the reaction at 298 K,For the reaction at 298 K,

PCl3 (g) + Cl2 (g) _PCl5 (g) ; PCl3 (g) + Cl2 (g) _PCl5 (g) ; KKc = 1.9c = 1.9 Low value of equilibrium constant shows thatLow value of equilibrium constant shows that

at equilibrium, the concentration of product at equilibrium, the concentration of product (PCl5) will be low. (PCl5) will be low.

. Similarly, for reaction at. Similarly, for reaction at 298 K,298 K, N2 (g) + O2 (g) 2 NO (g)N2 (g) + O2 (g) 2 NO (g) K = K = cc Very small value of Very small value of KKc implies that c implies that

reactantsreactants N2 and O2 will be dominant species in N2 and O2 will be dominant species in

the systemthe system at equilibrium.at equilibrium.