L16 Chemical Equilibrium

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PHYSICAL CHEMISTRY FOR

TEXTILE9. Chemical EquilibriumL16: chemical equilibrium,reversible reactions, law ofmass action, law of chemicalequilibrium

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CHEMICAL EQUILIBRIUM

In general equilibrium is a state whereall the forces acting on anything areequal and opposite or the equilibriumis a balance of forces.

For example a book lying on a table isin equilibrium since all the forcesacting downward and upward arebalanced and there is no motion.

When we consider such an equilibriumstate in chemical reactions, it is calledchemical equilibrium.

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Not all the chemical reactions that occur go to completion. Some reactions go to completion where all the reactants are

converted into products, for e.g. sodium bicarbonate reacts withhydrochloric acid to produce sodium chloride, water and carbondioxide. In this reaction the reactants are completely convertedinto products.

However some reactions stop when a certain amount ofreactants has converted into products under certain fixedconditions. These reactions are said to be in a state ofequilibrium.

For example CO2 dissolves in water under high pressure

according to the equation:

CO2 (aq) + H2O (l) HCO3- (aq)+ H+ (aq)

If high pressure is maintained the equilibrium lies to the

right, but on releasing pressure the equilibrium shifts to theleft.

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The equilibrium can be of two types: Static equilibrium: is that in which every type of change has

stopped when the system is in equilibrium. For instance if weconsider again the example of book lying on table on which allthe forces are balanced and absolutely no change is occurring.

Dynamic equilibrium: is that in which although the reaction asa whole has stopped and there is no more change in thequantities of reactants and products but the reaction in fact hasnot stopped. In these reactions a reverse reaction is alsooccurring and at equilibrium the rate of reverse reaction becomesequal to the forward reaction. Hence although no net change isoccurring but reactants and products are continuously convertinginto one another.

The position of equilibrium is defined as if at equilibrium thereactants are more than the products, the equilibrium is saidto lye to the left. However, if the products are more than thereactants, the equilibrium is said to lye to the right. 4


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REVERSIBLE REACTIONS

In organic chemistry most of the reactions arebidirectional i.e. they can proceed in bothdirections, from reactants to products or fromproducts to reactants. However the amount of

product formed depends on the relative speed ofboth forward and reverse reactions before theequilibrium is established.

The position of equilibrium, however depends alsoon the reaction conditions like temperature,pressure, concentration etc.

On the other hand if there is no reverse reaction,the reaction will go to completion and no reactantswill remain at the end of reaction. In organicchemistry such reactions are usually found inpolymerizations.

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KINETICALLY AND THERMODYNAMICALLYCONTROLLED REACTIONS

If a reaction is thermodynamicallypossible but it does not proceedreasonably rapidly (i.e. the rate of

reaction is very slow), it is called akinetically controlled reaction.

If on the other hand the reaction isreasonably rapid but proceeds to only

a small extent, it is calledthermodynamically controlled reaction.

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LAW OF MASS ACTION

For a homogenous system the rate of a chemicalreaction is proportional to the active masses of thereacting substances. The molecular concentrationof a substance in solution or in the gas phase istaken as a measure of its active mass.

If two substances A and B take part in a reversiblereaction in which the products are C and D, and a,b, c and d are their molar concentrations, thereaction may be written as follows:

aA + bB cC + dD

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LAW OF MASS ACTION

Then according to the law of mass actionthe rate of either of the forward or reversereaction is proportional to the product of

active masses of the various componentsinvolved in reaction, therefore:rate of forward reaction = kf [A]

a[B]b

rate of reverse reaction = kr [C]c[D]d

In these equations k represents the rateconstant and the square bracketsrepresent molar concentrations ofreactants and products.

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LAW OF MASS ACTION

When we combine the reactants A and B, theforward reaction starts immediately, but dueto formation of products, reverse reaction alsogets underway.

As the reaction proceeds the rate of forwardreaction diminishes, and the rate of reversereaction increases. Eventually both theforward and reverse reactions fall to the samereaction rate, and the reaction is said to be in

equilibrium. Hence:rate of forward reaction = rate of reversereaction

kf [A]a[B]b = kr [C]

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LAW OF CHEMICAL EQUILIBRIUM ANDEQUILIBRIUM CONSTANT

If we consider a homogenous reversible reaction,aA + bB cC + dD

the rates of which are kf[A]a[B]b and kr [C]

c[D]d respectively,then at equilibrium the rates of both the forward and

reverse reactions are the same, and hence:

kf [A]a[B]b = kr [C]

c[D]d

Or kf /kr = [C]c[D]d /[A]a[B]b = Kc

The constant Kc is called the equilibrium constant of thereaction. This equation, derived from the law of massaction, is referred to as the law of chemical equilibriumwhich states that whatever are the initialconcentrations of reactants and products, theconcentrations at equilibrium are related to eachother in such a way that the value of Kc remainsconstant.

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EQUILIBRIUM AND EQUILIBRIUMCONSTANT

S.

Initial concentration(mol/L)

Concentration at equilibrium

(mol/L)

Reactants Products

Equilibrium

No CH3

COOH C2H5OH CH3COOH C2H5OH CH3COOC2H5 H2O Cons-tant

1 1.00 0.18 0.829 0.009 0.171 0.171 3.9

2 1.00 1.0 0.333 0.333 0.667 0.667 4.0

3 1.00 2.0 0.142 1.142 0.888 0.888 4.5

4 1.00 3.0 0.034 2.034 0.966 0.966 3.911


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EQUILIBRIUM AND EQUILIBRIUMCONSTANT

When chemical reactions are carriedout in solution, the equilibriumconstant is given by the followingequation:

Kc = [C]c[D]d /[A]a[B]b

It follows from this equation that ifmore product is formed and less

reactants are left at equilibrium, Kc willhave a larger value. While if lessproduct is formed and more reactantsare left at equilibrium, the value of Kc

will be smaller.

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LAW OF CHEMICAL EQUILIBRIUM

We can draw the following importantconclusions from the above discussion:When Kc is large the equilibrium mixture will

contain a high proportion of products, i.e. thereaction will go nearly to completion.

When Kc is small the reaction does not proceedvery far under the given reaction conditions and

the concentration of product formed is low.The value of Kc does not change by the addition

of more reactants or products to the equilibriummixture if the reaction conditions are keptconstant.

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LAW OF CHEMICAL EQUILIBRIUM

Considering the following equation,aA + bB cC + dDKc = [C]c[D]d /[A]a[B]b

if some additional B is added the value of expression willnot stay equal to Kc, however, some of the B will reactwith A to produce more C and D until the proportion ofreactants and products again become equal toequilibrium value of Kc. If on the other hand some of theproduct is added to equilibrium mixture, it will convert to

the reactants to establish equilibrium Although there is no rule but for most practical purposes

the value of K within the rage from 0.01 to 100 isconsidered significant, which indicates that chemicallysignificant amount of all the components of the reactionsystem will be present in an equilibrium mixture and that

the reaction will be incomplete or reversible.

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SIGNIFICANCE OF KC

When Kc for a reaction is known, the relative proportions of thereactants and products at equilibrium can be calculated for anymixture of reactants used initially.

For the previously considered esterification reaction, the reactionequation will be as follows:

CH3COOH(l) + C2H5OH(l) CH3COOC2H5(l) +H2O(l)

Equation for Kc will be as follows:

Kc=[CH3COOC2H5(l)] [H2O(l)] /[CH3COOH(l)] [C2H5OH(l)]

If considering one mole each of acetic acid and alcohol at start ofreaction and Kc=4, the ratio of products and reactants will be:

Kc = 4 = 4/1 = 2/1

Ratio of products to reactants will be = [0.666] [0.666] /[0.333][0.333]

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UNCHANGING CONCENTRATION INEQUILIBRIUM EXPRESSIONS

Substances whose concentration do notundergo significant change in a chemicalreaction do not appear in equilibrium constantexpressions. There two such general cases

which are considerable:The substance is also a solventThe substance is a solid or a pure liquid phase.

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ONE OF REACTION COMPONENTS ISSOLVENT OF REACTION MIXTURE

Considering an acid base reaction,

HCl (aq) + NaOH (aq) NaCl (aq)+ H2O

we know that in acid base reactions, one ofthe products is water, therefore, if thereaction is carried out in aqueous medium,the concentration of water will not change

appreciably. Hence concentration of waterwill not appear in the expression ofequilibrium constant.

Kc = [NaCl(aq)] /[HCl(aq)] [NaOH(aq)]17


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COMPONENTS IS SOLVENT OFREACTION MIXTURE

In organic reactions if a condensationreaction is carried out in aqueousmedium, the concentration ofcondensate brings negligible changeand is therefore not considered inequation for Kc.

An example is the formation of watersoluble amides from organic acids andamines. Amide formation is acondensation reaction in which wateris produced as byproduct.

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SOLID REACTION COMPONENT ORPURE LIQUID PHASE

When one or more of the components of areaction are solid, their concentration cannotbe defined and cannot be included in theexpression of equilibrium constant. For

example the decomposition reaction of CaCO3to give CaO and CO2 the reaction is as follows:

CaCO3 CaO + CO2The equilibrium constant for this equation

does not include the solid components andhence the equation is:

Kc = [CO2(g)]

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L16 Chemical Equilibrium

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