   Chapter 15, Problem 69SCQ

Chapter
Section
Textbook Problem

Decide whether each of the following statements is true or false. If false, change the wording to make it true. (a) The magnitude of the equilibrium constant is always independent of temperature. (b) When two chemical equations are added to give a net equation, the equilibrium constant for the net equation is the product of the equilibrium constants of the summed equations. (c) The equilibrium constant for a reaction has the same value as K for the reverse reaction. (d) Only the concentration of CO2 appears in the equilibrium constant expression for the reaction CaCO3(s) ⇄ CaO(s) + CO2(g). (e) For the reaction CaCO3(s) ⇄ CaO(s) + CO2(g), the value of K is numerically the same, whether the amount of CO2 is expressed as moles/liter or as gas pressure.

Interpretation Introduction

Interpretation:

The given statements have to be identified for true or false.

Concept Introduction:

Equilibrium constantKC for a general reaction is:

aA(g)+bB(g)cC(g)+dD(g)KC=[C]c×[D]d[A]a×[B]b

Equilibrium constant in terms of pressure[Kp]: Equilibrium constant can be expressed in terms of partial pressures in atmospheres.

aA(g)+bB(g)cC(g)+dD(g)KP=PCc×PDdPAa×PBb

The activity of solid substance will not appear in equilibrium and numerically it is considered as one.

Le Chatelier’s principle: If equilibrium is disturbed by changing conditions, the system will moves the equilibrium to reverse the change.

Factor’s that effect chemical equilibria:

• Concentration – Equilibrium will be affected by changing the concentration of reactant or product. If we increase the concentration of reactant system will try to reverse the change by favouring forward reaction and thus increase the concentration of products. Likewise adding products increase yield of reactants.
• Temperature – When the temperature increases equilibrium will shift in the endothermic direction, in the direction that absorbs heat. When the temperature decreases equilibrium will shift in the exothermic direction, in the direction that releases heat.
• Pressure – If the reaction consists of only liquid and solid reactants and products, pressure has no effect in the equilibrium.

In gas reactions if the number of moles has no change then there will be no effect by pressure on equilibrium.

If pressure increases then equilibrium will shift to the direction having less number of molecules and if pressure decreases system will shift to the direction having more number of molecules.

Explanation

Reason for correct option:

(b) When two chemical reactions are added to give a net equation, the equilibrium constant for the net equation is the product of the equilibrium constants for the summed equations.

The statement can be explained using considering two simple equilibria:

Consider the reaction,

ABK1=[B][A](1)

and

BCK2=[C][B](2)

We can combine both the equations

ABBCAC

[B]=K1.[A]fromtheequationK1=[B][A]Substitutungthevalueof[B]inequation(2)K2=[C][B]=[C]K1[A]K1.K2=[C][A]

Thus the equilibrium constant for the combined equilibrium is the product of two equilibrium constants.

Hence the given statement is correct.

(d).Only the concentration of CO2 appears in the equilibrium constant expression for the reaction CaCO3(s)CaO(s)+CO2(g)

Given:

CaCO3(s)CaO(s)+CO2

In the given reaction the decomposition of calcium carbonate is given. In general if we change the concentration of reactant, equilibrium will shift to the direction that nullifies the change.

In the given reaction calcium carbonate and calcium oxide are in solid state and thus its pressure can be taken as unity. There is no role of calcium carbonate and calcium oxide in the equilibrium expression.

If we add pure substances like solids or liquids there will not be any change in equilibrium. Their activities will not appear in equilibrium and numerically it is considered as one.

Kp=[PCO2][SinceP(CaCO3)andP(CaO)hasnorole]

Hence the given statement is correct.

Reason for incorrect option:

(a) The magnitude of the equilibrium constant is always independent of temperature

According to Le Chatelier’s principle: If an equilibrium is disturbed by changing conditions, the system will moves the equilibrium to reverse the change

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