Factors Affecting the State of Equilibrium - Angela Tanghal

7/25/2019 Factors Affecting the State of Equilibrium - Angela Tanghal

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FACTORS AFFECTING THE

STATE OF EQUILIBRIUM :LE CHATELIER’S

PRINCIPLE



FACTORS AFFECTING THE STATE OF

EQUILIBRIUM

Chemical equilibrium is a very delicate system thatrepresents a perfect balance between forward and

reverse reaction. A small disturb in the equilibriummay shift the equilibrium position either to rightforming more products or to left forming morereactants. This reaction by the system is of coursetemporary and eventually the system will come back

to equilibrium. This phenomenon can be expressed inthe form of Le Chatelier’s rinciple.



LE CHATELIER’S PRINCIPLE

An important and very interesting qualitativeprinciple governing the equilibrium is the principle ofLe Chatelier. This principle! which is named after the"rench chemist Henry Louis Le Chatelier #$%&'($)*+,! may be stated as follows-

• if an external stress is applied to a system at

equilibrium, the system will tend to react in such way asto relieve the applied stress and tries to reestablish theequilibrium

n chemical reaction terminology! the /stress0means change in concentration! pressure! volume ortemperature. Le Chatelier’s principle can beunderstood either qualitatively or quantitatively

doing some problems. 1owever! we restrict



f the concentration of A increases! the system reacts to consumesome of it.

• f a reactant is added! the equilibrium position shifts to theright.

• f a product is added! the equilibrium position shifts to the

left.

f the concentration of 2 decreases! the system reacts to consume

some of it.• f a reactant is removed! the equilibrium position shifts to

the left.• f a product is removed! the equilibrium position shifts to the

right.

3nly substances that appear in the expression for Q canhave an e4ect.

A change in concentration has no eect on the value of K .

LE CHATELIER’S PRINCIPLE



The 54ect of Change in Concentration



The Effect of Added Cl2 on the PCl3-Cl2-PCl5 System

Concentration (M ) PCl3(g ) + Cl2(g ) PCl3(g )

Original equilibrium 0.200 0.125 0.600

Disturbance +0.075

New initial 0.200 0.200 0.600

Change - x - x + x

New equilibrium 0.200 - x 0.200 - x 0.600 + x

0.6!7"#



The effect of added Cl2 on the PCl3-Cl2-PCl5 system.

PCl3( g) + Cl2( g) PCl5( g)

When Cl2 ( yellow curve) is added, its

concentration increases instantly(vertical part of yellow curve) and then

falls gradually as it reacts with PCl3 to

form more PCl5. Equilibrium is re-

established at new concentrations but

with the same value of K.



6ample roblemPredicting the Efect o a Changein Concentration on theEquilibrium Poition

PROBLEM:To improve air quality and obtain a useful product,chemists often remove sulfur from coal and natural gas

by treating the contaminant hydrogen sulfide with O2:

What happens to

(a) [H2O] if O2 is added? (b) [H2S] if O2 is added?(c) [O2] if H2S is removed?

2H2S( g) + O2( g) 2S( s) + 2H2O( g)

PLAN:We write the reaction quotient to see howQc is affected by

each disturbance, relative to Kc. This effect tells us thedirection in which the reaction proceeds for the system to

reattain equilibrium and how each concentration changes.



SOLUTION:

Qc =[H2O]2[H2S]

2[O2]

(a) When O2 is added,Q decreases and the reaction proceeds to the

right untilQc = Kc again, so [H2O] increases.

(b) When O2 is added,Q decreases and the reaction proceeds to theright untilQc = Kc again, so [H2S] decreases.

(c) When H2S is removed,Q increases and the reaction proceeds to

the left untilQc = Kc again, so [O2] increases.

(d) The concentration of solid S is unchanged as long as some ispresent, so it does not appear in the reaction quotient. Adding

more S has no effect, so [H2S] is unchanged.



The e4ect of change in ressure#7olume,

ressure does not have any e4ect on concentrationsof species that are present in solid! liquid or solutionform. 3n the other hand! the change in pressure

a4ects the concentrations of gases. According toideal gas law! pressure and volume are inverselyproportional to each other8 the greater the pressure!the smaller the volume! and vice versa. Thus! it is

9ust enough to understand the a4ect of change in

pressure on the equilibrium system



Changing the c$ncentrati$n $% a gase$us c$m&$nent

causes the equilibrium t$ shi%t acc$r'ingl(. )''ing an inert gas has no effect $n the equilibrium

&$siti$n* as l$ng as the $lume '$es n$t change.

,his is because all c$ncentrati$ns an' &artial &ressures remain

unchange'. Changing the $lume $% the reacti$n essel will cause

equilibrium t$ shi%t i% ∆ngas 0.

Changes in &ressure $lume" hae no effect $n the

alue $% K .



When the volume of an equilibrium mixture of gases

is reduced, a net reaction occurs in the direction

producing fewer moles of gases. When the volume is

increased, a net reaction occurs in the direction

producing more moles of gases.

The e4ect of a change in pressure #volume, on asystem at equilibrium.



The e4ect of change n Temperature

To determine the e4ect of a change in temperature

on equilibrium! heat is considered a component ofthe system.

1eat is a product in an exothermic reaction #:H;rxn < ',.

1eat is a reactant in an endothermic reaction #:H;rxn = ',.

A change in concentration! pressure or volumealters the position of the equilibrium but not themagnitude #value, of the equilibrium constant.1owever! the change in temperature changes the

value of the equilibrium constant. To understandthe e4ect of temperature! we must know whetherthe reaction is endothermic #absorption of heat, orexothermic# release of heat,.



Raising the temperature of an equilibriummixture shifts the equilibrium condition in thedirection of the endothermic reaction.

Lowering the temperature causes a shift in thedirection of the exothermic reaction.

• A decrease in temperature removes heat! which favorsthe exothermic reaction.

• An increase in temperature adds heat! which favors theendothermic reaction.



The e4ect of a Catalyst in an 5quilibrium

•A catalyst speeds up a reaction by lowering its

activation energy. A catalyst therefore speeds up theforward and reverse reactions to the same extent.

•A catalyst causes a reaction to reach equilibrium morequickly! but has no eect on the equilibrium position.

The function of a catalyst is to speed up the reactionby lowering the activation energy. The catalyst lowersthe activation energy of the forward reaction andreverse reaction to the same extent. :ue to this!

there is no shift in equilibrium or the change in trhevalue of the equilibrium constant. Therefore! weconclude that the catalyst has no e4ect on theequilibrium system. Therefore-



Note: The presence of a catalyst does not change thenumerical value of the equilibrium constant.

The function of a catalyst is to change themechanism of a reaction to one having a loweractivation energy.

A catalyst has no effect on the condition ofequilibrium in a reversible reaction.



54ects of 7arious :isturbances on a6ystem at 5quilibrium

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Factors Affecting the State of Equilibrium - Angela Tanghal