Chemical Equilibria 1108 8AM

8/6/2019 Chemical Equilibria 1108 8AM

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A given chemical reaction system is defined bya ba lanced ne t chem ica l equ a t ion which is

conventionally written as

reac tan ts ? p ro d u c t s

The first thing we need to know about achemical reaction represented by a balancedequation is whether it can actually take place.

I f the reactants and products are all

substances capable of an independentexistence, then the answer is always "yes .


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This answer must always be qualified by thefollowing two considerations:

the reaction take place to a sufficientextent to produce useful (or evendetectable) quantities of products

the mechan ismof the reaction

What happens, at the atomic or molecular

level, when reactants are transformed into

products? What intermediate species areinvolved?


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A r eaction that is therm odynam ically possible butfor w hich no reasonably rapid m echanism is

available is said to be k in e t i ca l ly l im i ted .

One that occurs rapidly but only to a sm all extent is

t h er m o d y n am i cal ly l im i t e d .


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What is equilibrium?

Basically, the term refers to what we mightcall a "balance of forces".

A book sitting on a table top remains at rest

because the downward force exerted by theearth's gravity acting on the book's mass isexactly balanced by the repulsive forcebetween atoms that prevents two objects fromsimultaneously occupying the same space.

mechanical equilibrium


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I f we pick up the book and raise it above thetable top, the additional upward force exerted

by our arm destroys the state of equilibrium asthe book moves upward.

I f we w ish to hold the book at rest above the

table, we adjust the upward force to exactlybalance the weight of the book, thus restoringequilibrium.


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Another kind of equilibrium we all experienceis thermal equilibrium.

When two objects are brought into contact,heat will flow from the warmer object to thecooler one until their temperatures become

identical.

Thermal equilibrium is a "balance of forces" inthe sense that temperature is a measure of the

tendency of an object to lose thermal energy.


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Thermal equilibrium is something we oftenwant to avoid; this is why people insulate

buildings, perspire in the hot and wear heavierclothing in the w inter.


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Chemical equilibrium

When a chemical reaction takes place in acontainer which prevents the entry orescape of any of the substances involved inthe reaction, the quantities of these

components change as some are consumedand others are formed.

Eventually this change w ill come to an end,

after which the composition will remainunchanged as long as the system remainsundisturbed.


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The system is then said to be in its equilibriumstate, or more simply, "at equilibrium".

A chemical reaction is in equilibrium whenthere is no tendency for the quantities ofreactants and products to change.

The direction in which we write a chemicalreaction (and thus which components areconsidered reactants and which are

products) is arbitrary.


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Thus the two equations

H2 + I2 2 HI "synthesis of hydrogeniodide"

2 HI H2 + I2 "dissociation of hydrogen

iodide"

represent the same chemical reaction systemin which the roles of the components are

reversed, a n d b o th y i e ld t h e sa m e m ix t u r e o f com ponen t s w hen th e change is com p le ted .


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This is central to the concept of chemicalequilibrium.

It makes no difference whether we start w ithtwo moles of HI or one mole each of H2 and I2;once the reaction has run to completion, the

quantities of these two components will be thesame.

In general, then, we can say that the

composition of a chemical reaction system w illtend to change in a direction that brings itcloser to its equilibrium composition.


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Once this equilibrium composition has beenattained, no further change in the quantities of

the components will occur as long as thesystem remains undisturbed.


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Reversible reactions

A reversible reaction is one which can be madeto go in either direction depending on theconditions.

I f we pass steam over hot iron the steamreacts w ith the iron to produce a black,magnetic oxide of iron called triiron tetroxide,Fe3O4.


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The hydrogen produced in the reaction is sweptaway by the stream of steam.


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Under different conditions, the products of thisreaction w ill also react together. Hydrogen

passed over hot triiron tetroxide reduces it to

iron. Steam is also produced.

This time the steam produced in the reaction is

swept away by the stream of hydrogen.


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These reactions are reversible, but under theconditions normally used, they become one-

way reactions. The products aren't left incontact w ith each other, so the reversereaction can't happen.


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Reversible reactions happening in a c losedsys tem

A c losed sys t em is one in which no substancesare either added to the system or lost from it.Energy can, however, be transferred in or outat w ill.

In the example we've been looking at, wewould have to imagine iron being heated insteam in a closed container. Heat is being

added to the system, but none of thesubstances in the reaction can escape. The

system is closed.


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As the triiron tetroxide and hydrogen start tobe formed, they w ill also react again to givethe original iron and steam.

So, if we analysed the mixture after a while,what would we find?

We would find that we had established what isknown as a dyn am ic equ i l i b r i um.


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Th ink ing abou t r eact i on ra t es

This is the equation for a general reactionwhich has reached dynamic equilibrium

How did it get to that state? Let's assume thatwe started w ith A and B.


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At the beginning of the reaction, theconcentrations of A and B were at theirmaximum. That means that the rate of thereaction w as at its fastest.

As A and B react, their concentrations fall.

That means that they are less likely to collideand react, and so the rate of the forwardreaction falls as time goes on.


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In the beginning, there isn't any C and D, sothere can't be any reaction between them. As

time goes on, though, their concentrations in

the mixture increase and they are more likelyto collide and react.

With time, the rate of the reaction between Cand D increases.


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Eventually, the rates of the two reactions w illbecome equal. A and B w ill be converting into Cand D at exactly the same rate as C and Dconvert back into A and B again.


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At this point there won't be any further change

in the amounts of A, B, C and D in the mixture.

As fast as something is being removed, it isbeing replaced again by the reverse reaction.

We have reached a position of dynamicequilibrium.


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Summary

A dynamic equilibrium occurs when we havea reversible reaction in a closed system.Nothing can be added to the system or takenaway from it apart from energy.

At equilibrium, the quantities of everythingpresent in the mixture remain constant,although the reactions are still continuing.

This is because the rates of the forward andthe back reactions are equal.


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I f we change the conditions in a way whichchanges the relative rates of the forward

and back reactions we w ill change the

position of equilibrium

in other words, change the proportions ofthe various substances present in the

equilibrium mixture.


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Equilibrium Constant

Writing Equilibrium Constant Expressions

Concept of equilibrium constants is importantin chemistry

Equilibrium constants are the key to solving aw ide variety of stoichiometry problemsinvolving equilibrium systems


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q an industrial chemist who wants tomaximize the yield of sulfuric acid must

have clear understanding of equilibrium

constants for all steps in the process,starting from the oxidation of sulfur andending w ith the formation of acid

q a physician specializing in clinical cases of

acid-base imbalance needs to know theequilibrium constants of weak acids andbases

q an environmental chemist must has a

know ledge of equilibrium constants ofpertinent gas-phase reactions, w ill help himbetter understand the process of ozonedestruction in the stratosphere


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To use equilibrium constants, we must expressthem in terms of the reactant(s) and

product(s) concentrations

Our only guide is the law of mass action whichis the general formula for finding equilibrium

concentrations


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Law of Mass Action

States that for a reversible reaction atequilibrium and a constant temperature, acertain ratio of reactant and productconcentrations has a constant value K (the

equilibrium constant).In general, a reversible reaction is w ritten as:

aA + bB cC + dD

where a, b, c, and d are the stoichiometriccoefficients for the reacting species A, B, C,and D


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For the reaction at a particular temperature,the equilibrium constant is

K =

The above equation was formulated by two

Norw egian chemists, Cato Guldberg and PeterWaage in 1864.

Note that although the concentrations may

vary, as long as a given reaction is atequilibrium and the temperature does notchange, according to the law of mass action,the value of K remains constant.

[C]c[D]d

[A]a[B]b

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Chemical Equilibria 1108 8AM