1 Reversible Reactions & Equilibrium reversible reaction A reversible reaction is one which can be...

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1 1 Reversible Reactions & Reversible Reactions & Equilibrium Equilibrium A reversible reaction reversible reaction is one which can be made to go in either forward or reverse direction. Only the order of steps has changed.
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Transcript of 1 Reversible Reactions & Equilibrium reversible reaction A reversible reaction is one which can be...

Page 1: 1 Reversible Reactions & Equilibrium reversible reaction A reversible reaction is one which can be made to go in either forward or reverse direction. Only.

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Reversible Reactions & Reversible Reactions & EquilibriumEquilibrium

A reversible reaction reversible reaction is one which can be made to go in either forward or reverse direction. Only the order of steps has changed.

Page 2: 1 Reversible Reactions & Equilibrium reversible reaction A reversible reaction is one which can be made to go in either forward or reverse direction. Only.

22That is, the chemical mechanism is the same for

reactants --> products

or

products --> reactants.

Only the timing of steps is Only the timing of steps is in reverse order.in reverse order.

Page 3: 1 Reversible Reactions & Equilibrium reversible reaction A reversible reaction is one which can be made to go in either forward or reverse direction. Only.

33An irreversibleirreversible reaction can can not be made to occur not be made to occur in either the direction

products --> reactantsor

reactants --> productsby the same mechanism merely by changing the order by changing the order of reaction stepsof reaction steps.

Page 4: 1 Reversible Reactions & Equilibrium reversible reaction A reversible reaction is one which can be made to go in either forward or reverse direction. Only.

44We must be careful to distinguish between:

* “reversing a process” as a net result, and

* a reversible process.

Reversing a process undoes the result by whatever means needed.

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A reversible process occurs by A reversible process occurs by the same mechanism or steps in the same mechanism or steps in both directions: only the order of both directions: only the order of events in time has changed.events in time has changed.

IMPORTANT IDEA !!!!!

Page 6: 1 Reversible Reactions & Equilibrium reversible reaction A reversible reaction is one which can be made to go in either forward or reverse direction. Only.

66A common reversible process:water freezes, ice meltswater freezes, ice melts

This is a physical change:H2O remains H2O.

However, we can turn water into ice and ice into water merely by removing or adding heat from the H2O.

Page 7: 1 Reversible Reactions & Equilibrium reversible reaction A reversible reaction is one which can be made to go in either forward or reverse direction. Only.

77An irreversible process:An irreversible process:

dropping a carton of eggs, dropping a carton of eggs, thus breaking them.thus breaking them.

Reversing the results: Reversing the results:

buying a new carton of eggs.buying a new carton of eggs.

You can reverse the result but not by same mechanism!

Page 8: 1 Reversible Reactions & Equilibrium reversible reaction A reversible reaction is one which can be made to go in either forward or reverse direction. Only.

88A reversible reaction reaches equilibriumequilibrium when:

•the rates of the forward and rates of the forward and reverse reactions are reverse reactions are

equalequal.

As a result:

•the concentrations of concentrations of reactants reactants and products no and products no longer change longer change over time.

Page 9: 1 Reversible Reactions & Equilibrium reversible reaction A reversible reaction is one which can be made to go in either forward or reverse direction. Only.

99Consider the reversible reaction:

H2O + CO H2 + CO2.

Start with only H2O and CO:reaction begins to produce H2 and CO.Reaction slows down over time as H2O and CO used up:

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1010

timetime

[ ]

concentration of CO & Hconcentration of CO & H22OO

at start of forward reaction

Page 11: 1 Reversible Reactions & Equilibrium reversible reaction A reversible reaction is one which can be made to go in either forward or reverse direction. Only.

1111But consider reverse reaction:

CO2 + H2 CO + H2O.

At start of forward reaction, there was no CO2 & H2. Over time, the concentrations of CO2 and H2 increase, and so does the rate of the reverse reaction:

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1212

timetime

[ ]

concentration of COconcentration of CO22 & H & H22

at start of forward reaction

Page 13: 1 Reversible Reactions & Equilibrium reversible reaction A reversible reaction is one which can be made to go in either forward or reverse direction. Only.

1313When concentrations no longer changing, reaction at equilibrium:

timetime

[ ][ ]

COCO22 + + HH22

CO + HCO + H22OO

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Notice: at equilibrium at equilibrium the concentrations are not concentrations are not necessarily equal, but not changing anymorechanging anymore.

Reaction continues in both directions, but at equal rates to maintain constant concentrations.

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What are the concentrations of products and reactants once equilibrium is reached?

What is the “balance point”?

The equilibrium constant expresses these relationships.

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General reversible reaction

aa A + bb B cc C + dd D

A,B,C,D are chemical compounds;

a,b,c,d a,b,c,d are # moles of each compound in balanced equation

is symbol for reversible reaction

Page 17: 1 Reversible Reactions & Equilibrium reversible reaction A reversible reaction is one which can be made to go in either forward or reverse direction. Only.

1717ForFor

aa A + bb B cc C + dd D

Equilibrium constant isEquilibrium constant is

Keq =

product concentrationsproduct concentrations

[C]cc•[D]dd

[A]aa•[B]bb

reactant concentrationsreactant concentrations

=

Page 18: 1 Reversible Reactions & Equilibrium reversible reaction A reversible reaction is one which can be made to go in either forward or reverse direction. Only.

1818Thus, for the reaction

CO + H2O CO2 + H2 ,

Keq =[CO2] • [H2]

[CO] • [H2O]

Page 19: 1 Reversible Reactions & Equilibrium reversible reaction A reversible reaction is one which can be made to go in either forward or reverse direction. Only.

1919Equilibrium constants:

* are numerical values determined experimentally;

* are constant at constant temperature;

* change with changing temperature.

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2020

Size of Keq :

large, >>1 favors products

small, <<1 favors reactants

near to or = 1, does not favor either products or reactants.

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2121

What happens when a reaction at equilibrium is disturbed?

Le Chatelier’s Principle Le Chatelier’s Principle says:

When a system at equilibrium is When a system at equilibrium is acted upon by an external stress, acted upon by an external stress, the system acts to the system acts to counterbalance or relieve the counterbalance or relieve the stress.stress.

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2222

Here, a “system” means the chemical reaction mixture.

An “external stress” means:An “external stress” means:

* adding or removing * adding or removing reactantreactant

* adding or removing * adding or removing productproduct

* heating or cooling the * heating or cooling the reactionreaction

Page 23: 1 Reversible Reactions & Equilibrium reversible reaction A reversible reaction is one which can be made to go in either forward or reverse direction. Only.

2323Example:

CO + H2O CO2 + H2 .

If we:If we:*Add more CO, *Add more CO, then:then:reaction moves to right reaction moves to right -- -- creates more CO2 & H2 to relieve excess CO. This also uses up more H2O.

Page 24: 1 Reversible Reactions & Equilibrium reversible reaction A reversible reaction is one which can be made to go in either forward or reverse direction. Only.

2424CO + H2O CO2 +

H2 .

If we:If we:* add more CO* add more CO22, , then:then:reaction moves to left --reaction moves to left --attempts to use up extra CO2 by making more CO and H2O. This also uses up more H2.

Page 25: 1 Reversible Reactions & Equilibrium reversible reaction A reversible reaction is one which can be made to go in either forward or reverse direction. Only.

2525CO + H2O CO2 +

H2 .If we:If we:* remove H* remove H22, , then:then:reaction moves to the right --reaction moves to the right --tries to make more H2 to replace what was lost. This uses up CO and H2O and also makes more CO2.

Page 26: 1 Reversible Reactions & Equilibrium reversible reaction A reversible reaction is one which can be made to go in either forward or reverse direction. Only.

2626CO + H2O CO2 + H2 .

This is an exothermicexothermic reaction: this means heat heat is a product.is a product.

If we heat an exothermic If we heat an exothermic equilibrium reaction, then:equilibrium reaction, then:

the reaction is driven to left, towards reactants.

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For endothermicendothermic reactions, heat is added; heat is a heat is a reactantreactant.

If we heat an endothermic If we heat an endothermic equilibrium reaction, then:equilibrium reaction, then:

the reaction is driven in direction of forming products, to the right.

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Adding a catalyst speeds up an equilibrium reaction, but does not change the value of Keq = the position of balance.

With a catalyst, the reaction reaches the point of not changing sooner or faster than without a catalyst.