equilibrio en reacciones quimicas

8/13/2019 Equilibrio en Reacciones Quimicas

1/7

Reaction Rates & EquilibriumSpontaneous and Nonspontaneous

Changes

Reaction Rates

Molecular Collisions

Energy Diagrams

Factors Affecting Reaction RatesChemical Equilibrium

Factors that Influence Equilibrium

Reaction Rates andEquilibrium

This chapter considers three factors.

Thermodynamics If a reaction will occur.Kinetics How rapidly a reaction will occur.Equilibrium How much of a reaction will occur.

Spontaneous & nonspontaneous changes

Spontaneous process Takes place naturally with no apparent cause or stimulus.

Nonspontaneous process Requires that something be done in order for it to occur.

Spontaneous Nonspontaneous

When will a reaction be spontaneous?Spontaneity of a reaction can be determinedby a study of thermodynamics.

It can be used to calculate the amount ofuseful work that is produced by some chemicalreactions.

It also shows the two competing factors thatdetermine the spontaneity of a reaction.

Enthalpy and Entropy

First law of thermodynamics Energy and chemical bondsDuring a chemical reaction

Old bonds break. New bonds are formed. Energy is either consumed or produced.

Exothermic Energy is released.New bonds are more stable.

Endothermic Energy is required.New bonds are less stable.


2/7


3/7

Enthalpy vs. EntropyTwo competing forces.

EnthalpyStability gained from being ordered.

EntropyStability gained from being random.

Both affect how a reaction will proceed.

We need a term that considers both factors.

Free energy - GThe maximum amount of energy that canbe obtained from a reaction.

The total change in free energy is then

! G = G products - G reactantsAt constant pressure

! G = ! H - T ! S

Gibb

Free energyThe sign of ! G tells us a lot about the reaction

! G > 0 Not spontaneous , requires energy to occur.

! G < 0 Spontaneous , will occur on its own, releasingenergy.

! G = 0 ( ! H = T ! S)

At equilibrium , no desire to go one way or theother - why reactions stop at some point.

Chemical kineticsThe study of reactions as a function of time.

! GOnly tells us if a reaction will occur but not howlong it will take.

KineticsMeasures the time required for areaction tooccur.

Example - DiamondsLess stable than graphitebut very slow kinetics.

Effective collisionsFor reactants to make products

They must collide.

The energy of collision must be greaterthan the bond energy between the atoms

Activation EnergyThe minimum amount of energy required toproduce a chemical reaction.


4/7

Effective collision

A temporary state where bonds are in theprocess of reforming.

Activated

Complex

Energy diagrams

! Hactivation

energy E n e r g y

This type of plotshows the energy

changes duringa reaction.

Examples of energy diagrams

Exothermic reaction

Endothermic reaction

Examples

High activation energyLow heat of reaction

Low activation energyHigh heat of reaction

Factors that influence reaction rates

Can be affected by

reactant structure concentration of reactants temperature physical state of reactants presence of a catalyst

Can be used to describe an equilibrium- process that is established when the rate of theforward reaction is equal to the reverse reaction.

CatalysisCatalyst

A substance that changes the rate of areaction without being used in the reaction.Provides an easier way to react.Lower activation energy.Still make the same products.Enzymes are biological catalysts.

InhibitorA substance that decreases the rate ofreaction.


5/7

CatalysisTypes of catalysts

Homogeneous - same phaseCatalyst is uniformly distributedthroughout the reaction mixture

Example - I - in peroxide.

Heterogeneous - different phaseCatalyst is usually a solid and thereactants are gases or liquidsExample - Automobile catalytic converter

CatalystsWork by altering the mechanism of a reaction.The net effect is to reduce the activation energy.

CatalystsAll that matters is that the reaction goes from thereactants to the products. It does not matter how

you get there.

A B C+ AB + DAB*

A + B + C DCatalyst

Reactants

Products

Equilibrium

A state where the forward and reverse conditionsoccur at the same rate.

Equilibrium and reaction rates A point is ultimatelyreached where therates of the forwardand reverse reactionsare the same.

At this point, equilibrium is achieved.

Equilibrium

C o n c e n

t r a

t i o n

Time

Equilibriumregion

Kineticregion


6/7

Chemical equilibriumA dynamic process on the molecular level achievedwhen concentration of reactants and products remainconstant over time.

for a physical process: H2O(l) H2O(s) (reactant) (product)

the equilibrium process is indicated with anequilibrium arrow

Equilibrium constant (K)We can write an equilibrium expression for anyreaction at constant temperature.

[ ] represents the molar concentrations - mol/L

Always put products over reactants and raise each tothe power of their coefficients.

Le Chateliers principle Any stress placed on an equilibrium system will cause the system to shift to minimize the effect of the stress.

You can put stress on a system by adding orremoving something from one side of areaction.

N2 + 3 H 2 2 NH 3

What effect will there be if you added moreammonia? How about more nitrogen?

Le Chateliers principle

Adding ammonia - shift to left

Adding nitrogen - shift to right

N2 + 3 H 2 2 NH3

K eq =[ NH3 ]

2

[ N2 ] [ H 2 ]3

ExampleO2 transport in blood

Equilibrium equation

Equilibrium expression

K Hb =[Hb(O 2)4]

[Hb] [O 2]4

Hb + 4 O 2 Hb(O 2)4

O2 transport

Equilibrium equationEquilibrium equation

Hb + 4 O 2 Hb(O 2)4

Equilibrium expressionEquilibrium expression

K Hb =[Hb(O 2)4]

[Hb] [O 2]4


7/7

O2 transport

Hb + 4 O 2 Hb(O2)4

In the lungs there is an abundance of O 2 so:

Oxygen is picked up by the hemoglobin.

When blood reaches the cells, there is a lack of O2 so:

Oxygen is given up by the hemoglobin.

Hb + 4 O 2 Hb(O2)4


K Hb =[Hb(O 2)4]

[Hb] [O 2]4

= 5.0 x10 5

[Hb(O 2)4]

[Hb]K Hb x [O 2]

4 =

Lets say that

The ratio of oxygenatedto unoxygenatedhemoglobin is:

Le Chateliers principleIf the solubility of O 2 in blood at 37

oC = 0.1M. (in the lungs)

[Hb(O 2)4]

[Hb]= = 5.0x10 5 x 0.1 4

= 50= 50

[Hb(O2)

4]

[Hb]= = 5.0x10 5 x 0.001 4

= 5.0 x 10= 5.0 x 10 -7-7

In the capillaries, the O 2 level is about 0.001Mso -

K Hb x [O 2]4

K Hb x [O 2]4

Pressure will affect an equilibrium with gases asreactants or products.

3 H2(g) + N2(g) 2 NH3(g) 4 mol 2 mol of reactants of products

Increasing pressure causes the equilibrium toshift to the side with the least moles of gas.



Heat will also affect an equilibrium

NaOH(s) Na+(aq) + OH-(aq) + heat

Increasing the temperature causes less NaOHto dissolve.

equilibrio en reacciones quimicas

Documents