Chemical EquilibriumUnit 11

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Chemical Reactions

•We usually think of chemical reactions as having a beginning and an end.

reactants productsThe END?•Not always! Chemical reactions are

reversible when they can go forwards and backwards without any extra energy being used.

reactants products•This means that as products form, they can react and reform reactants.

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Changes in the Concentration

• The concentration of the reactants decreases and then remains constant.

• The concentration of the products increases and then remains constant.

• Chemical equilibrium is achieved when the concentration of the reactants and the products over time remains constant.

• A reaction may look "finished" when equilibrium is reached, but actually the forward and reverse reactions continue to happen at the same rate.

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Example of Chemical Equilibrium

Consider the following reactions:

CaCO3(s) + CO2(aq) + H2O(l) Ca2+(aq) + 2HCO3-(aq) (1)

and

Ca2+(aq) + 2HCO3-(aq) CaCO3(s) + CO2(aq) + H2O(l) (2)

• Reaction (2) is the reverse of reaction (1). • At equilibrium the two opposing reactions occur at the same

rate. • Concentrations of chemical species do not change once

equilibrium is established.

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Example of Chemical Equilibrium

2 NO2 N2O4

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Reaction Rate

Chemical Equilibrium:– The forward reaction rate is equal to the reverse

reaction rate.

– The concentrations of both the products and the reactants remain constant.

• What happens to the forward reaction rate (reactants → products) as we approach equilibrium?•It decreases, because we are running out of reactants.

• What happens to the reverse reaction rate (products → reactants) as we approach equilibrium?•It increases, because we are making more products

Equilibrium Constant

At equilibrium we have a certain concentration of products and reactions.

We can compare those and come up with a constant (K) for the reaction that describes this ratio for a specific reaction.

Each chemical reaction will have a unique equilibrium constant. These constants have been calculated for most reactions and are easily accessible.

The Equilibrium Constant• Consider the following equilibrium system

wA + xB ⇄ yC + zD

K =

• The numerical value of K is calculated using the molar concentrations of reactants and products that exist at equilibrium.

• Pure solids or liquids are not included in the calculation of the equilibrium constant. An example of a pure liquid is water, an example of a pure solid would be silver.

• Equilibrium Constants are normally calculated at 25 °C unless otherwise noted.

Brackets [ ] indicate that values are concentrations (molarity)

Physical Meaning of the Equilibrium Constant

•What does it mean to have a large equilibrium constant?

K[products]

[reactants]

=

It means that the equilibrium favors products, i.e. there are high concentrations of products and low concentrations of reactants at equilibrium.

It means that the equilibrium favors reactants, i.e. there are low concentrations of products and high concentrations of reactants at equilibrium.

K[products]

=[reactants]

Expressions for Equilibrium Constants

Examples:

N2(g) + 3H2(g) ⇄ 2NH3(g)

PCl5 (s) + H2O (g) ⇄ 2 HCl (g) + POCl3 (g)

2 NO2 (g) ⇄ N2O4 (g)

Find K for 2.00 moles of NO2 and 1.60 moles of N2O4 in a 4.00 L flask.

O]2

[H

]3

[POCl2[HCl] =K

1.60 2[0.5]

[0.4] =K

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What about reactions that are not at equilibrium?• Can we quantitatively describe reactions that are not at equilibrium?–Yes! We can use the same calculation as we did to get the equilibrium constant (K) to get a value, which is referred to as the reaction quotient (Q).

• Consider the following system that is NOT at equilibrium:

wA + xB ⇄ yC + zD

Q =

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What does the reaction quotient tell us?• All chemical reactions strive to reach equilibrium.• By comparing the reaction quotient (where the reaction is now) to the equilibrium constant (where it wants to go) we can make a

prediction about what will happen.• Lets look at an example; Consider the following reaction:

SO2Cl2(g)   SO2(g) + Cl2(g)      

What is Q before the reaction begins if the concentration of 0.100 M SO2Cl2, 0.070 M SO2, and 0.016 M Cl2? 

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What does the reaction quotient tell us?

If K = 0.078 at 100oC, which direction will the reaction proceed in order to establish equilibrium?

0.011 < 0.078

If the reaction quotient, Q, is going to increase, do we need to increase the concentration of our products or our reactants?

Q =[products]

[reactants]

*Increasing the products simultaneously decreases the reactants.If we need to increase the concentration of our products and decrease the concentration of reactants, the reaction must proceed in the forward direction.

reactants products

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What Factors have an effect on Equilibrium?To determine if a reaction is in chemical equilibrium, we must look at:•Concentration (Reactants and Products)•Time

Therefore, factors that effect chemical equilibrium include:•Concentrations of both the reactants and the products•Temperature•Pressure (for gases)

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What have we been saying all along?

A chemical reaction strives to reach equilibrium.

Le Chatelier’s Principle:

If a stress is put on a reversible reaction at equilibrium, the system will shift in such a way to relieve the stress.