Topic 7: Chemical Equilibrium

Or...why your new favourite letter is K!

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A look ahead...

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•The SL component of Eq’m is not in depth enough

•We will go into some HL material in order to prepare everyone

•SL students will not be tested on HL material

Two lovely volunteers, please!

• We have 2 bins of water, labelled “reactants” & “products”• Both of you will scoop water at the same rate, into the

other bin. • Of course, at first there are no products, so the reaction is

only going “forward” (reactants into products). • Eventually, when there is enough “products”, the reaction

can proceed in the “reverse” direction (products back into reactants).

• When water is going in both directions are the same rate, we have reached equilibrium

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Remember Kinetics?

•Equilibrium occurs when the RATE of the forward reaction is exactly equal to the RATE of the reverse reaction.

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In principle, every chemical reaction is reversible ... capable of moving in the forward or backward direction.

2 H2 + O2 2 H2O

Some reactions are easily reversible ...Some not so easy ...

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Equilibrium: the extent of a reactionIn stoichiometry we talk about theoretical yields,

and the many reasons actual yields may be lower.

Another critical reason actual yields may be lower is the reversibility of chemical reactions: some reactions may produce only 70% of the product you may calculate they ought to produce.

Equilibrium looks at the extent of a chemical

reaction.

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Let’s evaluate the reversible reaction of N2O4(g) 2 NO2(g)

• Consider colourless frozen N2O4. At RT, it decomposes to brown NO2: N2O4(g) → 2NO2(g)

• At some time, the colour stops changing and we have a mixture of N2O4 and NO2.

• reaction. • Chemical

equilibrium is the point at which the RATE of the forward reaction is equal to the rate of the reverse

The Concept of Equilibrium

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At that point, the concentrations of all species are constant.

NOTE - being constant is NOT the same as being EQUAL. • Using the collision model: – as the amount of NO2 builds up, there is a chance that

two NO2 molecules will collide to form N2O4. –At the beginning of the reaction, there is no NO2 so the

reverse reaction (2NO2(g) → N2O4(g)) does not occur.

The Concept of Equilibrium

• As the substance warms it begins to decompose: N2O4(g) → 2NO2(g)

• When enough NO2 is formed, it can react to form N2O4: 2NO2(g) → N2O4(g).

• At equilibrium, as much N2O4 reacts to form NO2 as NO2 reacts to re-form N2O4

• The double arrow implies the process is dynamic.

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Characteristics of Equilibrium •Observable

(Macroscopic) properties are constant.

•Equilibrium can only be reached in a CLOSED system and will remain in eq’m

•Equilibrium can be approached from either the forward or reverse direction

•You shouldn’t see any changes

•No outside influences should interfere

•Reactants breaking down or products forming

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The Concept of EquilibriumAs the reaction progresses–[A] decreases to a constant,–[B] increases from zero to a constant.–When [A] and [B] are constant, equilibrium

is achieved.–***[A] and [B] don’t have to be the same

concentration, just not changing!***

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The Equilibrium ConstantForward Reaction:

N2O4(g) → 2 NO2(g)

Forward Rate:kf[N2O4]

Reverse Reaction:2 NO2(g) → N2O4(g)

Reverse Rate:kr[NO2]2

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Keq = the equilibrium constant At equilibrium, there is a constant ratio between the

concentration of the products and reactants in any change.

You calculate the equilibrium constant by dividing the forward rate constant by the reverse rate constant

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Equilibrium Constant for any eq’m equation

The equilibrium equation is usually expressed in terms of molar concentrations (units: M/L or mol/L or mol/dm3).

So the equilibrium constant is both Keq and Kc

For the chemical equation:

aP + bQ cR+ dS

The equilibrium expression depends on the stoichiometry of the reaction and the concentration of the products & reactants.

Usually, units are not included when using or calculating the value of Kc.

Products(s)Reactant(s)

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Sample ProblemQUESTION:Write the equilibrium expression for the

following reaction:N2(g) + O2(g) 2NO(g)

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The Equilibrium Expression• Write the equilibrium expression for the

following reaction:

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The Equilibrium ConstantThe Magnitude of Equilibrium Constants• The equilibrium constant, K, is the ratio of

products to reactants.• Therefore, the larger K the more products are

present at equilibrium.• Conversely, the smaller K the more reactants are

present at equilibrium.• If K >> 1, then products dominate at equilibrium

and equilibrium lies to the right.• If K << 1, then reactants dominate at equilibrium

and the equilibrium lies to the left.

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The Equilibrium ConstantThe Magnitude of Equilibrium Constants• An equilibrium can be approached from any direction.Example:

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The Equilibrium ConstantThe Magnitude of Equilibrium Constants• However, for the reverse reaction...

• The equilibrium constant for a reaction in one direction is the reciprocal of the equilibrium constant of the reaction in the opposite direction.

• Look at the two K values. which reaction is favoured, forward or reverse?

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The Equilibrium ConstantHeterogeneous Equilibria• When all reactants and products are in one

phase, the equilibrium is homogeneous.• If one or more reactants or products are in a

different phase, the equilibrium is heterogeneous.

• Consider:

–experimentally, the amount of CO2 does not seem to depend on the amounts of CaO and CaCO3. Why?

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The Equilibrium Constant

Heterogeneous Equilibria• Neither density nor molar mass is a variable, the

concentrations of solids and pure liquids are constant. (You can’t find the concentration of something that isn’t a solution!)

• We ignore the concentrations of pure liquids and pure solids in equilibrium constant expressions.

• The amount of CO2 formed will not depend greatly on the amounts of CaO and CaCO3 present.

Kc = [CO2]

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What if you don’t know the equilibrium constant ? Let’s try some HL...

•You can determine some equilibrium concentrations if you know:▫ the initial concentration(s) of the reactant(s) & ▫ the concentration of one product at equilibrium

[which will tell you the change in concentration(s)].•Once you have the equilibrium concentrations, you can find the

equilibrium constant!!

•Use an ICE table to perform these calculations:▫ I = initial concentration▫ C = change in concentration▫ E = equilibrium concentration

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ICE Tables• Steps to Solving Problems:1. Write an equilibrium expression for the balanced reaction.2. Write an ICE table. Fill in the given amounts.3. Use stoichiometry (mole ratios) on the change in concentration

line.4. Deduce the equilibrium concentrations of all species.• Usually, the initial concentration of products is zero. (This is

not always the case.)

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Sample Problem

At 700 K, the equilibrium constant is 8.3. Suppose you start with 1.0 mol of CO(g) and 1.0 mol of H2O(g) in a 5.0 L container. What amount of each substance will be present in the container when the gases are at equilibrium at 700 K?

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