Chapter: 7 EquilibriumTitle: Lesson 1 Dynamic Equilibrium

Learning Objectives:– Outline the characteristics of a system under dynamic

equilibrium

– Simulate the formation of chemical equilibria

– Set-up an experiment for next week

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Writing equilibrium equations Equilibrium reactions are written using a

double arrow Each of the arrows only has a single-sided head

The unit will make a lot of reference to the rate of the: ‘forward reaction’ (reactants becoming

products) ‘back reaction’ (products becoming reactants)

REACTANTS PRODUCTS

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Physical systems and equilibriumBromine is a volatile liquid (b.p. close to r.t)

A significant number of particles will have enough energy to escape (liquidgas) (evaporation)

Concentration of vapour molecules will increase.Some vapour molecules will collide with the surface of the liquid (losing energy) and condense.Rate of condensation increases with increase in concentration of vapour.

Eventually the rate of condensation is equal to the rate of evaporation no net change in liquid or gas.

EQUILIBRIUM!

In an ordinary reaction; all reactants end up as products; there is 100% conversion

CONCENTRATION CHANGE IN A REACTION

As the rate of reaction is dependant on the concentration of reactants... the forward reaction starts off fast but slows as the reactants get less concentrated

FASTEST ATTHE START

SLOWS DOWN AS REACTANTS ARE USED UP

TOTAL CONVERSION TO PRODUCTS

THE STEEPER THE GRADIENT, THE FASTER

THE REACTION

Initially, there is no backward reaction but, as products form, it speeds up and provided the temperature remains constant there will come a time when the backward and forward reactions are equal and opposite; the reaction has reached equilibrium.

EQUILIBRIUM REACTIONS

In an equilibrium reaction, not all the reactants end up as products; there is not a 100% conversion.

BUT IT DOESN’T MEAN THE REACTIONIS STUCK IN THE MIDDLE

FASTEST AT THE STARTNO BACKWARD REACTION

FORWARD REACTION SLOWS DOWN AS REACTANTS ARE USED UP

BACKWARD REACTION STARTS TO INCREASE

AT EQUILIBRIUM THE BACKWARD AND FORWARD REACTIONS ARE

EQUAL AND OPPOSITE

It can be described as dynamic because both forward and backward reactions are still occurring.

The rate of the forward reaction is equal to the rate of the back reaction...

At equilibrium these are not zero… even though it looks like it on the graph… usually if we calculate… rate = change in concentration/change in time = zero…

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Why ‘dynamic’ equilibrium? The reaction hasn’t

stopped, it is still going, but the rate of the forward and back reactions are equal, so there is no overall change.

This is different to a static equilibrium (for example a see-saw) where there is no change at all.

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Example: TASK: Describe what you would observe and

what would be happening in terms of the forwards and backwards reaction...

(2 bullet points for observation, 4 bullet points on forwards and backwards reaction)

TASK: What would the graph of concentration vs time look like if you started off with a flask of:

(a) Mixture of H2(g) and I2(g) (b) HI(g)

OBSERVATION You would observe a purple colour owing to the production of iodine gas. Increase in colour would eventually stop.

THE REACTION Forward reaction is fastest at the start ([HI] greatest) Backwards reaction would start slowly from zero rate ([H2] and [I2] would be low) Eventually forwards and backwards reaction rate would become equal. EQUILIBRIUM MIXTURE (Concentrations of reactants and products would remain constant over time)

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TASK: What would the graph look like if you started off with a flask of:

(a) Mixture of H2(g) and I2(g) (b) HI(g)

Note: Even though concentrations of reactant and product are constant at equilibrium, in this case, the concentrations are not equal. E.g. [HI] is higher at equilibrium.

reactants

products

Equilibrium can be reached anywhere along the escalator as long as you maintain a constant running rate!

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Features of an equilibrium state

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Solutions

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The equilibrium constant Kc can be predicted from a reaction’s stoichiometry

Experiments with different starting concentrations of H2, I2, and HI.

Equilibrium is reached and the composition of each equilibrium mixture is measured.

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This equilibrium data has produced a constant value within the limits of experimental accuracy.This constant is known as the equilibrium constant, Kc.

Every reaction has it’s own value for Kc which can be derived from the reaction:

So equilibrium constant expression is:

for an equilibrium reaction of the form...

aA + bB cC + dD

then (at constant temperature) [C]c . [D]d = a constant, (Kc)

[A]a . [B]b

where [ ] denotes the equilibrium concentration in mol dm-3

Kc is known as the Equilibrium Constant

THE EQUILIBRIUM CONSTANT Kc

VALUE OF Kc

AFFECTED by a change of temperature

NOT AFFECTED by a change in concentration of reactants or productsa change of pressureadding a catalyst

The equilibrium constant describes where the position of equilibrium lies:

Minimum: 0, maximum: ∞, half-way: 1 Greater than 1: products favoured (as more products than reactants) Less than 1: reactants favoured (as less products than reactants)

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Solutions

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The reaction quotient, Q, enables us to predict the direction of reaction

If we take the concentrations of the reactants and products at one moment in time when the reaction is not in equilibrium, we can substitute these into the equilibrium constant expression, and find reaction quotient, Q.

As the reaction continues, the concentrations of all reaction components change and eventually reach equilibrium concentrations.

For the reaction:

Kc = 49.5 at 440oC (at equilibrium)

In exp 1: Q<Kc, so Q must increase as the reaction moves towards equilibrium. Reaction moves towards the right (products).

In exp 2: Q>Kc, so Q must decrease as the reaction moves towards equilibrium. Reaction moves towards the left (reactants).

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Ex: H2(g) + I2(g) ↔ 2HI(g)

k for this reaction at 450 C is 49. If 0.22 mol I2, 0.22 mol H2, and 0.66 mol HI are put into a 1.00-L container, would the system be at equilibrium? If not, what must occur to establish equilibrium.

]][I[H

[HI]Q

22

2

) )(0.22 (0.22

) (0.66

Lmol

Lmol

2L

mol

0.9Q < k forward reaction predominates until equilibrium is reached.

Ex: PCl3(g) + Cl2(g) PCl5(g) k=1.9 In a system at equilibrium in a 1.00 L container, we find 0.25 mol PCl5, and 0.16 mol PCl3. What equilibrium concentration of Cl2 must be present?

5c

3 2

[PCl ]K

[PCl ][Cl ]

]M)[Cl (0.16M) (0.25

9.12

2[Cl ] 0.82 mol/ L

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Relationships between Kc for different equations of a reaction Kc for the inverse reaction

The inverse reaction:

Defines the products as reactants and vice versa.

We can see that:

The equilibrium constant for a reaction is the reciprocal of the equilibrium constant for its inverse reaction.

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Relationships between Kc for different equations of a reaction Kc for a multiple of a reaction

The reaction:

We can see that:

Tripling of the stoichiometric co-efficients would lead to a cubing of the value of Kc, halving would lead to the square root of Kc etc…

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Summary of manipulations of the value of Kc:

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Solutions

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Modeling Equilibrium (20 mins) Complete the paper equilibrium activity.

Record your observations in a table like this:Step White

squaresBlack

squaresForward

rateBackward

rateEquilibrium

constant

0 100 0 n/a n/a ∞

1

2

…

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Measuring an Equilibrium Constant Next week we will be investigating the following

equilibrium:

Ethanol + Ethanoic Acid ⇌ Ethyl ethanoate + water

CH3CH2OH(aq) + CH3CO2H(aq) ⇌ CH3CH2OOCCH3(aq) + H2O(l)

However, it takes a while to establish itself, so we will set it up now.

Follow the instructions here Make sure you label your tube with your name, the

reaction number and the date Use cling-film and a rubber band to seal the tube

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Investigating Equilibrium (25 mins) Open the simulation here:

https://phet.colorado.edu/en/simulation/reactions-and-rates

Using the ‘Investigate the conditions under which a significant equilibrium is more and less likely to form

You will need to use the ‘Rate Experiments’ tab and select ‘Design your own’ for the reaction. Under ‘chart options’, select ‘bar chart’.

You should change: Enthalpy level of reactants Enthalpy level of products Activation energy Temperature Concentrations of reactants and products

You should find a systematic way to record your findings, and should try to reach some kind of conclusion.

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Key Points: At dynamic equilibrium:

Concentrations of reactants and products are constant

The rate of the forward and backward reactions are equal

The position of an equilibrium is given by Kc

Kc > 1 means products are favoured Kc < 1 means reactants are favoured