1. Title: Lesson 1 Dynamic Equilibrium
Chapter: 7 Equilibrium
Title: 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

2. 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

3. Physical systems and equilibrium
Bromine is a volatile liquid (b.p. close to r.t)
A significant number of particles will have enough energy to escape (liquidgas) (evaporation)
Concentration of vapour molecules will increase.
Eventually the rate of condensation is equal to the rate of evaporation  no net change in liquid or gas.
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.
EQUILIBRIUM!

4. 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 AT
THE START
THE STEEPER THE GRADIENT, THE FASTER THE REACTION
SLOWS DOWN AS REACTANTS ARE USED UP
TOTAL CONVERSION TO PRODUCTS
In an ordinary reaction; all reactants end up as products; there is 100% conversion

5. EQUILIBRIUM REACTIONS
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…
EQUILIBRIUM REACTIONS
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.
FASTEST AT THE START
NO BACKWARD REACTION
FORWARD REACTION SLOWS DOWN AS REACTANTS ARE USED UP
BACKWARD REACTION
STARTS TO INCREASE
In an equilibrium reaction, not all the reactants end up as products; there is not a 100% conversion.
BUT IT DOESN’T MEAN THE REACTION
IS STUCK IN THE MIDDLE
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.

6. 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.

7. 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)

8. 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)
reactants
products
Equilibrium can be reached anywhere along the escalator as long as you maintain a constant running rate!
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.

9. Features of an equilibrium state

11. Solutions

12. 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.

13. This equilibrium data has produced a constant value within the limits of experimental accuracy.
This constant is known as the equilibrium constant, Kc.
So equilibrium constant expression is:
Every reaction has it’s own value for Kc which can be derived from the reaction:

14. THE EQUILIBRIUM CONSTANT Kc
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
VALUE OF Kc
AFFECTED by a change of temperature
NOT AFFECTED by a change in concentration of reactants or products
a change of pressure
adding 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)

17. Solutions

19. 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).

21. Ex:. H2(g) + I2(g) ↔ 2HI(g) k for this reaction at 450 C is 49. If 0
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.
Q < k  forward reaction predominates until equilibrium is reached.

22. Ex:. PCl3(g) + Cl2(g)  PCl5(g) k=1
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?

23. 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.

24. 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…

25. Summary of manipulations of the value of Kc:

28. Solutions

30. Modeling Equilibrium (20 mins)
Complete the paper equilibrium activity.
Record your observations in a table like this:
Step
White squares
Black squares
Forward rate
Backward
rate
Equilibrium constant
100
n/a ∞ 1 2 …

31. 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

32. Investigating Equilibrium (25 mins)
Open the simulation here: 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.

33. 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