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Equilibrium A system/rxn is at equilibrium when the forward and reverse reactions occur at equal rates. Chemists draw arrows that look like this to show a system at equilibrium.
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Equilibrium
The story so far…• So far, we learnt about the kinetics of a rxn
– In other words, how fast or how slow it goes, what factors affect this rate and how we can calculate that rate given data.
• Today, we are only focused on one specific condition of rates “Equilibrium”
Equilibrium • A system/rxn is at equilibrium
when the forward and reverse reactions occur at equal rates.
• Chemists draw arrows that look like this to show a system at equilibrium.
Equilibrium Many chemical reactions are reversible if….. the activation energy is low enough , the system is closed and temperature is kept constant.
*Closed system= one where nothing can enter or escape (ideal)
Reactants ⇌ Products
Forward Rate Reverse Rate
reaction rate= kforward.[reactants]k is a rate constant that tells you how fast or slow the reaction is normally.
Approaching Equilibrium
Reactants ⇌ Products
Forward Reaction is left to right
Reverse Reaction is right to left
Let’s see what happens to some reactants if they are placed in a reaction vessel and allowed to react.
Reactants Products
Approaching Equilibrium
Forward Rate
Reverse Rate
Reactants Products
Reverse Rate
Forward Rate
Approaching Equilibrium
Reactants Products
Reverse Rate
Forward Rate
Approaching Equilibrium
Reactants Products
Reverse Rate
Forward Rate
Approaching Equilibrium
Reactants Products
Reverse Rate
Forward Rate
Approaching Equilibrium
Reactants Products
Reverse Rate
Forward Rate
Approaching Equilibrium
Reactants Products
Reverse Rate
Forward Rate
Are Equal
[concentration] is constant, but NOT EQUAL
Approaching Equilibrium
Reactants Products
Reverse Rate
Forward Rate
Rates are equal, but concentrations are constant( unchanged)
Approaching Equilibrium
How come the concentration of the reactants at equilibrium does not change even though the forward reaction continues to occur????
The reverse reaction continues at the same rate as the forward reaction in an equilibrium. In other words, the products that are made in the forward reaction are used up as fast as they are made due to the reverse reaction taking place at the same rate too! So all concentrations remain the same!
Rates and Concentrations at equilibrium
1- Rate of consumption = Rate of production
2- The forward and reverse rates are equal and remain constant unless the system is interrupted.
3-[reactants] differs from [products] but….
4- Concentrations of [reactants] and [products] is now CONSTANT/unchanged in time.
In general, any system which is not at equilibrium will tend to move towards establishing equilibrium
Recognize the difference• Forward and reverse rates : equal &
constant• Concentrations of reactants and products:
constant, but not necessarily equal.
How equilibrium is established Stage 1•As a reaction begins, high[reactants] generate a fast forward rate.
How equilibrium is established Stage 2•As the [reactants] falls so does the forward rate, meanwhile the reverse rate rises along with the [products]
How equilibrium is established Stage 3•This continues until the reverse rate equals the forward rate, thereby establishing equilibrium.
Concentration vs time
Concentration
Time
reactant
product
Equilibrium
Reactant concentrations start high and decrease as the reaction proceeds. The forward rate, which depends on collisions of the reactants, also decreases. Product concentrations start at zero and increase as the reaction proceeds. The reverse rate, which depends on collisions of the products, also increases. Eventually when the forward rate is equal to the reverse rate AND the concentrations are constant ( but not necessarily the same) This is equilibrium.
Rate vs time
Rate
Time
Forward rate
Reverse rate
Characteristics of a System at Equilibrium 1.The Forward rate = The Reverse rate
The equilibrium can be approached from starting with reactants or starting with products.
2.The Reactant and Product concentrations are constant/unchanging [ ] 3.The Macroscopic (observable) properties are constant
Large enough properties to be observed with the naked eyes. Ex; colour changes, total pressure,etc
Yet microscopic changes occur at the atomic or molecular level. 4.The system is in a Dynamic Equilibrium as the forward and reverse reactions continue.
5. Equilibrium will Only shift or change when conditions change -->later topic.
Reactants Products
Reverse Rate
Forward Rate
Approaching Equilibrium from Products
Reactants Products
Reverse Rate
Forward Rate
Approaching Equilibrium from Products
Reactants Products
Reverse Rate
Forward Rate
Approaching Equilibrium from Products
Reactants Products
Reverse Rate
Forward Rate
Approaching Equilibrium from Products
Reactants Products
Reverse Rate
Forward Rate
Approaching Equilibrium from Products
Reactants Products
Reverse Rate
Forward Rate
Rates Are Equal
[conc]’s are constant
Approaching Equilibrium from Products
If you start with products all aspects of the approach to equilibrium are reversed
Reactants and Forward Rate increases
Products and Reverse Rate decrease
Equilibrium (looking at reverse reaction)
Concentration
Time
reactant(originally products)
product
Rate Vs Time /Equilibrium
Rate
Time
Forward rate
Reverse rate
Conditions Necessary for Equilibrium
Ea is low so the reaction is reversible
Constant temperature
Closed system
Think...2NH3(g) N2(g) + 3H2(g)
This is how the above reaction occurs at equilibrium. It means:
__________________________________________________________________________________
2NH3(g) N2(g) + 3H2(g)
If there was a gas mixture of NH3, N2, H2 put in a container and after a few mins it was found that 2 moles of NH3 react for every 3 moles of products which react. Is this mixture at equilibrium?
Question 6 & 7 on page 40
Clearing out misconceptions• What we refer to is technically called
“Dynamic Equilibrium” as in, it is active and both the reactions continue to occur. They do not stop.
Hebden Practice Problems
Solve q’s 3-5 on page 39
Solve q’s 8 -13 on pages 42 and 43
I am here for help!
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