Equilibria, Le Chatelier's Principle and Kc Flashcards
Give examples of reactions that are reversible
the reaction between nitrogen and hydrogen to produce ammonia
What is dynamic equilibrium?
when the rates of the forward and backward reactions are the same
Why is it called DYNAMIC equilibrium?
Because although the concentrations of the composition are stable the forward and backward reactions are still occurring
Give a summary of the rate x time graph for getting to equilibrium
At the start: forward reaction is fast - no reverse reaction
After the start: reverse reaction speeds up as forward reaction slows down
At equilibrium: as quickly as the products are being made the products are changing back to reactants
What is the forward reaction?
reactants changing into products
What is the backward reaction?
products changing back into reactants
When does chemical equilibrium occur and when is it established?
when the concentrations of reactants and products remain constant and when the rate of the forward reaction equals the rate of the backward reaction
What is Le Chatelier’s Principle?
If a system at equilibrium is disturbed, the equilibrium moves in the direction that tends to reduce the disturbance. (if any factor is changed which affects the equilibrium mixture, the position of the equilibrium will shift so as to oppose the change)
What does a negative enthalpy sign mean?
heat is given out in the forward reaction - exothermic
If we increase the temperature of a reversible reaction at equilibrium which way will the equilibrium move?
moves in endothermic direction
If we decrease the temperature of a reversible reaction at equilibrium which way will the equilibrium move?
moves in exothermic direction
What is the definition of a catalyst?
A substance that increases the rate of a chemical reaction without itself undergoing any permanent chemical change.
What happens when the concentration of a reactant is increased?
The equilibrium will shift to the right
What happens when the pressure on the system is increased?
the equilibrium will shift to minimise the pressure increase which in effect means the equilibrium moves in the direction to produce the least moles
What is the effect of adding a catalyst to a reversible reaction at equilibrium? Why?
It makes no difference to the position of equilibrium because a catalyst speeds up the forward and back reaction to the same extent
Why use a catalyst for a reversible reaction?
A catalyst speeds up the rate at which a reaction reaches dynamic equilibrium
What is the temperature and pressure used at most ammonia plants?
200 atm and 450 degrees C
What is significant about the conditions used at most ammonia plants?
it is a lower pressure and a higher temperature than would give the maximum conversion (compromise conditions)
Why are compromise conditions necessary? (ammonia plants)
- higher pressures would be too dangerous/expensive
- 450 degrees C gives a moderate yield in a reasonable time (it takes too long to establish the equilibrium at 350 degrees C)
What is the atom economy of the Haber process? What is the atom economy for the conversion to ammonia?
100 % but conversion to ammonia is much less
Why is the atom economy of the conversion to ammonia much less than the Haber process?
1, Gases flow over the catalyst too quickly for equilibrium to be reached
2, An equilibrium process which tries to attain a moderate yield in a reasonable time
What is a homogeneous reaction?
A homogeneous reaction is where both products and reactants are present in the same phase.
What is the value of Kc unchanged by?
changes in concentration, pressure and use of a catalyst
What is the value of Kc changed by?
only temperature
For the reaction aA + bB (reversible reaction arrows) cC + dD, write the expression for the equilibrium constant Kc
Kc = [C]^c [D]^d / [A]^a [B]^b
What do the square brackets mean in the Kc expression?
Concentration in moldm-3
What is the significance of Kc?
It is a measure of how far a reaction goes. If Kc is exactly 1, then the position of the equilibrium must be exactly halfway between reactants and products.
Are products on the top or on the bottom of the Kc expression?
on the top
What does it mean if Kc is greater than 1?
then there are more products than reactants at equilibrium - the forward reaction is favoured
What does it mean if Kc is less than 1?
the reverse reaction is favoured - there are more reactants than products at equilibrium
How can you find Kc if you don’t have the concentration of reactants and products at equilibrium?
1, Initial amounts of each product or reactant
2, Change in each reactant
3, Equilibrium number of moles for each product and reactant
4, Equilibrium concentration of each reactant (moldm-3)
5, Kc expression (and calculation)
6, Units
What are the conditions for dynamic equilibrium?
1, the rate of the forward reaction = the rate of the reverse reaction
2, concentrations of reactants and products are constant
3, closed system
4, macroscopic properties remain constant i.e. temp, pressure
Give two features of a reaction in equilibrium
- the concentration of reactants and products remain constant
- the rate of the forward reaction is equal to the rate of the reverse reaction
Why do catalysts have no effect on the position of equilibrium?
- catalysts increase the rate of the forward and reverse reactions
- by an equal amount
Give the meaning of the term ‘dynamic’ in the context of ‘dynamic equilibrium’
The forward and reverse reaction are occurring at the same time
Give the meaning of the term ‘dynamic equilibrium’
- the concentration of reactants and products remain constant
- the rate of the forward reaction is equal to the rate of the reverse reaction
State a condition for a reaction in equilibrium
closed system
State Le Chatelier’s Principle
A reaction in equilibrium will act to oppose any change imposed upon it
State in general terms how a catalyst works?
- It provides an alternative reaction route
- With a lower activation energy
How does an increase in pressure alter Kc?
It remains unchanged
What is the only thing that affects the value of Kc?
temperature
