Topic 10 - Equilibrium I Flashcards
What is a reversible reaction?
A reaction that can go forward or backwards.
How do you show a reversible reaction in an equation?
Use an double arrow that points both ways in the equation.
See pg 118 of revision guide
What is dynamic equilibrium?
When the concentrations of reactant and products in a reversible reaction are constant, and the forward and backwards reactions happen at the same rate.
In dynamic equilibrium, what can be said about:
• The reaction rates
• The concentrations
- The forward and backwards reactions happen at the same rate
- The concentrations of the products and reactants stay constant (but not necessarily equal to each other)
In dynamic equilibrium, when there are more products than reactants, where does the position of the equilibrium lie?
To the right.
In dynamic equilibrium, when there are more reactants than products, where does the position of the equilibrium lie?
To the left.
Describe how dynamic equilibrium is reached from a system with just the reactants in at first.
- As the reactants get used up, the forward reaction slows down
- As more product is formed, the reverse reation speeds up
- Eventually, the two happen at the same rate and dynamic equilibrium is reached
Where can a dynamic equilibrium be reached?
In a closed system.
What is the equilibrium constant?
A measure of how far to the left or right the equilibrium is in a particular system.
What is the symbol for the equilibrium constant?
Kc
‘c’ is subscript
What is a homogeneous system?
One in which everything is in the same physical state,
What is a heterogeneous system?
One in which not everything is in the same physical state.
In a homogeneous system, which reactants and products are used in the equation for Kc?
All of them.
Write an equation for Kc for the reaction:
aA + bB dD + eE
Kc = [D]^d [E]^e / [A]^a [B]^b
In the equation for Kc, what goes on the top and bottom?
Top: Products
Bottom: Reactants
In the equation for Kc, what do the square brackets and powers mean?
- Square brackets: Concentration of each substance.
* Powers: Number of moles of each substance from equation.
In a heterogeneous system, which reactants and products are used in the equation for Kc?
All of them, except solids and pure liquids.
Why are solids and pure liquids not included in the equation for Kc in a heterogeneous system?
Their concentrations stay constant throughout the reaction.
Write an expression for the equilibrium constant for:
H2 (g) + I2 (g) 2HI (g)
Kc = [HI]^2 / [H2] [I2]
Write an expression for the equilibrium constant for:
H2O (g) + C (s) H2 (g) + CO (g)
Kc = [H2] [CO] / [H2O]
What are the units for the concentrations used in the eqution for Kc?
mol / dm3
What are the units for Kc?
- It depends.
* Divide the units on top of the euqtion by those on the bottom.
Remember to practise writing out dynamic equilibrium equations.
Pgs 118-119 of revision guide.
Do catalysts affect the equilibirum constant (Kc)?
No, they just speed up the rate at which dynamic equilibrium is reached.
If the equilibirum shifts to the left, you get more…
Reactants
If the equilibrium shifts to the right, you get more…
Products
State Le Chatelier’s principle.
If there’s a change in concentration, pressure or temperature in dynamic equilibrium, the equilibrium will move to help counteract that change.
What changes in conditions can cause a shift in the position of an equilibrium
- Temperature
- Concentration
- Pressure
How does increasing the concentration of a reactant affect equilibrium?
- Equilibrium tries to get rid of extra reactant
* Forward reaction is favoured -> So the equilibrium shifts to the right
How does decreasing the concentration of a reactant affect equilibrium?
- Equilibrium tries to get produce extra reactant
* Backwards reaction is favoured -> So the equilibrium shifts to the left
How does increasing the pressure affect equilibrium?
- Equilibrium tries to reduce the pressure
* Equilibrium shifts to the side with fewer moles of gas
How does decreasing the pressure affect equilibrium?
- Equilibrium tries to increase the pressure
* Equilibrium shifts to the side with more moles of gas
How does increasing the temperature affect equilibrium?
- Equilibrium tries to reduce the temperature
* Endothermic reaction is favoured -> So equilibrium shifts in that direction
How does decreasing the temperature affect equilibrium?
- Equilibrium tries to increase the temperature
* Exothermic reaction is favoured -> So equilibrium shifts in that direction
If the foward reaction is exothermic, the reverse reaction is…
Endothermic
Does changing the pressure always change the position of equilibrium?
No, only if there are gases involved.
When working out how a change in pressure will affect equilibrium, what must you look at?
The total number of MOLES of gas on each side, not just the number of gases on each side.
(i.e. Look at the numbers before each gas!)
What is the equation for the production of ethanol in industry?
C2H4 (g) + H2O (g) C2H5OH (g)
What are the conditions for the production of ethanol in industry?
- 60-70 atm
- 300*C
- Phosphoric(V) acid catalyst
C2H4 (g) + H2O (g) C2H5OH (g) ΔH = -46 kJ/mol
The reaction takes place at 60-70 atm, 300*C and with a phosphoric(V) acid catalyst. Explain the temperature used.
- Forward reaction is exothermic, so low temperatures favour the forward reaction -> Better yield
- But lower temperatures mean a slow rate of reaction
- So 300*C is a compromise between maximum yield and rate of reaction
C2H4 (g) + H2O (g) C2H5OH (g) ΔH = -46 kJ/mol
The reaction takes place at 60-70 atm, 300*C and with a phosphoric(V) acid catalyst. Explain the pressure used.
- High pressure favours the forward reaction -> Better yield
- High pressure also increases the rate of reaction
- But high pressure is expensive to maintain
- So 60-70 atm is a compromise between maximum yield (+ rate) and expense
Remember to revise how to word industry equilibrium consition questions.
Pg 121 of revision guide
