3.1.6 - Chemical Equilibria, Le Chatelier's Principle and Kc Flashcards
Define dynamic equilibrium
When the rate of forward reaction equals the rate of backwards reaction in a closed system
At dynamic equilibrium, concentrations of reactants and products ___ ____
stay constant
What is Le Chatelier’s Principle?
If a reaction at equilibrium is subjected to a change in concentration, pressure or temperature, the position of the equilibrium will change to oppose the change.
Le Chatelier’s Principle only applies to reactions where …
every species is in same physical state
What happens if you increase the concentration of a reactant in a reaction at equilibrium?
- Equilibrium tries to remove increase in reactant
- ∴ makes more product
- Equilibrium shifts to right
What happens if you increase the concentration of a product in a reaction at equilibrium?
- Equilibrium tries to remove increase in product
- Makes reverse reaction go faster
- Equilibrium shifts to left
Decreasing concentration has the _____ effect
opposite
What happens if you increase the pressure of a reaction at equilibrium?
- Shifts equilibrium to side with fewer moles of gas
- To reduce pressure/oppose the change
What happens if you decrease the pressure of a reaction at equilibrium?
- Equilibrium shifts to side with more moles of gas
- Raises pressure/opposes the change
What happens if you increase the temperature of a reaction at equilibrium?
- ↑ temp = adding heat
- Equilibrium shifts in endothermic (positive ΔH) direction to ↓ temp./oppose the change
What happens if you decrease the temperature of a reaction at equilibrium?
- ↓ temp = removing heat
- Equilibrium shifts in exothermic (negative ΔH) direction to produce more heat, to ↑ temp/oppose the change
Why do you get a higher yield at lower temperatures?
- ∵ it’s exothermic reaction, lower temps = forward reaction
- ∴ at lower temps = more ethene and steam is converted to ethanol
Why is the reaction carried out at a temperature of 300°C?
- It’s a compromise between reasonable yield and faster reaction
- Lower temps = slower rate of reaction
- No point in getting very high yield of ethanol if it takes you 10 years
Why is 60-70 atm used?
It’s a compromise - gives reasonable yield for lowest possible cost
Why is high pressure used?
- High pressure shifts equilibrium to side with fewer molecules = forward reaction, increases yield
- High pressure increases rate of reaction too
Why is not very high pressure used? Name 2 reasons why
- Very expensive ∵
- Power or energy required to provide high pressure
- Strong pressure vessel needed (to withstand high pressure)
State the expression that Kc can be written as for the equation:
aA + bB ⇌ dD + eE
If the temp change means there’s less product at equilibrium, what happens to Kc?
Kc ↓
State the units for this expression
mol–2 dm+6
