Chemical Equilibria Flashcards
as chem
Describe forward reactions in terms of time and concentration.
Initially reactants are used up quickly but then slow as their concentration drops.
Describe backward reactions in terms of time and concentration.
Initially reactants are reformed slowly but speed up as the concentration of products increases.
When is dynamic equilibrium reached?
When the rate of the forward reaction = the rate of the backwards reaction. The CONCENTRATION of each substance remains constant.
When can dynamic equilibrium only occur?
It can only occur in closed systems.
What is the Le Chatelier’s principle?
If a reaction at equilibrium is subjected to a change in pressure, temperature or concentration, the position of equilibrium will move to counteract the change.
What happens to equilibrium when concentration changes?
( only works in homogenous equilibria where reactants and products are in the same state)
If we increase the concentration of a reactant OR product the equilibrium will shift to try to reduce the concentration. (The opposite will happen if concentration is decreased)
(equilibrium shifts to the opposite side to where concentration increased)
What happens to equilibrium when pressure changes?
( only works in homogenous equilibria where reactants and products are in the same state)
Increasing the pressure means equilibrium will shift to the side with fewest number of gas particles this would reduce pressure.
Decreasing pressure would mean equilibrium will shift to side with most number of gas particles to increase pressure.
What happens to equilibrium when temperature changes?
(identify what the forward reaction is to figure out directions)
Increasing temperature means equilibrium will move in the endothermic direction to reduce temperature.
Decreasing temperature means equilibrium will move in the exothermic direction to increase temperature.
What is the effect of catalysts on equilibrium?
Catalysts have no effect on the position of equilibrium. A catalyst will speed up the rate of the forward and backward reaction equally.
A catalyst will speed up the rate at which equilibrium is reached but have no effect on yield.
What’s the link between Le Chatelier’s principle and making ethanol?
Conditions:
Pressure= 60 atmospheres
Temperature= 300*c
Catalyst= Phosphoric acid (h3PO4)
Temperature)
Forward reaction is exothermic so decreasing the temperature will mean equilibrium shifts to the right producing more ethanol.
However, a lower temperature means lower rate of reaction. 300°C is a compromise between yield and rate.
Pressure)
High pressure means equilibrium shifts right producing more ethanol. Less moles of gas on the right so equilibrium shifts right to reduce pressure.
High pressure increases rate too.
However, high pressure environments are expensive due to thicker more robust vessels and pipes needed. There is a compromise between yield / speed and cost.
What is KC?
Kc is a worked out value of the equilibrium constant and is worked out from a molar concentration of a reaction.
How is a KC expression written?
1) PRODUCTS/ REACTANTS
2) Square brackets used
3) If there is a number infant of species this turns into a power
4) substitute numbers into expression.
5) work out units by putting moldm-3 in the same format and cancel it out to see remaining moldm-3.
if it remains at the bottom then flip it up and invert the powers.
How do you work out concentrations to work out KC?
1) Draw an ICE table- to work out number of moles
Initial
Change
Equilibrium
2) moles for products at beginning would be 0
use mass/mr
3) Changes:
products = increases
Reactants= decreases
If there is a difference in moles x to reach amount
4) Then calculate concentrations
moles/ volume given= conc
5) substitute into kc expression written. to give kc value
6) work out units
How do you use KC to find equilibrium concentrations?
Substitute numbers in KC expression
Find remaining value and divide to fit number of reactants/products.
What is the affect of temperature on KC?
Kc is only valid for one temperature so changing temperature changes equilibrium concentration so kc would change.
If temp change causes equilibrium to shift right = KC INCREASES.
if temp change causes equilibrium to shift left= KC DECREASES.
What is the affect of concentration on KC?
Value of kc is unaffected by any changes in concentration
What is the affect of catalysts on value on kc?
Adding a catalyst has no effect on the value of Kc.
A catalyst speeds up the forward AND reverse reaction.
A catalyst speeds up the rate at which equilibrium is established.
