Equilibrium Flashcards
What is dynamic equilibrium?
Where the forward reaction as at the same rate as the backwards reaction
What can be said about the concentration of reactants and products for a reaction in dynamic equilibrium?
Because the forwards and backwards reactions are at the same rate, there is no overall change in concentrations of the reactants or products
What is the only type of system a dynamic equilibrium can occur?
Closed system at a constant temperature
Give an example of a physical system that an equilibria can be set up in
When liquid bromine is shaken in a closed flask, it turns into bromine gas. After a while, equilibrium is reached, where the exact amount of liquid turning into gas is the same as the exact amount of gas turning into liquid
Give an example of a chemical system that equilibria can be set up in
Hydrogen gas reacted with iodine gas to form hydrogen iodide
What is the equilibrium constant?
A ratio worked out from the concentration of the products and reactants once a reversible reaction has reached its equilibrium
For the general equation aA + bB ⇌ dD + eE, what is the equation for the equilibrium constant?
Kc = ([D]^d [E]^e) / ([A]^a [B]^b)
Lowercase letters are the number of moles of each reactant as shown in the balanced equation
What are the 2 steps to work out the equilibrium concentrations?
Find out how many moles of each reactant and product there are at equilibrium
Calculate the molar concentrations of each reactant and product by dividing the number of moles by the given volume
Why do you use square brackets for working out the equilibrium constant?
Because square brackets denote concentration (you won’t get mark in exam if use round brackets)
How can you work out unknown equilibrium concentrations if given the equilibrium constant?
Put all the values you know into the expression for Kc
Rearrange to solve for the unknown concentrations
What is a homogeneous reaction?
A reaction where all the reactants and products are in the same state
What is a heterogeneous reaction?
A reaction where reactants and products are in different states
What is the rule for putting in values for the equilibrium constant expression for a homogeneous reaction?
All the reactants and products are put into the expression for the equilibrium constant
What is the rule for putting in values for the equilibrium constant expression for a heterogeneous reaction?
Only gases and aqueous substances go into the expression for the equilibrium constant
Why do you not put solids and liquids into the equilibrium constant expression for a heterogeneous reaction?
Because they do not affect the reactant amount at equilibrium in the reaction
When you’ve found the equilibrium constant for a reaction, what else do you need to work out?
The units
How do you work out the units of the equilibrium constant
Make an equation out of the units for all the concentrations and then simplify
What is partial pressure?
The pressure exerted by a particular gas in a mixture of gases
How do you quite often see partial pressures written down as?
p(X) is the partial pressure of X
What is a mole fraction?
A proportion of a gas mixture that is made up of a particular gas
If you have 4 moles of a gas in total and 2 of them are gas A, what is the mole fraction of gas A?
4/2 = 0.5
What is the equation for the mole fraction?
Mole fraction = Number of moles of gas / Total number of moles of gas in mixture
What is the equation for partial pressure of a gas?
Partial pressure = Mole fraction of gas x Total pressure of mixture
What is the difference between Kc and Kp?
Kc is the equilibrium constant when it is found through the use of concentrations, whereas Kp is the equilibrium constant when it is found through the use of partial pressures
For the equilibrium aA(g) + bB(g) ⇌ dD(g) + eE(g), what is the equation for the equilibrium constant, Kp?
Kp = ( p(D)^d x p(E)^e) / ( p(A)^a x p(B)^b)
When using the equation for the equilibrium constant with partial pressures, what don’t you include in the equation?
Solids, Liquids and Aqueous substances
Why don’t you include solids, liquids and aqueous substances in the equation for Kp?
Because they have no effect on the pressure of the whole system
What is a particular value of Kp only valid for?
A specific temperature, as Kp is temperature dependant
When writing an expression for Kp for a heterogeneous equilibria, what don’t you include in the equation?
Solids or liquids
What is Le Chatelier’s Principle?
If there’s a change in concentration, pressure or temperature, the equilibrium will move to help counteract the change
Do you get more product or reactants if the equilibrium shifts left?
More reactants
Do you get more product or reactants if the equilibrium shifts right?
More products
What does Kc tell you about equilibrium position?
The size of Kc tells you where the equilibrium lies
Where is the equilibrium if Kc is a large value?
Further to the right (more product)
Where is the equilibrium if Kc is a small value?
Further to the left (more reactant)
What happens to the equilibrium position if you increase the temperature?
It’ll move in the endothermic (+ve ΔH) direction to absorb the heat
What happens to the equilibrium position if you decrease the temperature?
It’ll move in the exothermic (-ve ΔH) direction to try and replace the heat
For a equilibrium reaction, if the forward reaction is endothermic (+ve ΔH), what is the backwards reaction?
Exothermic (-ve ΔH)
What do the ΔH values given for a reversible reaction tell you?
ΔH of the forward reaction
If you change the concentration of one of the things in the equilibrium mixture, what do the other compounds do to keep the equilibrium constant the same for a given temperature?
They have to change to balance
What state does changing the pressure have an effect on the equilibrium?
For gases
What does the equilibrium do if you increase the pressure of the equilibrium mixture?
It moves to the side with the fewest moles of gas (fewest molecules of gas) to reduce the pressure
What does the equilibrium do if you decrease the pressure of the equilibrium mixture?
It moves to the side with the most moles of gas (most molecules of gas) to increase the pressure
What is the effect of adding a catalyst to the equilibrium position and the value of the equilibrium constant?
Adding a catalyst has no effect on either
Why does adding a catalyst have no effect on the equilibrium position and the value of the equilibrium constant?
Because it increases the rate of the forward and backwards reactions by the same amount (meaning equilibrium will be reached faster)
Is endothermic +ΔH or -ΔH?
+ΔH, because it takes in energy from its surroundings
Is exothermic +ΔH or -ΔH?
-ΔH, because it releases energy to its surroundings
What 3 factors have no effect on the equilibrium constants?
Changing pressure
Changing concentration
Adding a catalyst
