Equilibrium (topic 7/17) Flashcards
Le Chatelier’s Principle
When a system in equilibrium is stressed, it moves to oppose the stress. This principle predicts how a reaction/system will respond to one or more changes and what direction the equilibrium will shift.
Kc «_space;1
Equilibrium constant is smaller than smaller than 1, reactants are favoured
Kc»_space; 1
Equilibrium constant is bigger than bigger than 1, products are favoured
What can change Kc?
Only the temperature. Unless you change something in the equation then that would change the equlibrium constant
What can invert Kc?
Reversing the equation
What is Kc?
Equilibrium constant [equilibrium]
What is Q?
Reaction quotient [non-equilibrium]
Bond Breaking
Endothermic
Bond Making
Exothermic
2 conditions for chemical equilbrium to be established for a certain reaction
- Forward and backward reaction occur at the same rate.
- The concentration of reactants and products remain constant.
Can only happen in a closed system, where the reactants and/or products do not come in contact with the surroundings.
Chemical equilibrium at a microscopic level
The reaction does not stop as the reactants are continuously changed to products and the products are continuously changed to the reactants.
Chemical equilibrium at a macroscopic level
There is no change in the properties such as pressure, colour, pH, or concentration.
Physical equilibrium
Does not involve a chemical reaction, rather a change in the state, for example from liquid to gas and gas to liquid.
2 conditions for physical equilbrium to be established for a certain reaction
- The rate of forward process is equal to the rate of backward process.
- The volume of liquid remains constant.
Can only occur in a closed system.
Physical equilibrium at a microscopic level
Where the liquid is getting continuously evaporated and the gas is getting continuously condensed.
Physical equilibrium at a macroscopic level
The properties remain the same, such as color.
The equilbrium constant
Tells us the ratio between concentration of products and the concentration of reactants.
All concentrations have the unit
mol dm^-3
Large value of Kc
Tells us that the concentration of products is much greater than the concentration of reactants at equilibrium, meaning that the forward reaction is favoured and the mixture will contain mostly products.
Small value of Kc
Tells us that the concentration of reactants is much greater than the concentration of products at equilibrium and that the backward reaction is favoured and so the mixture contains mostly reactants.
If the value of Kc = 1
There are equal amounts of reactants and products.
If we take the reverse reaction of Kc
The value of Kc gets reciprocated. Instead of products / reactants, it would be reactants / products.
If Q < Kc
The concentrations of reactants is much greater than the concentration of products, and in order to bring the reaction to equilbrium, the concentration of reactants needs to be decreased.
This means that the forward reaction needs to be favoured in order to bring the reaction back to equilibrium.
If Q > Kc
The concentrations of products is much greater than the concentration of reactants, and in order to bring the reaction to equilbrium, the concentration of products needs to be decreased.
This means that the backward reaction needs to be favoured in order to bring the reaction back to equilibrium.
If Q = Kc
The reaction is at equilbrium.
Parameters that can influence the equilibrium of a reaction.
- Temperature
- Pressure
- Concentration
- Catalyst
Temperature
- If the enthalpy change (delta H) of a system is positive, in other words if it is an endothermic reaction:
- An increase in temperature leads to equilibrium shifting to the product side, and the forward reaction is favoured.
- A decrease in temperature leads to to equilibrium shifting to the reactant side, and the backward reaction is favoured.
- If the enthalpy change (delta H) of a system is negative, in other words if it is an exothermic reaction:
- An increase in temperature leads to equilibrium shifting to the reactant side, and the backward reaction is favoured.
- A decrease in temperature leads to equilibrium shifting to the product side, and the forward reaction is favoured.
Pressure
- If the number of gas molecules on the product side exceeds the number of gas molecules on the reactant side:
- An increase in pressure leads to the equilibrium shifting to the reactant side and the backward reaction is favoured.
- A decrease in pressure leads to the equilibrium shifting to the product side and the forward reaction is favoured.
- If the number of gas molecules on the reactant side exceeds the number of gas molecules on the product side:
- An increase in pressure leads to the equilibrium shifting to the product side and the forward reaction is favoured.
- A decrease in pressure leads to the equilibrium shifting to the reactant side and the backward reaction is favoured.
Concentration
- If the concentration of the reactants increases or the concentration of products decreases, this means that the reaction quotient (Q) is now less than the equilibrium constant (Kc), which means in order to bring the system back to equilibrium, the concentration of reactants needs to decrease, and so the forward reaction is favoured and the equilibrium shifts to the product side.
- If the concentration of the products increases or the concentration of reactants decreases, this means that the reaction quotient (Q) is now more than the equilibrium constant (Kc), which means in order to bring the system back to equilibrium, the concentration of products needs to decrease, and so the backward reaction is favoured and the equilibrium shifts to the reactant side.
Catalyst
- The addition of a catalyst leads to an increase in the rate of both the forward and backward reactions, so the position of the equilibrium does not change.
- However, equilibrium is attained faster.
Haber Process
N2(g) + 3H2(g) <=> 2NH3(g)
- The enthalpy change (delta H) of this reaction is -92 kJ mol^-1, meaning it’s exothermic.
- In order to maximise the yield of ammonia from this process, there are several conditions that need to met:
- Low temperature: since the reaction is exothermic, the product side will be favoured at low temperatures, and so an optimum temperature of 450 degrees celciusis used (as a lower temperature would slow down the rate of reaction substantially).
- High pressure: Since the number of gas molecules on the product side are less than the number of gas molecules on the reactant side, at high pressures equilibrium will favour the rate of the forward reaction, so an optimum pressure of 200 - 250 ATM is used (as a higher pressure would not be economically feasible).
- Concentration of product: the concentration of ammonia in the reaction is continuously decreased in order to shift the equilibrium to the product side and increase the rate of the forward reaction.
- Catalyst: Solid Iron (Fe) is used as a catalyst to speed up the rate of the reaction and reach equilibrium faster.
- The ammonia produced is usually used to make synthetic fertilisers, and in the synthesis of nitric acid, polymers and explosives.
Contact process
2SO2(g) + O2(g) <=> 2SO3(g)
- The enthalpy change (delta H) of this reaction is -197 kJ mol^-1, meaning it is exothermic as well.
- In order to maximise the yield of sulfur trioxide from this process, there are several conditions that need to met:
- Low temperature: since the reaction is exothermic, the product side will be favoured at low temperatures, and so an optimum temperature of 450 degrees celciusis used (as a lower temperature would slow down the rate of reaction substantially).
- High pressure: Since the number of gas molecules on the product side are less than the number of gas molecules on the reactant side, at high pressures equilibrium will favour the rate of the forward reaction, so an optimum pressure of 1 - 2 ATM is used (as this pressure is required for the process that occurs in order to convert the reactants to the product, and the reaction is about 97% efficient at this pressure).
- Concentration of product: the sulfur trioxide produced is continuously removed in order to shift equilibrium to the right and favour the product side.
- Catalyst: Vanadium Pentoxide (Va2O5) is used as a catalyst to speed up the rate of the reaction and reach equilibrium faster.
- The end product of the contact process, sulphuric acid (H2SO4), has many uses, including: fertilisers, dyes, detergent and soap, and paints and pigments.
The equilibrium law
- The equilibrium constant, Kc, can be calculated from the balanced chemical equation.
- The equilibrium law enables us to calculate Kc from given inital and final concentrations of reactants and products, as well as finding the concentrations of reactants/products at equilibrium.
ICE method
- Write the balanced equation for the described reaction
- Divide the values of concentrations for each component into 3 columns: initial, change, and equilibrium (ICE)
- Initial: represents the concenration of the compound before the reaction takes place; unless specified, initial product concentrations should be assumed to be 0.
- Change: represents the amount of the compound the reacts to reach the equilibrium concentration. This will always be negative for reactants and always be positive for the products. Changes will always occur in the specific stoichiometric ratios of the chemical equations, so given the change for one compounds, the other changes can be deduced.
- Equilibrium: this refers to the concentration of the compounds at equilibrium, and is equal to initial concentration +-change in concentration (This is the value that we have to find, and will directly be used to find Kc)
- Write the equilibrium expression for Kc, and plug in the equilibrium concentration values to find the value for Kc.
Gibbs Free Energy Equilibrium
Delta GØ = 0
When delta GØ < 0
The reactants have more energy than the products. Therefore equilibrium shifts to the right as forward reaction proceeds.
When delta GØ > 0
The products have more energy than the reactants. Therefore equilibrium shifts to the left as backward reaction proceeds.
When delta GØ = 0
The reactants and the products have the same energy, and hence the same collision frequencies, and the rate of the forward reaction equals the rate of the backward reaction. Therefore equilibrium is attainted.
A reaction which has delta GØ «< 0
Occurs spontaneously, and consists of an equilibrium mixture that contains mostly products.
A reaction which has delta Gػ_space;> 0
Occurs non-spotaneously and consists of an equilibrium mixture that contains mostly reactants.
The free energy of a reaction, and the equilibrium constant for that reaction at a particular temperature are linked by the following equation:
Delta GØ = -RT ln (Kc),
where
delta GØ = Standard Gibbs’ free energy
R = universal gas constant
T = specific absolute temperature of the reaction (in Kelvin)
Kc = numerical value of the equilibrium constant of the reaction
Free energy and Equilibrium mixture
Delta GØ < 0, ln (Kc) > 0, Kc > 1 - mostly products
Delta GØ > 0, ln (Kc) < 0, Kc < 1 - mostly reactants
Delta GØ = 0, ln (Kc) = 0, Kc = 1 - similar amounts of products and reactants
