Kc and Kp Flashcards
How do you calculate Kc from concentrations
The products are on the top of the fraction and the reactants are on the bottom, you then divide. if the reaction is 2A+B = C you would square A and multiply t with B , C would then divide this sum, as it would be on the top of the fraction.
How do you calculate the units for Kc
You would put it into the Kc equation squaring where appropriate then simplifying.
How do you figure out concentration for Kc
You calculate the change in mole for each substance then either add or subtract from the original amount of moles for each substance. If there is two of a substance then that change will be two times the amount of the others. You then find the concentration by dividing moles by volume.
What substances can you calculate Kc for
Gases and aqueous substances
Homogeneous equilibrium
Contains equilibrium species that are all in the same state
Heterogeneous equilibria
Contains equilibrium species that are in different states
Mole fraction
The proportion by volume of a gas to the total volume of gases in a gas mixture. Number of moles of substance A / Total number of moles in gas mixture added together
Partial pressure
The contribution that the gas makes towards the total pressure. The sum of the partial pressures equals the total pressure. Partial pressure = mole fraction of A x total pressure
What substances can you calculate Kp for
gases
How do you calculate Kp
You divide the final moles of the substance by the total moles of all the substances added together. You then times the moles fraction for each substance by the total pressure. You then out it into the equation
When does Kc or Kp change
Only with a change in temperature it does not change with modifications to concentration, pressure or a catalyst
Whats the effect of increasing temperature on an exothermic reaction to the kc
The equilibrium constant decreases with increasing temperature, to take in more heat energy in and minimise the increase in temperature. So when the Kc decreases the system is no longer in equilibrium. The ratio of the Kc equation is now greater than Kc. Therefore the concentration of the product must decrease and the concentration of the reactant must increase. The position of the equilibrium will then shift to the left
Whats the delta H value for an exothermic reaction
negative
Whats the effect of increasing temperature on an exothermic reaction to the kp
The equilibrium constant increases with increasing temperature and more products are produced. As the temperature increases the value for Kp increases, the system is no longer in equilibrium as the ration of the Kp equation is now less then Kp. The partial pressures which gave rise the old Kp value must now change to give rise to the new one. So the partial pressure of the product must increase and the partial pressure of the reactants must decrease. The position of equilibrium shifts towards the products
Equilibrium constants and concentration change
If the concentration of one of the reactants increases, the ratio of the Kc euation is now less then Kc and the system is no longer in equilibrium. The concentrations must change in order to return back to the Kc value. Therefore the concentration of the reactant must decrease and the concentration of the product must increase. A new equilibrium is established where the ratio equals the original Kc value.
How to compare the effect on Kc of pressure change
N2O4 –> 2NO2
If the total pressure is doubled, then there will be a doubling of the partial pressure and concentration of both reactant and product. The Kp equation will now be greater then Kp, the system is no longer in equilibrium. The partial pressures must change back to return the ratio back to the Kp value. The partial pressure of the product NO2 must decrease. The partial pressure of the reactant N2O4 must increase.
How does a catalyst affect equilibrium constants
Catalysts affect the rate of a chemical reaction but not the position, they speed up both the forward and reverse reaction
What is a dynamic equilibrium
It exists in a closed system, it is when the rate of the forward reaction is equal to the rate of the reverse reaction and the concentration of reactants and products does not change.
How does a catalyst affect the position of equilibrium
A catalyst increases the rate of both the forward and reverse reaction in an equilibrium by the same amount resulting in an unchanged position of equilibrium
What is le Chateliers principle
When a system in equilibrium is subjected to an external change the system readjusts itself to minimise the effect of that change
The effect of concentration change on equilibrium
If you were to add more reactant, the equilibrium position will shift to the right and more products will be formed.
Investigating changes to the position of equilibrium with concentration
Yellow chromate (CrO42-) ion react with H+ to form orange dichromate ions (Cr2O72-) and water, as they have different colours you can see change in equilibrium.
1) Add a solution of K2CrO4 to a baker
2) Add dilute sulphuric acid until there is no further change, the solution goes orange
3) Add NaOH, the solution goes yellow
Explanation of experimental change in position of equilibrium with concentration
By adding acid you are increasing the concentration of H+ ions, thus increasing the rate of the forward reaction causing a shift to minimise the H+ concentration. This shift decreases the concentration of H+ reactant. The position of equilibrium shifts to the right, making more products so it goes orange. Its the same but the other way when you add NaOH
Investigating changes to the position of equilibrium with temperature
Cobalt chloride dissolves in water to form a pink soloution, this produces an equilibrium between two complexes of cobalt that are different colours The forward reaction is endothermic
[Co(H2O)6]2+(pink) + 4Cl- –> CoCl4-2(blue) +6H2O
1) Dissolve cobalt chloride in water in attest tube, add HCl, place in some iced water, the solution is pink
2) Put the test tube in a boiling water bath, the solution turns blue
What direction does the equilibrium shift when there is an temperature change
An increase in temperature shifts the equilibrium position in the endothermic direction delta H is positive, to take heat energy in and minimise the increase in temperature. A decrease in temperature will shift the reaction towards the exothermic side
The effect of pressure change on equilibrium
Increasing the pressure of the system will shift the position of the equilibrium towards the side with the fewer molecules, reducing the pressure of the system
The effect of catalyst on equilibrium
A catalyst does not change the position of equilibrium as it speeds up the rate of both the forward and reverse reaction equally. A catalyst will increase the rate at which equilibrium is established
The compromises needed in establishing operational conditions in the chemical industry with the example of the haber process
In the haber process N2(g) + 3H2(g) –> 2NH3(g) delta H is negative. A low temperature would produce a high yield of product but it would be a slow rate. A high pressure would increase the yield but this requires a lot of energy increasing the cost. The compromise is to give a high enough temperature to give a reasonable rate without shifting the equilibrium position to far to the reactants. Ensuring a good yield of ammonia is achieved quickly, cheaply and safety.
