Equilibria II Flashcards
How do you calculate Kc from equilibrium concentrations?
The Kc equation is Kc =[reactants]^mol reactants / [products]^mol products
e.g. for the equilibrium aA + bB —> cC + dD Kc = [C]^c[D]^d/[A]^a[B]^b
The values for the equilibrium concentrations (in mol dm-3) can be substituted into the equation for Kc
How do you work out the units of Kc?
The units of Kc are found by cancelling out the units of the concentrations (e.g. if there are 2 mol reactants and 2 mol products, there are no units, if there is 1 mol reactants and 2 mol products, the units are mol dm-3 and so on).
How can you find equilibrium concentrations using starting moles/concentrations?
Use an ICE table (initial, change, equilibrium) to work out the initial moles of each substance, how much each will change by and therefore the equilibrium moles using stoichiometric ratios. Calculate equilibrium concentrations by dividing each by the total volume for each species and sub into Kc equation (if Kc is unitless you can use moles instead of concentrations as the volumes will cancel out).
What is partial pressure of a gas?
The pressure that a gas would exert on its container if it were the only gas in the container.
How do you calculate the partial pressure of a gas?
Partial pressure = total pressure x mole fraction
(Mole fraction is the proportion of the total number of gaseous moles that are of that gas)
How do you formulate a Kp expression?
Kp=(partial pressure products)^mol products/(partial pressure reactants)^mol reactants
For the equilibrium aA(g) + bB(g) —> cC(g) + dD(g) Kp = (p(C)^c x p(D)^d)/(p(A)^a x p(B)^b)
Square brackets are not used as the values are partial pressures, not molar concentrations.
How do you calculate Kp?
Use an ICE table to calculate equilibrium moles of each species then calculate the partial pressures of each. Sub into the Kp expression to calculate Kp. The units of the partial pressures cancel out to give the Kp units (e.g. if you had 2 moles of products and 3 moles of reactants and the pressures were in atm, the units would be atm-1).
When calculating Kp for heterogenous equilibria, are solids and liquids included?
No, only gases are present in the Kp expression.
How do concentration and pressure changes affect the equilibrium constant?
They have no effect on the equilibrium constant as the position of equilibrium will shift to restore the Kc/Kp value (the ratio of reactants to products remains the same).
What effect does the addition of a catalyst have on the value of Kc/Kp?
No effect as catalysts do not affect the position of equilibrium, but equilibrium will be reached faster.
What affect does temperature have on the value of the equilibrium constant?
Increasing the temperature will shift equilibrium in the endothermic direction while decreasing the temperature will favour the exothermic direction. If changing the temperature causes less product to form, the equilibrium moves to the left and the equilibrium constant decreases. If more product is formed, equilibrium shifts to the right and the equilibrium constant increases.
