YR12 & YR13 Equilibrium - Kc and Kp
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What is the definition of dynamic equilibrium?
A dynamic equilibrium occurs when the forward and backward reaction happen at the same rate, the concentrations of products and reactants are constant and the reaction is continuous.
What is meant by Le Chatelier’s principle?
If an external condition is changed in an equilibrium system, the system will shift to try and reverse the change.
Using Le Chatelier’s principle, explain the effect of increasing the temperature on the position of equilibrium.
If temperature is increased, the equilibrium shifts to oppose this and moves in the endothermic direction to decrease temperature. This causes the equilibrium to shift towards the left, giving a lower yield of ammonia.
How does changing the pressure effect the position of equilibrium, according to Le Chatelier’s principle?
The equilibrium shifts to the side with less molecules when the pressure is increased. The equilibrium shifts to the side with more molecules when the pressure is decreased.
If given an equilibrium system where the moles of gas are equal on both sides of the equilibrium, how does changing the pressure effect said system?
Changing the pressure will have no effect on the equilibrium system
What effect does changing the concentrations in a system have on the equilibrium position?
• If a given substance is added, the equilibrium will move to remove that substance • If a given substance is removed, the equilibrium will move to regenerate that substance.
What effect does a catalyst have on the position of an equlibrium?
A catalyst has no effect on the position of equlibrium, but it will speed up the rate that said equlibrium is achieved. It speeds up the backward and forward reaction equally.
• Why might a low temperature not be used when it, in theory, produces the optimum yield of a desired product in an equilibrium • State an example of an industrial chemical manufacturing process where a compromise temprature is used.
• The rate of reaction can still be decreased significantly by using a lower temp. A compromise temp is therefore used, generating both an acceptable yield and rate. • Haber process.
Why might a high pressure not be used when it, in theory, produces the optimum yield of desired products?
Maintaining too high a pressure requires a significant amount of energy, which may make producing the chemical unprofitable.
Why, when an equlibrium involves an alkene, may it be inappropriate to use a high pressure even when it would theoretically produce the highest yield of desired products?
The alkene would readily polymerise, reducing atom economy
How is an expression for the equilibrium constant, Kc, written?
• Products divided by reactants in a fraction • All species must be shown in [square brackets] • The balancing number in front of each species is the power, which is placed outside of each bracket
When given a Kc expression, how are the units for Kc calculated?
• Each species in the Kc expression should be substituted with mol dm⁻³ • Cancel out units where applicable. If no units are left on the top of the fraction, this becomes the number 1.
How is the equlibrium constant, Kc, calculated?
• Work out the moles of each species at equlibrium by using the ICE table. Initial moles - change in moles = equlibrium moles. Using the equation given and ratios will allow you to find the initial moles for all species. • Then, work out the equlibrium concentrations by doing concentration = (moles x volume)/1000 • Put the concentrations into the Kc expression and calculate the numerical value as well as units, if required.
What is different about a Kp expression compared to a Kc expression?
The species must be inclosed in round brackets as the equation is considering pressure, not concentration.
In a gaseous equilibrium system, what is important about partial pressures?
• The total pressure must equal the partial pressures combined. • If the moles of the species are equal at equilibrium, the partial pressures will also be equal.
When given moles of a gaseuous equlibrium system, how is the value for Kp calculated?
• Work out the moles of each species at equlibrium by using the ICE table. Initial moles - change in moles = equlibrium moles. Using the equation given and ratios will allow you to find the initial moles for all species. • Add the equlibrium moles together to find the total moles. Calculate the mole fraction of each species by doing moles of species/total moles. • Calculate the partial pressure of each species by doing mole fraction x total pressure of system. • Subsitiute the partial pressures into the Kp expression, calculating the numerical value and the units if neccesary. The units for Kp are whatever pressure unit is used in the question.
How can the values of Kc/Kp be used to predict the yield in an equilibrium system?
The larger the value of Kc/Kp, the greater the amount of products. If Kc/Kp is small, the system favours the reactants.
How can the value of Kc/Kp be changed by influencing the conditions of the equilibrium system?
• Kc/Kp only change with temperature. • Changing pressure, concentration or adding a catalyst have no effect on Kc.