Equilibrium Constants Flashcards
What does it mean when a reaction mixture has reached a point of dynamic equilibrium?
Reactants are being changed into products at the same rate as products are being changed into reactants
What is a reaction in which only one phase is present described as?
Homogeneous
If there is a mixture of phases what is the reaction described as?
Heterogeneous
What is the equilibrium constant given by?
Kc = [C]c[D]d/[A]a[B]b
What is daltons law?
The partial pressures of all the gases in a system add up to the total pressure
Ptotal = PA +PB +PC +….
In homogeneous gaseous reactions what are the quantities of reactants and products given in terms of?
Pressure rather than concentration
What is the mole fraction x of a substance?
The number of moles of the substance divided bu the total number of moles in the mixture:
xA = moles of A/ total number of moles
What does the sum of all the mole fractions of reactants and products in an equilibrium mixture always equal?
1
In a mixture of gases what does the mole fraction of each gas equal?
Its partial pressure divided by the total pressure:
xA = PA/Ptotal
How is the equation for the equilibrium constant of a gaseous reaction written?
The same way as the equation for a reaction that takes place in a solution with products on the top and reactants on the bottom. However while square brackets are used to show concentrations of reactants and products in solution, the symbol P is used for partial pressures of reactants and products in the gas phase
Kp = P(C)c/ P(A)aP(B)b
What are the values used to calculate Kp?
The partial pressures or the gases and not concentrations, mole fractions or any other values
What are the units of partial pressure?
Usually pascals (Pa) or kilopascals (kPa) 1000 Pa = 1 Kpa
How can the units of Kp be calculated?
By replacing each of the terms in the equation by their units and then cancelling out duplicated units
How do you use ICE to calculate equilibrium constants?
1) Construct an ICE table with a seperate column for each substance
2) Insert all known values of initial amounts of substances. The values must be in moles
3) Use the stoichiometru of the reaction to show the changes in the amount of substance (e.g. one mole = x 2 moles = 2x)
4) Use the initial values and changes to produce an equation containing X to show the equilibrium amounts of each substance
5) solve for the value of X using the data provided for the equilibrium amount of one substance
6) calculate equilibrium numbers of moles of each substance
7) calculate the partial pressure of each substance using the total pressure muliplied by the mole fraction
8) use the partial pressures to calculate the equilibrium constant and round the value of Kp to two significant figures
What reaction condition changes the equilibrium position and the value of the equilibrium constant?
Changing the temperature of the system
Why does the equilibrium constant not change when the pressure changes?
Because the pressure is used in the calculation of the equilibrium constant and the increased yield is cancelled out by the increased pressure
Why does using a catalyst not change the equilibrium constant?
As it only affects the rate at which the reaction mixture reaches equilibrium and not the actual position of equilibrium
What effect does increased pressure or concentration have on rate, rate constant, yield and equilibrium constant?
Rate: increase
Rate constant: no change
Yield: depends on stoichometry
Equilibrium constant: no change
What effect does increased temperature have on rate, rate constant, yield and equilibrium constant?
Rate: increase
Rate constant: increase
Yield: depends on enthalpy change
Equilibrium constant: depends on enthalpy change
What effect does the addition of a catalyst have on rate, rate constant, yield and equilibrium constant?
Rate: increase
Rate constant: increase
Yield: no change
Equilibrium constant: no change
Which an exothermic reaction what would an increase in temperature lead to?
The equilibrium constant decreasing, the equilibrium position shifting to the left and a decrease in the equilibrium yield of the product
