Equilibrium Constants

Question 1

What does it mean when a reaction mixture has reached a point of dynamic equilibrium?

Answer 1

Reactants are being changed into products at the same rate as products are being changed into reactants

Question 2

What is a reaction in which only one phase is present described as?

Answer 2

Homogeneous

Question 3

If there is a mixture of phases what is the reaction described as?

Answer 3

Heterogeneous

Question 4

What is the equilibrium constant given by?

Answer 4

Kc = [C]c[D]d/[A]a[B]b

Question 5

What is daltons law?

Answer 5

The partial pressures of all the gases in a system add up to the total pressure

Ptotal = PA +PB +PC +….

Question 6

In homogeneous gaseous reactions what are the quantities of reactants and products given in terms of?

Answer 6

Pressure rather than concentration

Question 7

What is the mole fraction x of a substance?

Answer 7

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

Question 8

What does the sum of all the mole fractions of reactants and products in an equilibrium mixture always equal?

Answer 8

1

Question 9

In a mixture of gases what does the mole fraction of each gas equal?

Answer 9

Its partial pressure divided by the total pressure:

xA = PA/Ptotal

Question 10

How is the equation for the equilibrium constant of a gaseous reaction written?

Answer 10

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

Question 11

What are the values used to calculate Kp?

Answer 11

The partial pressures or the gases and not concentrations, mole fractions or any other values

Question 12

What are the units of partial pressure?

Answer 12

Usually pascals (Pa) or kilopascals (kPa) 1000 Pa = 1 Kpa

Question 13

How can the units of Kp be calculated?

Answer 13

By replacing each of the terms in the equation by their units and then cancelling out duplicated units

Question 14

How do you use ICE to calculate equilibrium constants?

Answer 14

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

Question 15

What reaction condition changes the equilibrium position and the value of the equilibrium constant?

Answer 15

Changing the temperature of the system

Question 16

Why does the equilibrium constant not change when the pressure changes?

Answer 16

Because the pressure is used in the calculation of the equilibrium constant and the increased yield is cancelled out by the increased pressure

Question 17

Why does using a catalyst not change the equilibrium constant?

Answer 17

As it only affects the rate at which the reaction mixture reaches equilibrium and not the actual position of equilibrium

Question 18

What effect does increased pressure or concentration have on rate, rate constant, yield and equilibrium constant?

Answer 18

Rate: increase
Rate constant: no change
Yield: depends on stoichometry
Equilibrium constant: no change

Question 19

What effect does increased temperature have on rate, rate constant, yield and equilibrium constant?

Answer 19

Rate: increase
Rate constant: increase
Yield: depends on enthalpy change
Equilibrium constant: depends on enthalpy change

Question 20

What effect does the addition of a catalyst have on rate, rate constant, yield and equilibrium constant?

Answer 20

Rate: increase
Rate constant: increase
Yield: no change
Equilibrium constant: no change

Question 21

Which an exothermic reaction what would an increase in temperature lead to?

Answer 21

The equilibrium constant decreasing, the equilibrium position shifting to the left and a decrease in the equilibrium yield of the product