Chapter 19
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What are the two main types of equilibria?
1) homogeneous 2) heterogeneous
What is homogeneous equilibrium?
Equilibrium species that all have the same state or phase
What is heterogeneous equilibrium?
Equilibrium species that have different states or phases
In heterogeneous equilibria, what states are omitted from the Kc expression?
1) solids 2) liquids
Why are solids + liquids omitted from the Kc expression?
They are essentially constant
How would you experimentally determine a value for Kc in the laboratory?
1) In a conical flask, mix together 0.100mol of both reactants 2) Add 0.0500mol of HCl(aq) as an acid catalyst to the flask - the total volume of the mixture in the flask is 20.0cm³ 3) Add 0.0500mol of HCl(aq) to a second conical flask as a control 3) Stopper both flasks and leave for a week to reach equilibrium 4) Carry out a titration on the equilibrium mixture using a standard solution of sodium hydroxide 5) Repeat the titration with the control to determine the amount of acid catalyst that had been added
When experimentally determining a value for Kc in the laboratory, how could you modify the method to ensure that equilibrium has been reached?
Set up another identical experiment at the start and sample it after 2 weeks. If the result is the same, then it is safe to assume that equilibrium had been established after 1 week
What is Kp?
Equilibria involving gases
For gases, is it easier or harder to measure pressure than concentration?
Easier
What is the mole fraction of a gas?
Its proportion by volume to the total volume of gases in a gas mixture
mole fraction x(A) =
number of moles of A / total number of moles in a gas mixture
What is partial pressure, p?
The contribution that a gas makes towards the total pressure
sum of partial pressures =
total pressure
partial pressure p(A) =
mole fraction of A x total pressure P x(A) x P
What volume does 1 mole of gas occupy at RTP?
24.0cm³
sum of mole fractions =
1
What are suitable units for partial pressures?
1) kilopascals (kPa) 2) pascals (Pa) 3) atmospheres (atm)
If concentration of a species is increased, where does the position of equilibrium shift?
The equilibrium position shifts in the direction that reduces the concentration
If the pressure is increased, where does the position of equilibrium shift?
The equilibrium position shifts towards the side with fewer gaseous moles
If the temperature is increased, where does the position of equilibrium shift?
The equilibrium position shifts in the endothermic direction
What does the magnitude of an equilibrium constant K indicate?
The extent of a chemical equilibrium
What does K = 1 indicate?
An equilibrium halfway between reactants and products
What does K = 100 indicate?
An equilibrium well in favour of the products
What does K = 1 x 10⁻² indicate?
An equilibrium well in favour of the reactants