Topic 11 Equilibrium 2 Flashcards
What is the definition of dynamic equilibrium?
When the forward and backwards reactions in a reversible reaction continue at equal rate so that there is no overall change.
What is the symbol for the equilibrium constant?
K - This is most commonly seen a K꜀ (concentration) or Kₚ (pressure)
What is the equation to calculate the K꜀?
Reaction - aA + bB –> cC + dD
K꜀ = [C]ᶜ + [D]ᵈ / [A]ᵃ + [B]ᵇ
K꜀ = Conc of products / Conc of reactants
What do [X] mean when calculating K꜀?
[X] means the concentration of X
How do you work out the units for K꜀?
If K꜀ = [C]ᶜ + [D]ᵈ / [A]ᵃ + [B]ᵇ
Then Units = (mol dm⁻³)ᶜ + (mol dm⁻³)ᵈ / (mol dm⁻³)ᵃ + (mol dm⁻³)ᵇ
E.g. If a=1, b=5, c=2, d=1 Then Units = (mol dm⁻³)² + (mol dm⁻³)¹ / (mol dm⁻³)¹ + (mol dm⁻³)⁵ = (mol dm⁻³)³ / (mol dm⁻³)⁶ = 1 / (mol dm⁻³)³ = 1 / mol³ dm⁻⁹ = mol⁻³ dm⁹
What are the 3 steps required to find the equilibrium constant by experiment?
- Mix measured quantities of reactants and/or products. Then allow the mixture to reach equilibrium under steady conditions.
- Analyse the mixture to find the equilibrium concentration of one of the chemicals at equilibrium.
- Use the reaction equation for the reaction and the information from steps 1 & 2 to work out all the equilibrium concentrations of all the atoms, molecules or ions. Then substitute the values into the equilibrium constant equation and calculate the K꜀.
What do you need to keep in mind when calculating an equilibrium constant?
Only gaseous or aqueous atom, molecules or ions should be included. If there is a solid or liquid present in the reaction, these should be ignored when calculating the equilibrium constant.
E.g. H₂O(g) + C(s) –> H₂(g) + CO(g)
K꜀ = [H₂] + [CO] / [H₂O]
What is the definition of pressure?
The force per unit area.
The SI unit for pressure is the Pascal (Pa) which is the pressure of 1 neutron per square meter.
The Pascal is a very small unit so pressure is normally seen in kilopascals (kPa). Standard atmospheric pressure is 101.3 kPa
What is the definition of partial pressure?
The partial pressure of a gas is a measure of its concentration in a mixture of gases. It is the pressure that the gas would exert if it was the only gas present.
How can partial pressure be calculated?
If the reaction = A ⇌ B + C
Total Pressure = Partial Pressure A + Partial Pressure B + Partial Pressure C
Or
Partial Pressure A = Total Pressure x Mole Fraction of A
What is a mole fraction?
If the reaction = A ⇌ B + C
Mole Fraction of A = Moles of A / Total number of Moles
How do you find equilibrium constants by experiment?
- Mix measured quantities of reactants and/or products. Then allow the mixture to reach equilibrium under steady conditions.
- Analyse the mixture to find the equilibrium concentration of one of the chemicals at equilibrium.
- Use the equation for the reaction and the information from steps 1 and 2 to work out the values for the equilibrium concentration of all the atoms, molecules or ions. Then substitute these values into the expression for K꜀.
What is a heterogeneous equilbrium?
When the substances involved in the equilibrium are not in the same phase.
e.g. When steam is heated with carbon in a closed container.
What is a homogenous equilibrium?
When the substances involved in the equilibrium are in the same phase.
e.g. 2SO₂ + O₂ ⇌ 2SO₃
What do different values of K꜀ mean to the direction and extent of change to the equilibrium?
K꜀ < 1 x10⁻¹⁰ - Reaction does not go.
K꜀ ≈ 0.01 - Reaction reaches an equilibrium in which the reactants are predominant.
K꜀ = 1 - Roughly equal amounts of reactants and products at equilibrium.
K꜀ ≈ 100 - Reaction reaches an equilibrium in which the products are predominant.
K꜀ > 1 x10¹⁰ - Reaction goes to completion.
