What is the definition of an equilibrium law?
The equilibrium law states that for the equilibrium aA + bB ⇔ C + dD
Kc = [C]c[D]d/[A]a[B]b
What is the definition of a homogeneous equilibrium?
A homogeneous equilibrium is an equilibrium in which all the species making up the reactants and products are in the same physical state.
What is the definition of a heterogeneous equilibrium?
is an equilibrium in which species making up the reactants and products are in different physical states.
How are the three meanings of K presented differently?
The equilibrium constant is represented by a capital K, followed by a subscript letter to indicate which equilibrium constant you are referring to a lower case k represents the rate constant and K is on its own is the symbol for potassium.
What would Kc, and it's units be for the reaction 2SO2 (g) + O2 (g) ⇔ 2SO3(g) ?
- Kc = [SO2(g)]2/ [SO2(g)]2 [O2(g)]
- Units (moldm-3)2/ (moldm-3)3 = dm3mol-1
What needs to be known to determine concentrations at equilibrium?
To determine a value of Kc, the concentration of reactants and products need to be known.
Chemists determine the concentrations in a number of ways including a titration and using a colorimeter.
What are the drawbacks of using titrations to determine concentrations at equilibrium?
- Titrating a reactant or product against a known concentration will show how much of the reactant or product os present.
- However the reactant or product has to be removed from the reaction mixture, and this will alter the position of equilibrium.
- Other substances may also be present, affecting the results (catalysts).
What can monitor the concentrations of product molecules throughout reactions to determine the value of Kc?
A colorimeter can be used to monitor the concentrations of a reactant or product throughout.
This works because coloured substances alter the amount of light that can pass through.
Calculating unknown equilibrium concentrations;
What can you tell from a balanced equation?
- the reacting quantities needed to prepare a required quantity of a product
- the quantities of products formed by reacting other known quantities of reactants.
What are the steps involved in calculating Kc from unknown equilibrium concentrations?
- Find the concentrations of all three components in the equilibrium mixture
- Work out the equilibrium amounts of the reacting molecules.
- You can use the balanced equation and stiochiochemistry to find out the moles of a reactant used a product formed.
- Finally, convert the amounts to concentration.
Follow these steps on the example on page 25.
What are the steps of calculating Kc from equilibrium concentrations?
- Calculate the moles at equilibrium by giving the moles at equilibrium by the volume of substance or container.
- Write the expression for Kc and work out the units. Then calculate Kc under these conditions.
Example on pg 24
If you are given equilibrium amounts in moles and you know the total volume, what must you calculate first?
concentrations = no. moles / volume in dm3
What is affected by an equilibrium being heterozygous, instead of homozygous?
When a heterogeneous equilibrium is present, molar concentrations for solids and pure liquids do not change because their volume remains constant.
This means that the expression for Kc changes:
- the concentrations of solid substances are not included in the expression
- the concentrations of pure liquids are not included in te expression
What is the expression for Kc for the reaction P4 (s)+ 5O2 (g) ⇔ P4O10 (s)?
Kc = [O2]5
What is a mole fraction?
The mole fraction of a substance is a measure of how much of a given substance is present in a reaction mixture.
What is the partial pressure?
The partial pressure of a substance is the pressure an individual gaseous substance would exert if it occupied a whole reaction vessel on its own.
What symbol is given to the mole fraction?
It is given the symbol X.
How is the mole fraction of a substance calculated?
mole fraction, XA = number of moles of substance A / total number of moles of all substances
What is the pressure of any reaction mixture involving gases within a sealed vessel the sum of?
The pressure of any reaction mixture involving gases within a sealed vessel is effectively the sum of being exerted by each of the gaseous substances involved.
The amount of pressure being exerted by an individual species within a reaction vessel is called the partial pressure.
What symbol is the partial pressure given?
partial pressure is given the symbol, P, followed by a subscript denoting the species to which is refers.
How do you calculate partial pressure from the total pressure?
If we know the total pressure of a reaction along with the mole fraction of a given substance, we can calculate the partial pressure of that substance.
For substance A: partial pressure, PA = mole fraction x total pressure.
What equilibrium constant is used for reactions involving gases?
Equilibrium expressions can be written using partial pressures instead of concentrations.
What would the equilibrium constant, Kp, be for the reaction aA + bB ⇔ cC + dD?
kp = (pC)c (PD)d / (PA)a (PB)b
How does the expression for kp change in a heterogenous reaction?
- Solids will not be included in the expression
- pure liquids will not be included in the expression
What does the magnitude of the eqilibrium constants kp and kc indicate?
The extent of a chemical reaction.
A equilibrium constant, k, with a value of 1 would indicate of 1 would indicate that the position of equilibrium is halfway between reactants and products.
What side of the reaction does equilibrium favour when K is less than 1?
- the reaction is reactant favoured
- the reactants on the left-hand side predominate at equiibrium.
What side of the reaction does equilibrium favour when kp is less than one?
When kp is less than 1:
- the reaction is reactant-favoured
- the reactants on the left hand side predominate at equilibrium
How do changes in temperature affect the enthalpy value?
- An increase in temperature shifts the position of equilibrium in the endothermic direction (ΔH + ve)
- A decrease in temperature shifts the position of equilibrium in the exothermic direction (ΔH - ve)
How do the ΔH values for the forward and reverse reactions in equilibrium compare?
They have the same magnitude but have opposite signs.