Topic 10 - Equilibrium I Flashcards
What is the definition of a reversible reaction?
One where both: the reactants react to form the products whilst the products simultaneously break down to reform the reactants
What is used to show reversible reaction?
2 half arrows pointing to the products and the reactants
The formation of products is called the…
The forwards reaction
The formation of reactants is called the…
The backwards reaction
What conditions must there be for dynamic equilibrium?
A closed system
What features are characteristic of dynamic equilibrium?
- Rate of the forwards reactions = Rate of the backwards reaction
- Both the forwards and backwards reaction are proceeding
- The concentration of both the reactants and products remains constant
Are all equilibrium mixtures the same?
No
What is the definition of an equilibrium mixture?
The amounts of products and reactants present when dynamic equilibrium is reached
State Le Chateliers Principle
If a dynamic equilibrium is subject to a change in conditions, the position of equilibrium movea to counteract that change
What are the 3 factors that change the position of equilibrium?
- Temperature (reversible reactions have an exothermic and endothermic direction)
- Concentration (if you feed off your products or change the concentrations of one of your products or reactants
- Pressure (to do with gases and shifting to the side with less/more moles)
Does the addition of a catalyst change the position of equilibrium?
No
What happens when a catalyst is added to a reversible reaction?
- The rate of both the forwards and backwards reaction is increased
- Dynamic equilibrium is reached more quickly
What is the definition of a homogenous equilibrium?
An equilibrium where everything is present in the same phase/state
What is the definition of a heterogenous equilibrium?
An equilibrium where compounds are present in more than one phase
What is Kc?
The equilibrium constant
What is the definition of Kc?
A numerical value associated with the equilibrium concentrations of each product and reactant in a reaction AT A GIVEN TEMPERATURE
What is the only condition that changes the value of Kc?
Temperature
How do you find the units for Kc?
- Each species has the units moldm(-3)
- Use indices rules in order to determine the units of Kc for each individual equilibria
NB: sometimes an equilibria will have no units
What is the Kc expression for this equilibrium:
mA + nB <=> pC + qD
[A]^m[B]^n / [C]^p[D]^q
What do the reacting mole numbers become in Kc expressions?
The powers in front of the species
Write the equation for the contact process and, therefore, its Kc expression and units
2SO2 (g) + O2 (g) <=> 2SO3 (g)
Kc = [SO3]^2 / [SO2]^2[O2]
units - dm^3mol^-1
Why is it important that you write down the equation next to your Kc expression?
As Kc expression can be written either way round as they describe reversible reactions. Therefore, you need to write down the reaction direction you are taking as forward (and therefore the backwards reaction too)
Write the equation for the contact process and, therefore, its Kc expression and units
N2 (g) + 3H2 (g) <=> 2NH3 (g)
Kc = [NH3]^2 / [N2][H2}^3
units - dm^6mol^-2
What is the effect on Kc if the POE is in the centre?
Kc = 1
What is the effect on Kc if the POE lies to the right?
Kc > 1
What is the effect on Kc if the POE lies to the left?
Kc < 1
Which state is not included in any Kc expressions? Why?
Solids - they do not have a concentration
If solids are present in an equilibria what do you do to them in the Kc expression?
get rid of them
If you are given the moles in a Kc question what do you need to do?
- Determine starting moles and then equilibrium moles of all species in the reaction
- Use the volume provided to determine the concentrations of each of the species at equilibrium
- Plug the concentrations into a Kc expression to determine Kc
If gases are reacted in a container that is 5dm^3 what is the volume you use when determining concentration of gases at equilibrium?
5 dm^3 - as gases fill the volume of whatever container they are in
What is the main factor you need to consider when determining equilibrium moles?
Reacting moles in the equation
What does a titration of a sample of the equilibrium solution help to determine?
The equilibrium concentration of a species - form that you can determine equilibrium moles of all your species and, therefore Kc
