3.2.3, 5.1.2 Equilibrium constant Kc Flashcards
what is equilibrium constant used for
Kc is used to measure th eposition of equilibrium
state the equation for Kc
Kc = [products]/[reactants]
state the expression for Kc using
aA +bB ↔ cC + dD
Kc = ([C]^c x [D]^d)/ ([A]^a x [B]^b)
^ = to the power of
What does the value of Kc give us
It tells us about the position of equilibrium
What does
What does Kc > 1 tell us
Equilibrium favour the products
What does Kc < 1 tell us
Equilibrium favours the reactants
What does Kc = 1 tell us
Equilibrium lies in the middle
How are the units of Kc worked out
- subsitute units into the expression for Kc (usually mol dm⁻³)
- cancel common units and show the final units on a single line
what happens if the units of Kc are ( mol dm⁻³)/ (mol dm⁻³) and so equal 1
if Kc units = 1 then no units are used
Outline the steps to calculate Kc
- expression for Kc
- equilibrium concentrations
- calculate Kc
- work out units
What changes with calculating Kc if there are no units
When there are no units mol ratio can be used instead of concentrations
How do you calculate Kc from initial moles when there are units
exam level question
Create a table, across the top will be the molecules and down the left will be:
- Initial moles
- equilibrium moles
- change
use mole ratio to work out the change and use that to work out the equilibrium moles. (dont forget to use the right +/- signs)
Using the equilibrium moles work out the concentration using c=n/v
Insert into the Kc expression
then work out units
(products = +) (reactants -)
the initial moles of a product will be 0.00
when using the table always go to x.xx at least
What factors can change Kc
ONLY temperature
How does temperature change Kc
If: A↔B. 𝚫H = -ve
The increase in temperature favours the endothermic reaction (here the backwards one)
↑[A] and ↓[B]
Kc = ↓[B] /↑[A]
Therefore Kc changes (decreases here)
How does Kc not change when subjected to changes in concentration and pressure
If: A↔B. 𝚫H = -ve
Equilibrium shifts to minimise the changes and restores Kc.
what are the two types of equilibria
Homogeneous
Heterogenous
what is homogeneous equilibria
A homogeneous equilibrium contains equilibrium species that all have the same state or phase.
What is heterogeneous equilibria
A heterogeneous equilibrium contains equilibrium pecies that have different states or phases.
In heterogeneous equilibria the concentrations of solids and liquids are essentially constant. so any solids or liquids are omitted from the Kc expression.
How do you determine Kc from experimental results
- determine the equilibrium amount of 1 reactant from the results of 2 titrations.
- use the table with initial moles, equilibrium moles and change to calculate the equilibrium amount of each component of the table
- find the equilibrium concentrations in moldm⁻³
- write the expression for Kc, substitute the values, and calculate Kc and units
Suggest a method to ensure that equilibrium was reached when using experimental results for Kc
Set up an identical experiment and leave for longer e.g if the first was left for a week to reach equilibrium leave this one for 2 weeks.
Work out Kc and if the results are the same it is safe to assume equilibrium was reached after week 1
