3.2.3, 5.1.2 Equilibrium constant Kc

Question 1

what is equilibrium constant used for

Answer 1

Kc is used to measure th eposition of equilibrium

Question 2

state the equation for Kc

Answer 2

Kc = [products]/[reactants]

Question 3

state the expression for Kc using

aA +bB ↔ cC + dD

Answer 3

Kc = ([C]^c x [D]^d)/ ([A]^a x [B]^b)

^ = to the power of

Question 4

What does the value of Kc give us

Answer 4

It tells us about the position of equilibrium

Question 5

What does

What does Kc > 1 tell us

Answer 5

Equilibrium favour the products

Question 6

What does Kc < 1 tell us

Answer 6

Equilibrium favours the reactants

Question 7

What does Kc = 1 tell us

Answer 7

Equilibrium lies in the middle

Question 8

How are the units of Kc worked out

Answer 8

1. subsitute units into the expression for Kc (usually mol dm⁻³)
2. cancel common units and show the final units on a single line

Question 9

what happens if the units of Kc are ( mol dm⁻³)/ (mol dm⁻³) and so equal 1

Answer 9

if Kc units = 1 then no units are used

Question 10

Outline the steps to calculate Kc

Answer 10

1. expression for Kc
2. equilibrium concentrations
3. calculate Kc
4. work out units

Question 11

What changes with calculating Kc if there are no units

Answer 11

When there are no units mol ratio can be used instead of concentrations

Question 12

How do you calculate Kc from initial moles when there are units

exam level question

Answer 12

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

Question 13

What factors can change Kc

Answer 13

ONLY temperature

Question 14

How does temperature change Kc

If: A↔B. 𝚫H = -ve

Answer 14

The increase in temperature favours the endothermic reaction (here the backwards one)
↑[A] and ↓[B]
Kc = ↓[B] /↑[A]
Therefore Kc changes (decreases here)

Question 15

How does Kc not change when subjected to changes in concentration and pressure

If: A↔B. 𝚫H = -ve

Answer 15

Equilibrium shifts to minimise the changes and restores Kc.

Question 16

what are the two types of equilibria

Answer 16

Homogeneous
Heterogenous

Question 17

what is homogeneous equilibria

Answer 17

A homogeneous equilibrium contains equilibrium species that all have the same state or phase.

Question 18

What is heterogeneous equilibria

Answer 18

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.

Question 19

How do you determine Kc from experimental results

Answer 19

1. determine the equilibrium amount of 1 reactant from the results of 2 titrations.
2. use the table with initial moles, equilibrium moles and change to calculate the equilibrium amount of each component of the table
3. find the equilibrium concentrations in moldm⁻³
4. write the expression for Kc, substitute the values, and calculate Kc and units

Question 20

Suggest a method to ensure that equilibrium was reached when using experimental results for Kc

Answer 20

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