Kc

  • With a known molar concentration, of each substance at equilibrium then Kc can be determined
  • Kc is only specific to that particular temperature

aA + bB ⇋ cC + dD

Kc = [C]c [D]d
[A][B]b

  • ­Lowercase letters represent the number of each substance
  • Square brackets refer to the concentration (mol dm-3)
  • The products go on the tops and the reactants at the bottom

e.g. 2Mg + O -> 2MgO
Kc = [MgO]2 
[Mg][O2]

Method:

  • Find the amount of moles of each reactant and product there are at equilibrium (find this by balancing the equation if not given)
  • Calculate the molar concentration of each reactant:

Concentration (mol dm-3) = Number of moles (mol)
Volume (dm-3)

  • Place these values into the Kc equation
  • To determine the units: each value in the brackets is ‘mol dm-3’ and multiply it by the amount power and cross off the the units that are on both sides of the fraction e.g.

Kc = [MgO]2 (mol dm-3 ) (mol dm-3)
[Mg][O2] = (mol dm-3) (mol dm-3) (mol dm-3)

Unit = mol dm-3

  1. If there are an equal number of ‘mol dm-3’ then there is no units
  2. If there are 2 or more mol dm-3 then multiply it by how many there are e.g.

2 x mol dm-3  = (mol-2 dm-6)

Using Kc

With the value of Kc known, then an unknown equilibriums concentration can be determined;

  • Put all of the values you know into the expression of Kc
  • Rearrange the equation and solve it for the unknown values

 

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