Kc

K_c

• 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]^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 + O₂  -> 2MgO
Kc = [MgO]² 
[Mg]² [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]² [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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