- 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
- 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
- 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)
- 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
- If there are an equal number of ‘mol dm-3’ then there is no units
- 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)
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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