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Help with a question: equilibria

Hi! I’m struggling with question 4.4
I don’t understand why equilibrium changes when the overall conc changes, as all the concentrations change equally.
I thought that equilibrium only changes when you change the conc of only one substance.

Reply 1

alex2433

its a bit like pressure i think, where if you decrease pressure the eq shifts to the reaction that creates more moles of gaseous products. However since these are liquids, we are talking about concentration, but the concept is the same, so if you decrease the concentration, then the equilibrium will still shift to the reaction that creates more moles of liquid molecules.

As for this. "I thought that equilibrium only changes when you change the conc of only one substance." these changes affect the system overall, not just individual substances, so the eq does shift.

Reply 2

Tagsmags

If it helps to think about this mathematically, imagine you have the equilibrium k = [a]^2 / . When the concentration of both values is halved the value of [a]^2 decreases to a greater extent than due to the exponent ; so that the value for [a]^2 / now less than k, therefore [a] needs to increase and needs to decrease for the value of k to be restored.