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VICChemistryQuick questions
Unit 3: How can design and innovation help to optimise chemical processes?
Quick questions on Dynamic equilibrium, Kc and Le Chatelier's principle: VCE Chemistry Unit 3
11short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.
What is dynamic equilibrium?Show answer
A reaction is at dynamic equilibrium when:
What is the equilibrium expression and Kc?Show answer
the equilibrium law expression is:
What is q vs Kc?Show answer
The reaction quotient Q has the same algebraic form as Kc but uses current concentrations (which may or may not be at equilibrium).
What is le Chatelier's principle?Show answer
Le Chatelier's principle: if a system at equilibrium is disturbed, it shifts to partially counteract the disturbance and restore equilibrium.
What is visualising on a concentration vs time graph?Show answer
A common SAC/exam graph plots concentration on the y-axis and time on the x-axis. Reactant and product concentrations level off when equilibrium is reached. A disturbance shows up as a vertical jump or drop in the concentration of the disturbed species, then a re-equilibration as the system shifts.
What is saying equilibrium means "the reaction has stopped"?Show answer
It has not. Forward and reverse rates are equal but non-zero.
What is including solids or pure liquids in Kc?Show answer
Only (g) and (aq) species appear in the expression.
What is saying a catalyst increases yield?Show answer
A catalyst affects rate only, not the equilibrium position or Kc.
What is confusing "shift left" with "decrease Kc" for concentration changes?Show answer
Only temperature changes Kc. A concentration disturbance shifts the position but the system returns to the same Kc.
What is forgetting to compare moles of gas for pressure changes?Show answer
The shift direction depends on which side has fewer moles of gas, not on which side has more "atoms" or "mass".
What is applying Le Chatelier without checking ΔH?Show answer
For temperature changes you must know the sign of ΔH for the forward reaction. Without it, you cannot predict the direction.