Back to the full dot-point answer
QLDChemistryQuick questions
Unit 3: Equilibrium, acids and redox reactions
Quick questions on Dynamic equilibrium and closed systems (QCE Chemistry Unit 3)
9short 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 reversible reactions?Show answer
Some reactions go essentially to completion (combustion of hydrocarbons in excess oxygen). Others are reversible: products can recombine to reform reactants. Reversible reactions are written with a double arrow.
What is why equilibrium develops?Show answer
Consider starting with pure N2O4 in a sealed flask.
What is macroscopic vs microscopic behaviour at equilibrium?Show answer
A common exam trap is to claim that "the reaction has stopped". It has not. Only the net change has stopped.
What is why the system must be closed?Show answer
A closed system is one in which matter cannot enter or leave. Equilibrium cannot be established in an open system because:
What is recognising equilibrium on a concentration-vs-time graph?Show answer
Equilibrium is the time region where every curve has zero slope. The curves do not have to be at the same value: reactant concentrations and product concentrations typically differ at equilibrium, but each one is individually constant.
What is saying the reaction has stopped?Show answer
It has not. Net change is zero, but molecules are reacting continuously.
What is claiming concentrations are equal at equilibrium?Show answer
They are constant, not equal. Forward and reverse rates are equal, which is what makes the concentrations constant; the actual values depend on Kc.
What is forgetting the closed-system requirement?Show answer
If matter can leave, equilibrium cannot be reached. Always check whether the question describes a sealed system.
What is confusing equilibrium with completion?Show answer
A reaction that goes essentially to completion has Kc very large, but if it is reversible it still has a tiny reverse rate at equilibrium. Equilibrium and completion are different concepts.