4CH1

Reversible Reactions and Equilibria

Physical Chemistry

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 6% of your exam marks.

stable
Low
Stable6%

Haber process conditions and Le Chatelier's principle regularly examined.

What equilibrium looks like

  • Place a reversible reaction in a sealed container with no escape route for any chemical — a
  • At first only the forward reaction occurs (no products to react in the reverse direction yet); its rate is highest
  • As the products build up, the reverse reaction starts and speeds up
  • The forward rate falls (reactants becoming scarcer) and the reverse rate rises (products becoming more abundant)
  • Eventually the two rates become equal: the system is at
Graph of reaction rate against time: the forward rate starts high and decreases while the reverse rate starts at zero and increases, until the two lines meet and level off where equilibrium is reached with both rates equal
Source: Dynamic equilibrium by Save My Exams

Two defining features

  • The rate of the equals the rate of the
  • The concentrations of reactants and products stay constant from this point onward (provided temperature and pressure do not change)

Why "dynamic", not "static"

  • At equilibrium the reactions have not stopped — both still happen at full speed
  • Each forward conversion of a reactant into a product is matched by a reverse conversion of a product back into a reactant
  • The chemicals interchange constantly; only the overall amounts stay fixed

Closed-system requirement

  • Equilibrium can only be reached when nothing can leak out (or leak in)
  • Open systems lose products as they form (a gas escaping into the air, for instance), so the reverse reaction never has enough material to keep up and equilibrium is never reached
Iodine in an open versus a closed test tube: in the open system iodine vapour escapes so the change is one-way, I2(s) → I2(g); in the sealed closed system the vapour cannot leave and recondenses, so a reversible equilibrium I2(s) ⇌ I2(g) is set up
Source: Dynamic equilibrium by Save My Exams
Exam tip

Define dynamic equilibrium

Stating what dynamic equilibrium means is a regular 2-marker, so you need to know: the forward and reverse reactions happen at the same rate, and the concentrations of reactants and products stay constant. "Constant" means unchanging over time — not that the two concentrations are equal to each other.