Learning Objectives

Reversible reactions

When both the Forward and Reverse reaction can take place under given conditions, the reaction is reversible.

For a reversible reaction to occur, the activation energy of both reactions must be small (so it can occur naturally) and there must be a low enthalpy change (so the activation energies are of a similar size).

When the activation energy for the reverse reaction is not substantially higher than the forward reaction, the equation will generally be reversible.

Physical processes are generally reversible, as energy change is usually low (compared to chemical changes).

In reversible reactions there is a forward and reverse reaction

Forward

⇒ Left to Right

⇒ Reactants to Products

Reverse

⇒ Right to Left

⇒ Products to Reactants

Equilibrium

In a reversible reaction, conditions may favour the rate of one reaction, when the forward reaction occurs at a faster rate than the reverse reaction, the reaction is a “net forward reaction”.

The net forward reaction decreases over time as the rates balance out again.

When there is no net change in rate of reaction or concentration of particles, the reaction is said to be in a “state of equilibrium”.

Equilibrium ⇒ no net change (balanced)

⇒ FRR = RRR, & rate of change for [F] = roc, for [R]

Dynamic equilibrium ⇒ equilibrium achieved despite ongoing change (reactions one way cancelled out by reactions the other way)

If all reactants are consumed, then “position of equilibrium lies toward the right hand side” (left for less products produced)

⇒ Reaction rate (dynamic equilibrium is reached when rates cancel out)