Equilibrium (1) Properties of a dynamic equilibrium

Edexcel A level Chemistry (2017)

Topic 10: Equilibrium I:

Here are the first two learning objectives:

10/I/1. Students should know that many reactions are readily reversible and that they can reach a state of dynamic equilibrium in which:

i) the rate of the forward reaction is equal to the rate of the backward reaction. 


ii) the concentrations of reactants and products remain constant.

What is a dynamic equilibrium?

Look at a reagent bottle half full of water at room temperature.

Inside the sealed bottle there are two processes happening simultaneously.

Water molecules from the liquid state enter the gaseous state at the rate r1.

Simultaneously, water molecules from the gasous water vapour enter the liquid state at the identical rate r2.

r1  =  r2

How do we know that r1 = r2?

The water level never changes at constant temperature.

If we left the bottle on the lab bench for 10,000 years the water level would remain the same at constant temperature (that’s unlikely by the way given the rate of enhanced global warming!!)

And yet the water level is constantly changing its composition as molecules leave and enter the liquid state!!!

The level looks static yet it is changing all the time!!.

The contents of the bottle look static yet they are in dynamic equilibrium.

The water molecules (billions upon billions of them it has to be said) are in constant motion entering and leaving the liquid state. 

The concentration of water in the bottle never changes of course.

This is what it is if the volume of water is 500ml.

500ml weighs 500g at rtp. because water’s density is 1g/cm^3.

The molar mass of water is 18 g/mol.

So the number of moles of water is 500/18  =  27.78 moles.

That's for half a litre of water so the concentration of water in moles per litre will be 27.78  × 2   =   55.6M.

The concentration of water  [H₂O]  =   55.6M and is constant at constant temperature.

And the two opposing rates of evaporation and condensation are equal to each other  r1   =  r2.

All these conditions are true only if the stopper remains on the bottle but take it out and there is no longer a sealed system and no longer a perfect dynamic equilibrium.

These are the exclusive features of all perfect dynamic equilibria.

• constant temperature (think about what would happen if the bottle above was stood in a beaker of boiling water)

• sealed system (no material leaves or enters the reaction)

• equal and opposite reaction rates

• unchanging constant concentrations of reactants and products (sometimes referred to as the position of equilibrium)

Many reactions are reversible but very few exist in a perfect equilibrium state.

Chemists often assume the perfect equilibrium state exists for a reaction when in reality a very imperfect one exists especially in industrial processes where material is constantly being added at the start and removed at the end of the process!!

Some Reversible Reactions:

CH₃CH₂OH    +    CH₃COOH     ⇌     CH₃COOCH₂CH₃    +   H₂O

N₂    +   3H₂   ⇌     2NH₃

2SO₂    +    O₂    ⇌    2SO₃