Many processes can be reversed by changing the conditions. This applies to both physical and chemical change.
Syllabus reference R2.3.1Reactivity 2.3.1 - A state of dynamic equilibrium is reached in a closed system when the rates of forward and backward reactions are equal.
- Describe the characteristics of a physical and chemical system at equilibrium.
Guidance
Tools and links
Kinetic theory
The name 'kinetic' comes from the Greek word for motion 'kinesis'. The kinetic theory postulates that all particles are in constant motion, vibration, rotation and translation. The motion of is caused by energy.
However this energy is not distributed evenly over all of the particles, instead it is distributed in a completely random fashion that is described statistically as a Maxwell-Boltzmann distribution.
It can be seen in the diagram that there are very few particles with low energy and the number of particles with high energy tails off in a curve. The average or mean energy of the particles is directly proportional to the absolute temperature.
When the temperature is increased, three things happen to the energy distribution of the particles.
- 1 The curve flattens
- 2 The curve becomes wider
- 3 The mean or average energy increases.
This energy distribution is very important to understand the behaviour of particles in any system. It will be referred to regularly - it is a learning essential
Vapour and gas
A
vapour consists of particles of a substance, normally a liquid under the conditions
specified, but which exists in the gaseous form.
For example: There is water vapour in air at room temperature, 25ºC.
This consists of free water molecules mixed with the molecules of air, even though the boiling point of water is much higher than 25ºC.
The reason that these molecules do not turn back to water is that they are separated enough from each other to not be attracted to one another. As the amount of water vapour in the atmosphere increases, there is more possibility of the water molecules coming together and 'precipitating' as water droplets or rain.
Clouds are simply accumulations of water droplets formed by particles of water vapour coming together.
Evaporation
When water is left to stand in the open, it slowly evaporates. It does so at temperatures well below the boiling point of water. How can this happen?
Evaporation is the process of turning from a liquid into a vapour. It is not boiling. Evaporation happens to liquids at all temperatures. Look at the Maxwell-Boltzmann distribution of energies over particles. You can see that there are always some particles with large amounts of energy. These 'energetic' particles escape from the attractions of the other particles and become vapour.
These energetic particles leave the body of liquid. As the energetic particles leave, the average energy of the liquid bulk decreases.
Energy then flows into the liquid from the surroundings and the original shape of the distribution curve is retained (although now with fewer particles). This continues until eventually all of the liquid particles leave and the liquid has evaporated.
Condensation
When a body of air that contains water vapour is cooled down, the particles move slower on average and forces of attraction between them become more important. The particles form drops of liquid. We say that the vapour has condensed.
This is, of course, the reverse of evaporation. In one case the liquid absorbs energy and the water evaporates and in the other case the vapour loses energy and, if there are enough vapour particles in close proximity, water condenses.
water + energy → vapour
vapour → water + energy
Closed systems
A closed system is one in which neither matter nor energy can enter or leave.
If a body of water is sealed in a closed system, a dynamic equilibrium is established. Initially, particles of water in the liquid leave to form vapour above the body of the liquid. Occasionally, due to collisions, some of these vapour particles will lose energy and rejoin the body of liquid.
As more particles gather in the space above the liquid, there will be more possibility of particles losing energy and returning to the liquid. There is a two-way flow:
|
liquid water → water vapour |
We now have two processes happening simultaneously. Liquid particles turning to vapour and vapour particles turning to liquid.
Reversible reactions
These are reactions that can go in either direction.
Reactants → Products
Products → Reactants
Although they might not seem to be the norm, they are in fact very common. Simply changing the conditions changes which direction in the reaction proceeds.
|
Example: The decomposition of copper(II) sulfate pentahydrate. The compound is a blue crystalline solid that loses water of crystallisation on heating according to the equation: CuSO4.5H2O → CuSO4.H2O + 4H2O The product is a white powder, anhydrous copper(II) sulfate NOTE Although the term 'anhydrous' is used, there is still one water of crystallisation that is very hard to remove. If water is added dropwise to the product, there is an exothermic reaction and blue crystals are produced. The reaction has been reversed. CuSO4.H2O + 4H2O → CuSO4.5H2O The first process requires energy and the second reaction releases energy. The reaction is reversible and could be written: CuSO4.5H2O ⇋ CuSO4 + 5H2O |
If the reversible reaction can proceed in both directions at the same time, a situation may develop in which the reactants and products concentrations become constant, although both the forward and reverse reactions continue as before.
This is called dynamic equilibrium.
In the previous example of hydrated copper(II) sulfate, if the heating were carried out in a sealed tube then the water vapour produced in the reaction would create a reversible system, as it would tend to react with the anhydrous copper sulfate produced.
The extent of the equilbrium depends on the temperature used.
Dynamic Equilibrium
Eventually in the closed system example above, the rate at which the particles move from the liquid to the vapour phase equals the rate at which the particles move from the vapour phase to the liquid:
Vaporisation rate = Condensation rate
When this situation develops the concentration of vapour particles in the air space is constant even though there is movement in both directions. We call this situation "Dynamic Equilibrium".