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Recognising Equilibrium States
We are able to identify systems in equilibrium. Sometimes we can be lucky and the reactants and products have different colours, thus making identifying of equilibrium easier. In general we can say that a state of equilibrium exists whenever:
- there is a closed system (i.e. we can’t add material to the system or remove it)
- the reaction is reversible
- there is a constancy in macroscopic properties
- the temperature is constant
To help you decide which factor you should monitor, a good idea is to always include the state of all chemical species involved.
For Example
So if we monitor the pressure of the system we should be able to monitor the reaction. Equilibrium would be reached when the pressure of the system reaches a constant value.
Another example is the reaction between chlorine gas and hydrogen iodide. If we look closely we can see that we could again measure the pressure. It would decrease until it reached a constant value. But why not simply watch the colour change? We see that one of the products is iodine, a purple solid. So surely monitoring a colour change would be an easier way to determine the progress of the reaction towards completion.
Factors affecting the progress of a reactionIn general two factors will have an impact upon the "drive" of a reaction towards completion. These factors are entropy and enthalpy, and both will be covered in more detail in the Energetics section. A summary of their effect is given in the table below.
Entropy | Enthalpy | |
Affect on a reaction | Entropy of the tendency of chemicals to achieve randomness. We know that chemicals will try to reach stability. Higher degrees of order mean higher energy involved in maintaining the order. So reactions will be "pushed" towards forming products of lower entropy. | Enthalpy is the heat content of the chemicals, and in general terms a reaction will try to lower the heat content of the chemical species.We know that an exothermic reaction typifies the idea and the push will be to form products.However, if the reaction is endothermic then the reverse reaction will be favoured and the push will be towards forming reactants. |
Factors affecting Equilibrium Position
An equilibrium can be shifted forward or backward by a variety of changes to its conditions. By changing any of these conditions we force the reaction to move to a new equilibrium condition.
a) Concentration of products or reactants
b) Volume changes
c) Pressure changes
- Increasing the pressure by decreasing the volume is similar to above.
- Increasing the pressure by adding a reactant is similar to concentration change.
- Increasing the pressure by adding an inert gas (a non reactant) has no effect.
d) Temperature changes
e) Catalysts
A catalyst increases only the rates of both reactions. So both rates will increase equally. This means that there is no change in the equilibrium. For Example- Add hydrogen gas to the system - will result in the reverse reaction occurring at a greater rate than the forward reaction. This will mean that [HF] will increase and the [F2] will decrease.
- Remove fluorine gas as it forms - will result in the forward reaction rate being greater than the reverse reaction. This will mean that [HF] will decrease and [H2] will increase
- Increase the volume of the reaction vessel - will have no effect. The fact that both sides have equal moles of gas, means that a change in volume, and the corresponding change in pressure will have no effect.
- Double the pressure of the system - will have no effect. The fact that both sides have equal moles of gas, means that a change in pressure will have no effect.
- Raise the temperature of the reaction vessel - will result in the forward reaction rate being greater than the reverse reaction. We can see that the forward reaction is endothermic (likes heat) and the reverse reaction is exothermic (hates heat). This will mean that [HF] will decrease and [H2] will increase
- Cool the reaction vessel in an ice bath - will result in the reverse reaction occurring at a greater rate than the forward reaction. We can see that the forward reaction is endothermic (hates being cooled) and the reverse reaction is exothermic (loves being cooled). This will mean that [HF] will increase and the [F2] will decrease.
Le Chatelier's Principle
Le Chatelier examined systems in equilibrium and noted that whenever a stress is applied to a system at equilibrium, the system will respond so as to reduce the stress. In real terms that means that if you change the reaction conditions of a system in equilibrium, then the reaction will move in such a way as to again reach an equilibrium.Let's use some examples to illustrate Le Chatelier's Principle in action
a) Concentration change after equilibrium is attained
Stress: the stress is the increase in [H2]; Reaction to stress: the equilibrium shifts away form the stress, H2; Result: the products are favoured as the excess/added H2 reacts with the I2 to produce more HI.
b) Volume
and Pressure change after equilibrium is attained
Stress: the stress is too many molecules. If you apply moles to the equation we see that 4 moles of reactants and only 2 moles of product; Reaction to stress: the equilibrium shifts away from the stress of trying to fit 4 moles into the smaller volume that we know will be easier to fit 2 moles of gas into; Result: the products are favoured.
c) Temperature change after equilibrium is attained
Stress: the stress applied is too little heat or a drop in heat; Reaction to Stress: the equilibrium shifts to produce
heat (i.e. favour the exothermic reaction) - we see that the forward reaction is exothermic and that means that the reverse reaction will be endothermic; Result: then the products are favoured as the exothermic reaction will be able to handle the drop in heat.
d) Catalysts
have no effect on the equilibrium as there is no stress to the equilibrium.