What will happen to the position of equilibrium when the temperature of the vessel is increased?

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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

Look for a property that can be monitored. When this property remains constant, a state of equilibrium exists. 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

If we look carefully at the decomposition of phosphorus pentachloride gas. We can see that there are more moles of product than reactant, so we would expect the pressure of the system to increase as the concentration of products increased. This of course is dependent upon the volume of the reaction vessel remaining constant as well as the temperature of the system.

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

Increasing the concentration of a species in an equilibrium increases its collision frequency and its rate of reaction. The equilibrium then favours the formation of this specie's products; the equilibrium shifts away from the increased concentration. Remember that we can also reduce the concentration of species in reactions by removing them from the system. So adding NO2 shifts the equilibrium toward the product side, producing additional N2O4.

b) Volume changes

Decreasing the volume of a container increases the collision frequency. If there are more gas molecules of reactants than products in the reaction equation, the equilibrium shifts so as to reduce the number of gas molecules. This is similar to changes in pressure! So reducing the volume shifts the equilibrium toward the product side, producing additional NH3.

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

Increasing the temperature will increase both the forward reaction rate and the reverse reaction rate. However, the ENDOTHERMIC rate increases more than the exothermic rate, hence the endothermic reaction is favoured and the equilibrium shifts away from the added heat. So adding heat will favour the endothermic reaction and shift the reaction towards make more product, H2O(g).

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

If we consider the reaction that is shown, then we see that if we:

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

If H2 is added to the equilibrium

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

If the volume is decreased

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

If the temperature is decreased

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.