Le-Chatliers Principle

1.Commentry LeChatliers Principle

The French chemist Henri Le Chatelier studied how the equilibrium position shifts as a result of changing conditions

Le Chateliers principle describes the behavior of chemical reaction system(amixture of reactants and products)in its equilibrium state after it is subjected to change that up sets the equilibrium condition.

The present collage shows how stress on equilibrium reaction in form of increase in concentration of reactants/product, increase of pressure, increase of temperature changes the equilibrium reaction.

In middle part of collage, chemical reaction is shown in the form of see-saw, where fulcrum is shown in form of chemical equilibrium, left side of see saw is loaded with reactants and right side is loaded with products. Keq =[products]/[Reactants].

Right Side of Collage

Effect of Temperature Changes: The intrinsic value of Keq does not change when we increase concentrations or pressures of components in a reaction. However ,almost every equilibrium constant(Keq)change in response to changes in temperature.

Le Chatelier’s rule can be applied to predict the effect of temperature changes upon chemical reactions.

1.Most chemical reactions have some heat change associated wih the reaction( The energy of a reaction can be used to either do work –or to change temperature. If we consider reaction conditions under which no work is done ,and therefore all energy changes associated with reactions will be manifested by temperature changes.)

2.Exothermic reactions are associated with heat release when the reaction proceeds in the forward direction

3.Endothermic reactions are associated with heat release when the reaction proceeds in the opposite direction( i.e. heat is absorbed in forwaed direction).

4.These two types of reactions and their associated heat changes can be written as :

Reactants <=> Products + Heat( Exothermic)

Reactants + Heat ó Products( Endothermic)

1.If temperature is increased ,the equilibrium will shift so as to minimize the effect of the added heat.

2.The reaction will shift in the appropriate direction such that the added heat is absorbed.

Efeect of T on K

When heat is added to exothermic reaction at equilibrium , products will be consumed to produce reactants.(Shifts to the LEFT).Therefore value of Keq will be lowered.

When heat is added to endothermic reaction at equilibrium, reactants will be consumed to produce products (Shift to the RIGHT).Therefore value of Keq will be increased.

 Thus, the effect of increasing temperature on an exothermic reaction is to lower the value of Keq.

 Conversely, the effect of increasing temperature on an endothermic reaction is to increase the value of Keq

Description :Le Chatelier’s principle is demonstrated by invoking a colour change inside a sealed tube containing NO₂ (brown) and N₂O₄(colourless) gases at equilibrium. The equilibrium shifts when the temperature changes inside the vessel.

Figure showing how NO₂(redish brown ) is converting into N₂O₄(colourless) with time.

Discussion :Nitrogen dioxide is a redish brown gas while N2O4 is colorless.

According to thermodynamic data for this system, the dimerisation of NO2 (shown below) is an exothermic reaction..

2 NO2(g) Û N2O4(g) + ∆H

As temperature is increased ,the above equation shifts to the left, generating a higher concentration of NO₂, resulting in the darkening of the redish brown colour inside the tube. Conversly , cooling the reaction shifts the equilibrium to the right, producing more N₂O₄ which is colourless

Summary

· Increasing the temperature of a system in dynamic equilibrium favours the endothermic reaction. The system counteracts the change you have made by absorbing the extra heat.

· Decreasing the temperature of a system in dynamic equilibrium favours the exothermic reaction. The system counteracts the change you have made by producing more heat.

Left side of Collage

In order to try to figure out how to optimize the production of ammonia from hydrogen and nitrogen. Haber studied the equilibrium concentration of ammonia in his famous process .

N2 (g) + 3H2(g) ó 2 NH3(g)

He noted the equilibrium concentration of ammonia at different temperatures(while

keeping pressure constant)

He also noted the equilibrium concentration of ammonia at different pressures( while keeping the temperature constant)

Figure above shows the 3 Dimensional graph showing effect of temperature and pressure on concentration of NH₃ gas formed in Haber process.

Figure above shows three molecules of H₂ are reacting with one molecule of nitrogen forming two molecules of ammonia(equilibrium reaction).Bottom part of above figure shows that to start with pressure in reaction chamber was four units. As nitrogen and hydrogen interact to form ammonia, gas pressure is reduced to two units and combination reaction gives out heat. According to Le Chatlier’s principle, to make the reaction proceed left to right high pressure and low temperatures are needed.

A brief summary of the Haber Process

The Haber Process combines nitrogen from the air with hydrogen derived mainly from natural gas (methane) into ammonia. The reaction is reversible and the production of ammonia is exothermic.

A flow scheme for the Haber Process looks like this:

http://www.chemguide.co.uk/physical/equilibria/haber.html

The pressure

Equilibrium considerations

Notice the above equation there are four molecules on the left-hand side of the equation, but only two on the right side of equation.

According to Le Chatelier's Principle, if you increase the pressure the system will respond by favouring the reaction which produces fewer number of molecules. That will cause the pressure to fall again.

A chemical system in equilibrium can respond to the effects of pressure also. According to Le Châtelier's Rule, if the pressure is increased on a system, it will respond by trying to reduce the pressure. How does it do this?

If we concentrate on homogeneous gaseous reactions

The stoichiometry of the reaction may lead to a greater number of molecules on one side of the equation.

For example, in the Haber reaction, N₂(g) + 3H₂(g) <=> 2NH₃(g) there are twice as many moles of reactants as products

If the Haber reaction were in equilibrium, and the pressure was increased, This would reduce the overall number of moles in the reaction, and therefore, lower the pressure

An interesting point about pressure effects is that they do not cause a change in the value of the equilibrium constant, K (as long as T is held constant). Their affects are upon concentration of reactants and products

Summary

In order to get as much ammonia as possible in the equilibrium mixture, you need as high a pressure as possible. 200 atmospheres is a high pressure, but not amazingly high.

The temperature

Equilibrium considerations

You need to shift the position of the equilibrium as far as possible to the right hand side in order to produce the maximum possible amount of ammonia in the equilibrium mixture.

The forward reaction (the production of ammonia) is exothermic.

According to Le Chatelier's Principle, this will be favoured if you lower the temperature.(shown in figure below) The system will respond by moving the position of equilibrium to counteract this - in other words by producing more heat.

The above figure shows that haber process is favoured by low temperature(298 K)

Summary

In order to get as much ammonia as possible in the equilibrium mixture, you need as low a temperature as possible. Due to kinetic consideration , Haber process is carried out at 400 – 450^oC

Bottom of Collage

A Change in Reactant or Product Concentration

At equilibrium the rate of the forward reaction is equal to the rate of the reverse reaction.According to Le Chatelier’s principle states that if the concentration of one of the component of the reaction (either product or reactant) is changed ,the system will respond in such a way as to counteract the effect.

If a substance (either reactant or product) is removed from a system,the equilibrium will shift so as to produce more of that component(and once again achieve equilibrium).

If a substance(either reactant or product) is added to a system, the equilibrium will shift so as to consume more of that component(and once again achiev equilibrium ).

• As an example, consider the Haber reaction:

N₂(g) + 3H₂(g) Û 2NH₃(g)

What would happen if we start with a Haber reaction at equilibrium and suddenly added some H2 (g) to the reaction mixture?

The following is a graphical representation of how the concentrations of the individual components, and the overall system, would react in response to adding H₂ (g).

http://www.mikeblaber.org/oldwine/chm1046/notes/ChmEquil/LeChat/LeChat.htm

What has happened to the equilibrium of the system in response to the added H₂ (g).? Prior to the addition of H₂ (g) the system is in equilibrium.

The addition of H₂ (g) causes the balance of the system to shift in favour of the forward reaction (i.e. the production of NH₃(g) from N₂(g) and H₂(g).Thus,some of added H2(g) is consumed and thus the system responds to counteract the perturbation caused by added H2(g).

N₂ + 3H₂ 2NH₃

(reaction is driven "to the right" by the effects of added H₂)

After some time,the system reaches a new state of equilibrium. It will not be identical to the original state, however . Although the system has responded to resist the effects of the added H₂ (g) the new equilibrium state contains a slightly higher concentration of NH₃(g)and slightly lower concentration of N₂(g)( as well as a slightly higher concentration of H₂ (g).

The overall ratio of [NH3]² / ([N2]*[H2]³ is the same as before at equilibrium (ie the value of Kc does not change)

Figure above shows the example of the reaction 2HI óH₂ + I₂, which is in equilibrium, if concentration of H₂ is increased more concentration of HI will form to keep Keq constant.

The Effect of Catalysts

A catalyst lowers the activation energy barrier, E_a

Although the activation energy barrier is a different magnitude for the forward and reverse reactions, the change in the activation energy (∆E_a) is the same for both the forward and reverse reactions

Therefore, a catalyst changes the rate at which equilibrium is achieved, but does not change the composition of the equilibrium mixture (i.e. does not alter the equilibrium constant, Keq)

Figure above shows that catalyst lowers Ea for both forward and reverse reaction.

Message

Collage is shown in form of Earthen diya is a symbol of eliminiating darkness of knowledge ,It counteracts the stress of storm and leads to new equilibrium so that the light of knowledge spreads in world , proving the Le Chateliers principle in real sense.

References: http://www.chemguide.co.uk/physical/equilibria/haber.html

http://www.daviddarling.info/encyclopedia/L/Le_Chateliers_principle.html