O Level Chemistry & IP Chemistry Notes by 10 Year Series Author – Chemistry Specialist

IP Chemistry – Chemical Equilibrium and Le Chatelier’s Principle

First of all, this topic on Chemical Equilibrium is not being tested for GCE O-Level Pure Chemistry (syllabus code 6092) in Singapore. It is only being tested in IP Chemistry, IGCSE Chemistry and IB Chemistry.

At the end of the blog post, you will be able to answer three important questions:

Just like all the Chemistry tuition classes and holiday revision workshops which I have been personally conducting over the last 16+ years, I would like you to have an overview in what you be learning today.

The key points in this post are:

A) Reversible Reactions

Many chemical reactions can proceed in one direction only. i.e. they cannot be reversed and they go towards completion.

e.g. Neutralisation reaction between potassium hydroxide & hydrochloric Acid

KOH(aq) + HCl(aq) → KCl(aq) + H2O(l)

Some chemical reactions can be reversible .i.e. reactions can go either directions and they reached an equilibrium, instead of going towards completion.

At equilibrium, the forward and backward reactions do not stop; they continue, but at the same speed. Hence, there is no overall change in the amounts of reactants and products.

At the end of reaction, a mixture of reactants and products is present and they are known to be in equilibrium.

e.g. Haber Process:           N2(g) + 3H2(g) ⇌ 2NH3(g)

By altering conditions of temperature, pressure and  use of catalysts, these reactions can be adjusted to favour more reactants or more products.

Whether a reaction is reversible or irreversible depends on activation energy.

If the activation energy of the reverse reaction (i.e. Eb) is exceptionally high, then this reaction will be unfavourable and the reaction is described as irreversible.

B) Dynamic Equilibrium

Let’s consider a reversible reaction whereby:

A + B ⇌ C + D

When the mixture of A and B reacts to become C and D, the concentrations of A and B (reactants) decrease with time, while the concentrations of C and D (products) increases with time.

As the concentrations of reactants decrease, the rate of the forward reaction also decreases with time

At the start, the rate of backward reaction is zero because there is no C and D.

As the reaction proceeds, concentrations of C and D increase. Hence, rate of backward reaction also increases.

After a period of time, Dynamic Equilibrium is reached.

Dynamic Equilibrium refers to a reversible reaction in which the rates of forward and reverse reactions have become equal and there is no change in the concentrations of the reactants and the products.

C) Le Chatelier’s Principle & Position of Equilibrium

Summaries the effect of external factors (changes in temperature, concentration or pressure) on a system at equilibrium.

Le Chatelier’s Principle states that if a change is made to a system in equilibrium, the system reacts in such a way as to tend to oppose the change, and a new equilibrium is formed.

Basically, whatever is done to a system in equilibrium, the system does the opposite to it!

Let’s consider 4 external factors: concentration, pressure, temperature and addition of a catalyst.

C.1 Changes in Concentration

If reactants are added (or products removed) in an equilibrium, a new equilibrium is produced which contains a higher proportion of the products i.e. position of equilibrium shifts to the right hand side.

At the new equilibrium, concentrations of the reactants and products are not the same as those in the previous equilibrium.

C.2 Changes in Pressure

Only applicable for systems containing gas(es) only.

If the pressure of an equilibrium mixture is increased (or volume decreased), the mixture will try to reduce the pressure by reducing the number of moles (or molecules) of gases.

C.3 Changes in Temperature

This is linked to the concepts you have learned in the topic of Energy Changes (also known as Chemical Energetics or Thermodynamics).

C.4 Presence of Catalyst

This is linked to the concepts on catalysts which you have learned in the topic of Energy Changes as well as Rate of Reaction.

D) Haber Process (Case Study)

N2 (g) + 3H2( g) ⇌ 2NH3(g)                      ∆H = – 92 kJmol-1

Note that we have covered Manufacturing of Ammonia using Haber Process previously in this blog, which touch on the key concepts and associated keywords necessary for students taking GCE O-Level Pure Chemistry (syllabus code: 6092). We have described how nitrogen gas (from fractional distillation of liquefied air), and hydrogen gas (from cracking of crude oil), as raw materials for producing ammonia in the Haber Process. We have also looked into the essential /optimum conditions for the process. We shall not talk about those here today, so, it’s really good that you refer to that previous blogpost.

Instead, we shall look at how chemical engineers and chemists can use Le Chatelier’s Principle to maximise the yield of ammonia in the Haber Process, by manipulating the conditions in it.

Conditions (Optimum)

1. Temperature: 450 oC

2. Pressure: 200 atm

3. Catalyst: Finely divided Fe catalyst

4. Continual removal of ammonia

I hope you find the content easy for your understanding and if you have any questions, leave me a comment below. Feel free to share this blog post with your friends.

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