Overview of Week 1 Inquiry Question

From this point onwards, I will assume that you have read all the materials from the Year 11 HSC Chemistry Syllabus Notes. If not, please do so now. 

Getting the fundamentals of chemistry will set you up for success. 

You will spend less time and effort revising for your chemistry exams. 

Before we hop on the materialistic train and start digging into the course material, please allow me a minute to walk you through what you should keep in mind as the ‘major highlights’ of this week’s material. 

The inquiry question for this week deals with the chemical reactions that do not proceed to completion. 

In the Year 11 HSC Chemistry Syllabus Notes, most of the chemical reactions that you wrote are those that proceeded to completion. 

For example, your teacher may have burned a thin strip of magnesium where you saw a bright white glow, resulting in the magnesium metal turning into what you see as white solid (magnesium oxide, MgO).

However, there are chemical reactions that DO NOT proceed to completion and in fact the products can react with each other and reform the initial reactants! Wow? Yes.

These will be chemical reactions, which we call equilibrium reactions, that do not proceed to completion as the reactants and products are constantly reacting with themselves, converting to between each other. 

Within the category of equilibrium reactions, there is static and dynamic equilibrium systems. 

A system is an environment where there is one or more chemical reactions taking place. 

These equilibrium systems may be opened or closed. 

Open systems are subjected to influence of environmental factors such as temperature, pressure, foreign substances entering the system, thus, altering the chemical reaction equation. 

Closed systems are not subjected to the influence of environmental factors, however, we can manipulate the environmental conditions of the closed system using various methods and technologies.

Every chemical system, including equilibrium systems, has its own entropy and enthalpy. 

Although we have talked about entropy and enthalpy in preliminary HSC Chemistry, we will explore what these two terms mean again here and assess the impact these concepts have on a chemical reaction in dynamic equilibrium.

At the final section of this week’s notes, we will cover the effects of the collision theory and reaction rates on equilibrium reactions.

Learning Objective #1 – Distinguish between static & dynamic equilibrium

What is equilibrium?

It is important to note that the word ‘equilibrium’ always come with the symbol, ⇌

Whenever you see this symbol, ⇌, you should automatically note to yourself that anything on the left of the chemical equation are being converted into whatever that is on the right of the equation and vice versa. 

For example, suppose we have an equilibrium reaction (denoted by ⇌)

Compound A + Compound B  ⇌ Compound C + Compound D 

You can interpret the above equilibrium reaction as Compound A is reacting with Compound B to produce Compounds C and D. 

However, at the SAME TIME, when the reaction has reached or is at equilibrium, Compounds C and D are also reacting to produce Compounds A and B at the SAME RATE.

This means that the rate at which A and B are being consumed is equal to the rate at which A and B are being formed (from C and D reacting together).

This also means that at the state of chemical equilibrium, the rate of the forward and reverse reaction occur at the same rate. 

Rate of forward reaction = Rate of reverse reaction.

This is always true for all the types of equilibrium that we will explore later on , so remember it.

Thus, the rate of change and reversal of the change are the same! 

You may ask, wait.. does this mean that if you observe a reaction that is AT chemical equilibrium (visually from your naked eye), you will not see any changes in visual appearance of the system? YEP!

However, at a molecular level, there will definitely be movement of compounds, i.e. chemical bonds breaking and forming. 

It is also important to note that the CHANGE IN CONCENTRATION of Compound A, B, C and D in the equilibrium reaction are CONSTANT. 

Using the same generic equilibrium reaction we used above, if one mole of compound A is reacted with one mole of compound B to form products (compounds C and D), then the products will react together to form one mole of A again. This means that the concentration of A in the equilibrium system will always be constant. 

That is, at equilibrium, compound A will decrease and increase by the same amount that is consistent with its mole ratio at any given time. This is true for all other reacting species (reactants and products) that is involved in the equilibrium reaction, i.e. compound B, C and D.

Constant change in concentration DOES NOT necessarily mean that there is an equal change in concentration for  all species (reactant and products) participating in the equilibrium reaction. This is because the increase/decrease in concentration amount depends on the molar ratio of the participating species! 

For example, say there is the following equilibrium reaction:

2A + B  ⇌ 4C + D

Here, two moles of substance A is required to react with one mole of substance B to form four moles C and one mole of D. 

Vice versa, four moles of substance C is required to react with one mole of substance D to form 2 moles of substance A and one mole of substance B.

Hence, the change in concentration is not in equal amount for every substance as their molar coefficients are different. Therefore, when species A change by 2 moles, species C will change by 4 moles. 

Although the change in A’s concentration (2 moles <-> 2 moles) and change in C’s concentration (4 moles <-> 4 moles) are same, the change in A’s concentration does not equal to the change in C’s concentration (2 does not equal 4).

The reason for this is because molar ratio between the species are FIXED for each particular chemical reaction. So, although all species do not necessarily change by the same amount (in terms of moles), each species’ change in concentration is constant and consistent with their mole coefficient.

Equal and constant change in concentration mean completely different things! 

Do not get them mixed up. 

Also, note that the examples above are examples of dynamic equilibrium NOT static equilibrium reaction. 

We will compare the similarities and differences between static and dynamic equilibrium in the next learning objective.

If you do not completely understand this, you can watch the lecture video at the end of this week’s notes. We will have a visual diagram illustrating this to help with clarification.