Introduction to Chemical Equilibrium Chemistry Tutorial

Key Concepts

• A chemical system can be described as being either open or closed⁽¹⁾:

  ⚛ Open system: matter and energy can enter or leave the system.

  ⚛ Closed system: matter cannot enter nor leave the system.

• Equilibrium refers to a state of balance : chemical equilibrium refers to the balance achieved in the concentration of reactant and product molecules.
• A chemical system at equilibrium can be described as being either static or dynamic:⁽²⁾

  ⚛ Static equilibrium: no changes are taking place at the macroscopic level and no changes are taking place at the molecular level.

  ★ rate of forward reaction = rate of reverse reaction = 0

  ⚛ Dynamic equilibrium: no changes appear to be taking place at the macroscopic level but constant changes are taking place at the molecular level.

  ★ rate of forward reaction = rate of reverse reaction ≠ 0

• For a chemical system in a state of dynamic equilibrium:

  ⚛ It is a closed system.

  ⚛ The chemical reaction is reversible.

  ★ Both reactants and products are present at the same time in the system.

  ⚛ At the macroscopic level the macroscopic properties of the system are constant:

  ★ The concentration of reactant molecules is constant, and, the concentration of product molecules is constant.

  ★ The colour of the system is constant.

  ★ The density of the system is constant.

  ★ The temperature of the system is constant.⁽³⁾

  ★ The pressure of the system is constant (relevant to gaseous systems in particular).

  ⚛ At the molecular level reactant molecules are continually being broken apart to form product molecules and product molecules are continually being broken apart to re-form reactant molecules.

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Open and Closed Systems

After you wash your socks, you hang them on the outside clothesline to dry. The wet socks dry because the water evaporates, that is, liquid water changes into gaseous water and this gaseous water escapes into the atmosphere.
This is an example of an open chemical system because matter, water, is leaving the chemical system, wet socks.

If, instead of hanging your wet socks on the clothesline, you put your wet socks into a glass jar and screwed the lid on tight, your socks will not dry.
In this case water will still evaporate but instead of escaping into the atmosphere it is now trapped inside the sealed jar. The gaseous water that forms when water evaporates will collect inside the jar where it will condense back into liquid water. Droplets of liquid water will continually fall back onto your socks keeping them wet.
This is an example of a closed system, matter (water) cannot enter or leave the system because there is a lid on the jar.

If the reactants and/or products cannot escape from the chemical system then it is a closed system.
If reactants and/or products can escape from the chemical system then it is an open system.

For chemical reactions (including physical changes such as water evaporating and condensing) the system must be enclosed in order for it to be a closed system.
If the chemical reaction is not enclosed, then it will be described as an open system.

Defining Equilibrium

Equilibrium is defined as a state of balance.

For a chemical system to be in equilibrium the concentration of the reactant molecules and the product molecules must be in balance.

Let's go back to those wet socks.
When you dry the socks on the clothesline, the chemical system is never in balance. Each time a water molecule escapes the liquid state and enters the gas state it is whisked off into the atmosphere and lost to the chemical system. The reaction is only going in one direction, liquid water becoming gaseous water.
Wet socks drying on the clothesline represents a system that is NOT at equilibrium because the products of the reaction are continually escaping. Eventually there will be no water left in the liquid state to be evaporated and the socks will be dry.
Ofcourse, if it happens to rain while your socks are hanging on the clothesline then water from the atmosphere will wet your socks and you will have to wait for the sun to come out again before your socks can begin to dry again.
An open system cannot achieve equilibrium because reactants and/or products can leave the system and go into the surroundings, or particles from the surroundings can enter the system.

What about wet socks in a sealed jar?
In this case some water molecules in the liquid phase are escaping into the gas phase while, at the same time, some water molecules in the gas phase are entering the liquid phase.
In the overall chemical system there is a balance between reactant molecules (water in liquid phase) and product molecules (water in gas phase).
The reaction is reversible: foward reaction produces gaseous water while reverse reaction produces liquid water.
This chemical system can achieve a state of equilibrium in which the number of water molecules in the liquid phase does not appear to change over time, and the number of water molecules in the gas phase does not appear to change over time. This is a chemical system in balance, we say that the system is in equilibrium.
Even if it rains on the sealed jar, the tightly fitted lid prevents water from the atmosphere entering the jar, so the chemical system inside the jar can achieve a state of equilibrium because reactants (water) cannot enter the system, and products (water) cannnot leave the system. This is still a chemical system in balance, that is, a chemical system at equilibrium.

Static Equilibrium and Dynamic Equilibrium

For a closed chemical system in a state of equilibrium both reactant and product molecules are present at the same time and their concentrations appear to be constant.

There are two different ways to achieve this:⁽²⁾

• No changes are occurring in the system and the concentration of reactant molecules and product molecules is constant.

  This is known as static equilibrium.

• The concentration of reactant and product molecules is not changing but some individual reactant molecules are being broken apart and the atoms rearranged to produce product molecules at the same time and at the same rate at which some individual product molecules are being broken apart and the atoms rearranged to re-form reactant molecules.

  This is known as dynamic equilibrium.

Forget your wet socks for now and think about what happens when you add vegetable oil to water.
Vegetable oil and water do not react, but mixing them together does form a chemical system.
Lets add 50 mL of vegetable oil to 50 mL of water in our glass jar then screw the lid on tight to form a closed system.
Now for the fun part ... shake that sealed jar vigorously so that small droplets of vegetable oil are spread evenly throughout the water.
Observe what happens.
Droplets of vegetable oil, being less dense than water, will make their way to the water/air boundary where they will stick together and form a layer so now we have a water/oil boundary.
Over time, more and more of the vegetable oil drops reach the boundary and join the oil layer, until eventually there are no vegetable oil drops left suspended in the water.
The number of water molecules in the volume of fluid has not changed so its concentration is constant.
The number of oil molecules in the volume of fluid has not changed so its concentration is also constant.
We have now reached a state of static equilibrium, the concentration of oil and water is constant and there is no movement of oil molecules back into the liquid water layer and no movement of the oil molecules from the liquid water layer into the oil layer.
The rate of the forward "reaction", oil droplets rising up to the oil layer, is zero. The rate of the reverse "reaction", oil droplets falling down from the oil layer and into the water layer, is also zero.

Yes, back to the wet socks!

When sealed in the glass jar to form a closed system, we noted that some water molecules are in the liquid phase while some water molecules are in the gas phase.
At the beginning of the experiment, we have only wet socks with the water molecules in the liquid phase.
Then some water molecules gain enough energy to escape into the gas phase (evaporate).
For some time, the number of water molecules entering the gas phase (evaporation) is greater than the number of gaseous water molecules losing energy and condensing back into liquid water, that is, the concentration of water molecules in the gas phase is increasing. During this time, this system is NOT at equilibrium because the concentration of water molecules in the gas phase is increasing.
Eventually, a time will come when the number of water molecules entering the gas is the same as the number of water molecules condensing back into liquid water. That is, the concentration of water molecules in the gas phase will appear to be constant. At this time equilibrium has been achieved.
The rate at which water molecules leave the liquid phase and enter the gas phase is the same as the rate at which water molecules leave the gas phase and enter the liquid phase. The rate of the forward reaction (evaporation) is the same as the rate of reverse reaction (condensation).
Note that this equilibrium is very different to the one described above for oil and water (static equilibrium in which rate of forward "reaction" = rate of reverse "reaction" = 0).
Water particles in the closed jar system are constantly entering and leaving the gas and liquid phase, they are dynamic (moving), so this is referred to as a state of dynamic equilibrium.
In a state of dynamic equilibrium, the macroscopic properties such as concentration of reactants and products is constant, but on a molecular level the molecules are constantly changing between being reactant molecules or product molecules.
Dynamic equilibrium occurs when: rate of forward reaction = rate of reverse reaction and that rate is NOT zero.

Dynamic Equilibrium and Chemical Changes

Physical changes such as changes of state are reversible, so, in a closed system these changes of state can achieve a state of dynamic equilibrium.

Reversible chemical changes that occur in a closed system can also achieve a state of dynamic equilibrium.

Nitrogen dioxide is a brown gas.
Dinitrogen tetraoxide is a colourless gas.⁽⁵⁾
If you heat a sealed vessel of brown nitrogen dioxide gas it will decompose to produce some colourless dinitrogen tetraoxide gas and the colour of the gas in the container will become less brown.
If you then cool this sealed vessel, some of the colourless dinitrogen tetraoxide will decompose to produce brown nitrogen dioxide and the brown colour of the gas in the container will increase.
We know there has been a chemical change because the colour in the sealed vessel is changing.
We know that the chemical reaction is reversible, that is heating produces more dinitrogen tetraoxide while cooling produces more brown nitrogen dioxide.
The intensity of the brown colour is telling us the concentration of brown nitrogen tetraoxide relative to the concentration fo colourless dintrogen tetraoxide, that is, the more intense the brown colour the greater the concentration of nitrogen dioxide, and the less intense the brown colour the lower the concentration of nitrogen dioxide.
When the system is in a state of dynamic equilibrium the concentrations of nitrogen dioxide and of dinitrogen tetraoxide will be constant and the colour of the gas inside the sealed vessel will therefore remain constant.

20 mL of an aqueous solution of iron(3+) chloride (iron(III) chloride) is yellow.⁽⁶⁾
20 mL of an aqueous solution of sodium thiocyanate of the same concentration is colourless.
If you pour all the yellow iron(3+) chloride solution into the colourless sodium thiocyanate solution they will react to produce a deep red aqueous solution of iron(2+) thiocyanate (iron(II) thiocyanate).
We know a chemical change has taken place because there is a colour change when the new product is formed.
But how do we know that the system is in a state of dynamic equilibrium?
Recall that a system in a state of dynamic equilibrium will contain some reactants and some products, but the concentration of each reactant and product will be constant.
So, if we add a bit more of one reactant, for example we add a bit more iron(3+) chloride, then there will be some thiocyanate ion in the solution which is available to react and form more iron(2+) thiocyanate and the colour of the solution will become an even deeper red. If we were to add a bit more of the sodium thiocyante solution instead, then the colour of the final solution would also go a deeper red because there would have been some iron(3+) available in the solution to react.
Therefore we have demonstrated that the reaction between iron(3+) chloride and sodium thiocyanate to produce iron(2+) thiocyanate is reversible (because each reactant and product must be present in the final solution).
The colour of the solution also indicates the concentration of the iron(2+) thiocyanate.
When this chemical system is NOT at equilibrium we will see the colour of the solution changing.
When this system is in a state of dynamic equilibrium then the concentration of each reactant and of each product will be constant so the colour of the solution will not be changing, the colour of the solution will remain constant.

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