Introduction to Chemical Equilibrium Chemistry Tutorial
- 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.
Open system: matter can enter and leave the chemical system.
Closed system: matter cannot enter nor leave the chemical system.
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.
For a chemical system to achieve a state of equilibrium:
- The chemical reaction (or physical change) must be reversible.
- The system must be a closed system.
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)
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.
Question: Chris the Chemist set up 2 test tubes labelled A and B.
Chris added the same mass of purplish-black solid iodine to each test tube.
Test tube A was left unsealed.
Test tube B was sealed with a tightly fitting rubber bung.
Both test tubes were placed in the fume cupboard and left overnight.
In the morning, Chris noted that test tube A was empty, but test tube B contained a purple gas as well as small purple crystals.
Chris measured the concentration of purple gas in test tube B at 9 am, 10 am and 11 am and found that it did not change.
Which test tube represents a chemical system in dynamic equilibrium?
(using the StoPGoPS approach to problem solving)
||STOP! State the Question.
||What is the question asking you to do?
Determine which test tube represents a chemical system in dynamic equilibrium.
||PAUSE to Prepare a Game Plan
||(1) What information (data) have you been given in the question?
Initially: Test tubes A and B contain the same amount of dark purple solid iodine.
Conditions of the experiment: Test tube A is not sealed, test tube B is sealed.
Both test tubes left in the same place for the same length of time under the same conditions of temperature and pressure.
Results: Test A is empty, test tube B contains some solid purple iodine and some purple iodine gas and the concentration of purple gas did not change.
(2) What is the relationship between what you know and what you need to find out?
For a system to be in a state of dynamic equilibrium:
(a) The system must be a closed system.
(b) The chemical reaction must be reversible: both reactants and products must be present in the reaction vessel.
(c) The concentration of reactants and of products must be constant.
||GO with the Game Plan
(a) Test tube A is not sealed so matter can enter and leave the test tube, this is an open system not a closed system. Test tube A cannot therefore achieve a state of dynamic equilibrium.
Test tube B is sealed with a rubber bung so matter cannot enter or leave the test tube. Test tube B is a closed system. Test tube B could achieve a state of dynamic equilibrium.
(b) In test tube B both reactants (solid purple iodine) and products (purple iodine gas) are present, so this is a reversible reaction that could achieve a state of dynamic equilibrium.
(c) The concentration of the purple iodine gas in Test tube B was constant so this represents a system in a state of dynamic equilibrium.
||PAUSE to Ponder Plausibility
||Have you answered the question?
Yes, we have determined that the chemical system in test tube B has achieved a state of dynamic equilibrium.
Is your answer plausible?
Confirm that test tube A has not achieved dynamic equilibrium:
(i) The chemical reaction is reversible, solid iodine can become gaseous iodine and gaseous iodine can become solid iodine (as evidenced by test tube B), so it is possible for this system to achieve a state of dynamic equilibrium.
(ii) There is no evidence of the presence of either reactant (solid iodine) or product (gaseous iodine) in test tube A, so the system has NOT achieved a state of dynamic equilibrium.
(iii) Test tube A has not achieved a state of dynamic equilibrium because every time an iodine molecule escapes the solid phase and enters the gas phase it escapes from the unsealed test tube and enters the atmosphere, that is, the reaction is only occurring in one direction (solid turning into gas).
Conversely, test tube B is a closed system that contains both solid iodine and gaseous iodine and the concentration of gaseous iodine is constant so this is a system in dynamic equilibrium.
||STOP! State the Solution
The contents of test tube B are in a state of dynamic equilibrium.
(1) A chemical system can also be described as isolated.
In an isolated system neither matter nor energy may enter or leave the system.
Isolated systems are very important in calorimetry, that is, when we are measuring heats of reaction because we need to prevent energy from entering or leaving the system.
To do this we insulate the reaction vessel.
(2) There is another way to achieve equilibrium, steady state equilibrium.
In steady state equilibrium the rate at which reactants enter the system is the same as the rate at which reactants are consumed by the chemical reaction, and, the rate at which products leave the system is the same as the rate at which product molecules are produced.
Using a bunsen burner is a good example of a steady state equilibrium in that the rate at which the gas enters the system (the burner) is the same as the rate at which the gases (oxygen gas and natural gas) are consumed, and, the rate at which the products of combustion are produced is the same as the rate at which products leave the system. Therefore the bunsen burner continues to burn for as long as you supply oxygen gas from the atmosphere and a gas (such as natural gas) to burn, and allow the products of combustion to enter the atmosphere.
Fuel cells can also be considered an example of a steady state equilibrium system.
Steady state equilibrium is an important concept in chemical engineering, industrial chemistry, etc, but of less relevance to students in a chemical laboratory at school performing chemical reactions so it is often overlooked.
(3) Temperature is a measure of the total kinetic energy of the particles in the system. Adding heat may increase the kinetic energy energy of the particles in the system and hence the temperature of the system, or it may not.
For example, the temperature of liquid water stays constant while it boils even though heat energy is being added to convert liquid water to gaseous water.
If the temperature of the system is not changing the system is said to achieve thermal equilibrium.
(4) Note that this definition is only about "matter". Another type of equilibrium is about heat energy, when the temperature of a chemical system is not changing we refer to this as a system in thermal equilibrium.
(5) You will see dinitrogen tetraoxide also referred to as dinitrogen tetroxide.
See the introductory tutorial Naming Inorganic Non-metallic Binary Covalent Compounds for more information about IUPAC nomenclature.
(6) Why have we given two names for the same compounds?
IF the compound formed between iron and chlorine is ionic, then it should be named as an inorganic binary ionic compound (salt) and the formal charge (charge on the iron ion) should be represented by the charge number in brackets, that is, iron(2+) chloride, or iron(3+) chloride.
IF the compound formed between iron and chlorine is covalent, then we could represent the same compounds using the oxidation state of the iron in Roman numerals, that is, iron(II) chloride, or iron(III) chloride, although more systematically we should refer to iron dichloride and iron trichloride (see Naming of Covalent Inorganic Compounds above).
In aqueous solution, however, neither of the above is strictly accurate since the compound will exist in a hydrated form.
Inorganic chemistry is a fascinating study, and there is still a lot we don't understand about even the most apparently "simple" inorganic compounds (either in the solid state or in solution).