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Reversible and Irreversible Changes in Chemical Systems Chemistry Tutorial

Key Concepts

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Reversible Changes

Draw a line on the floor to mark your starting position then take one step forward. If you now take one step backward you arrive back at the original starting position. You have just demonstrated a reversible change!
If you take a step forward you can reverse the process by taking a step backward.
If you take a step backward you can reverse the process by taking a step forward.
The process is reversible because you can go both in the forward direction or in the reverse direction, dependent on the conditions (whether you take a step forward or backward).

Many chemical and physical changes can be reversed, they can proceed in both the forward and reverse directions, depending on the conditions of the system.

Reversible Physical Changes

Recall that a physical change involves a change in the physical state or appearance of the substance but does NOT involve producing any new substances.

Consider a piece of ice, solid water. If you take the ice out of the freezer and place it in a plastic cup on the kitchen bench it will melt and we will have a pool of liquid water (a physical change because we have not made any new substances, it is still water but now it is a liquid instead of a solid).

We can reverse this physical change by placing our cup of liquid water in the freezer so it changes back into solid water.

Overall, this chemical system is reversible depending on the conditions which, in this case, is the temperature of the system (adding heat or removing heat by cooling).
If we increase the temperature of the system by adding heat to the system, solid water changes into liquid water.
If we decrease the temperature by removing heat from the system (cool it), liquid water changes into solid water.

Similarly, if we heat liquid water we can change liquid water to gaseous water.
To reverse this physical change and change gaseous water back into liquid water we have to remove heat (cool it).
Overall this physical change is reversible depending on the conditions, which, in this case, is the temperature of the system (adding heat to the system or removing heat from the system).

If we heat solid iodine (a very dark purple colour) we produce purple iodine gas (this is an example of sublimation, no liquid iodine is formed first).
If we remove heat from this iodine gas (cool it), we re-form the solid iodine.
Overall this physical change is reversible depending on the conditions, which, in this case, is the temperature of the system (adding heat to the system or removing heat from the system).

Changes of state are reversible physical changes.

Reversible Changes of State
Chemical system Forward reaction Reverse reaction
solid/liquid solid to liquid
liquid to solid
liquid/gas liquid to gas
gas to liquid
solid/gas solid to gas
gas to solid

Adding or removing heat (changing the temperature of the system) is not the only method we can use to induce a physical change.
If we increase the pressure of some gaseous systems we can get the gas to condense to a liquid.
For example, LPG (liquid petroleum gas) is used as fuel for some motor vehicles. It can be produced by increasing the pressure of the 'petroleum gas' (a mixture of gases) to condense it into a liquid. When the pressure is decreased, by opening the gas bottle and exposing the contents to the much lower atmospheric pressure, the liquid evaporates and becomes a gas once more.
This physical change is reversible dependent on the condition of pressure, because we could re-capture the gas and increase the pressure to condense it back into a liquid.

Other physical changes are also reversible.
Imagine you have some solid blue copper sulfate in a beaker and you add just enough boiling hot water to dissolve it, producing a hot blue solution.
If you leave the hot solution to cool, blue crystals of copper sulfate will appear.
You can reverse this change by heating the solution again to re-dissolve the crystals.
Dissolving a salt like copper sulfate in water is an example of a reversible physical change.(2)

Reversible physical changes are very important in chemistry. We often use reversible physical changes to purify, or separate, substances in a mixture.
For example, if we have a mixture of ethanol (the alcohol in wine, beer and spirits) and water, we can separate the ethanol from the water by heating it to form gaseous ethanol, then remove the heat by cooling it to re-form the liquid ethanol (this process is called distillation and is an important experimental technique in chemistry).
For example, if a chemical reaction produces a salt that is dissolved in water, we can recover the salt by heating the mixture to evaporate off the water and leave the solid salt behind (this process is called crystallisation and is also an important chemical technique). If we wanted to recover the water, we could cool the gaseous water to condense it back to liquid water and re-use it in another chemical reaction.

Reversible physical changes are also important to society.
For example, one method of producing table salt is to evaporate the water off seawater. When we add this table salt to water when we cook, we are reversing the process when the salt dissolves.
Some plastics can be recycled by chopping them up into small pieces, placing the small pieces in a mold and heating so that the plastic melts and takes on the new shape when it is cooled.(3)

Reversible Chemical Changes

We saw above that physical changes are reversible, but many chemical changes, or chemical reactions, are also reversible (depending on the conditions).

Recall that a chemical change is one in which new substances are formed.

When I put hot coffee in my favourite white mug, the word "hot" appears in pink, and as the coffee cools, the pink word "hot" fades. If I put cold water in the same mug then the word "cold" appears in blue-green, and as the water warms up, the blue-green word "cold" fades. The ink on the different words responds to changes in heat (these are called thermochromic inks(4)). The word "hot" is written in an ink that is white when cool but changes to pink when it is hot, while the word "cold" is written in an ink that is white when hot but blue-green when it is cold.
Similar inks are used on strip thermometers, each number written in an ink that responds differently to changes in heat and hence displays the temperature.
We know there has been a chemical reaction because the change in colour indicates that a new substance has been formed.
The overall chemical reaction for each ink is reversible and is dependent on the condition of the system, temperature (whether we add heat to the system or remove heat from the system).

On my desk I have a "humidity indictor", a plaster of paris deer which is coated in a substance called cobalt chloride. When the humidity is low, there is very little water in the atmosphere, this substance is blue, but when the humidity is high and there is lots of water vapor in the atmosphere, the coating turns pink.
We know there has been a chemical reaction because the change in colour indicates that a new substance has been formed.
The overall chemical reaction for this substance is reversible and is dependent on the condition of how much water vapor is present in the atmosphere (NOT dependent on the temperature).

You have probably used an acid-base indicator to test whether a substance is an acid or a base. When you add a drop of this indicator to a base it is one colour, but a different colour when added to a base.
If you add a drop of litmus indicator to an acid it will be red, but if you add lots of base to the acid you can change the colour of the litmus indicator to blue.
If you then add more acid to this mixture, you can change the litmus indicator back to red.
We know there has been a chemical reaction because the change in colour indicates that a new substance has been formed.
This is an example of a reversible chemical change which is dependent NOT on heat or water, but on the "acidity" of the chemical system.

The stalactites and stalgmites in limestone caves are also produced by a reversible chemical reaction. The limestone reacts with water and the carbon dioxide in the atmosphere to produce a colourless solution. This solution forms drops from which the water evoparates and carbon dioxide is removed as a gas as the solution warms, reforming the solid limestone out of the drop.
The overall chemical process is reversible and is dependent on several conditions, the amount of carbon dioxide in the atmospheric water and the temperature of the system.

Understanding reversible chemical reactions is extremely important in chemistry. Many chemical reactions used in industry are reversible.
For example, the production of ammonia which is used to make fertilisers, is a reversible reaction between nitrogen gas and hydrogen gas. We need to understand the conditions that make the reaction reversible so that we can maximise the amount of ammonia that is produced from the reactants.(5)
Some of the flavour chemicals we add to our food are also produced in commercial quantities using reversible chemical reactions. Once again, we need to understand the conditions under which we can maximise the amount of flavour chemical produced(6).

Your health and well-being is also dependent on reversible chemical reactions. Many of the medicines you take when you are sick, such as aspirin, paracetamol and ibuprofen, and are made using reversible chemical reactions. We need to undertstand the conditions under which these reactions take place in order to maximise the amount of useful medicine we produce on a commercial scale.

A rechargeable battery, like the one in your laptop, tablet computer, phone, or car, also uses reversible chemical reactions.
When you use the battery the chemical reactions proceed in one direction to make electricity, but when you re-charge the battery you supply electricity to force the chemical reactions to go in the reverse direction.
These reactions are reversible and dependent on the condition of "electricity".(7)

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Irreversible Changes

Physical changes are reversible, some chemical changes are reversible, but some chemical changes are NOT reversible.
Chemical changes that are not reversible are called irreversible changes.
Irreversible chemical changes are chemical reactions that proceed ONLY in the forward direction.

Set a small piece of paper alight so that it burns until there is nothing left but a small pile of black ash. If you try to remove the heat by cooling the ash, all you will get is cold ash, you will not re-form the original paper.
This is because burning the paper is an irreversible chemical reaction, a chemical reaction that proceeds only in the forward direction.
Burning any substance, paper, candle wax, toast, fuels like petrol (gasoline) or ethanol (alcohol), is an irreversible chemical reaction.

At some stage you probably made a model of a volcano to which you added some sodium bicarbonate and vinegar to produce a frothy white mass which rolls down the sides of your model volcano (see acid and carbonates). The "froth" is made when carbon dioxide gas is produced during the chemical reaction, and it will all eventually escape and cause your "froth" to collapse into a sticky white mess on the table top. If you collect up all this "froth" and try to mix it with water, or heat it, you will not re-form the original sodium bicarbonate and vinegar because this chemical reaction is irreversible.

You have probably made hydrogen gas in the laboratory by adding a small piece of magnesium metal to some hydrochloric acid or water (see acid and metals). Once all the magnesium has reacted you are left with a colourless solution. You cannot reform the magnesium metal by heating the solution, or cooling it, or adding water, or doing anything else because this is an example of an irreversible reaction so the reaction proceeds only in the forward direction to produce hydrogen gas.

Irreversible chemical reactions are also extremely important to society.
Many of the plastics we use everyday are produced by irreversible chemical reactions. If you take a plastic bottle and heat it, it will probably soften and change shape, but you won't produce the original reactants. The chemical reactions that make many plastics are irreversible, they proceed in only one direction. This causes enormous problems in terms of their disposal, every rubbish tip in the world is full of plastics that will not change and will exist in landfill for many thousands of years. (Note: do not confuse the irreversible production of plastics from reactants with the reversible physical change used to recycle some plastics!).(8)

Similarly the burning of fossil fuels such as coal, petrol (gasoline) and natural gas, is an irreversible chemical reaction. When we burn the fossil fuel we produce carbon dioxide gas which escapes into the atmosphere, so this is an irreversible chemical change because the chemical reaction proceeds in only one direction (to produce the carbon dioxide gas). As more and more carbon dioxide enters our atmosphere the temperature of our atmosphere is increasing, resulting in an overall "global warming" and "climate change".(9)

A chemical change is irreversible if the chemical reaction proceeds only in the forward direction.

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Worked Example

The Problem:
A test tube contains a yellow solution of potassium chromate.
A student adds a drop of acid to the test tube and the solution changes colour to orange.
Then the student adds a drop of base to the test tube and the solution changes back to yellow.

Is this a reversible or an irreversible chemical reaction?

The Solution to the Problem

(using the StoPGoPS approach to problem solving)

STOP STOP! State the Question.
  What is the question asking you to do?

Decide whether the chemical reaction is reversible or irreversible.

PAUSE PAUSE to Prepare a Game Plan
  (1) What information (data) have you been given in the question?

(a) acid added to yellow solution turns solution orange

(b) base added to this orange solution turns solution yellow

(2) What is the relationship between what you know and what you need to find out?

(i) Reversible changes: changes to a chemical system that can proceed in both the forward and reverse directions depending on conditions.

(ii) Irreversible changes: changes to a chemical system that proceed in ONLY the forward direction.

GO GO with the Game Plan
  For the chemical system described in the question:

Forward direction: yellow to orange

Reverse direction: orange to yellow

Condition to change direction of reaction is "acidity" (adding acid or base)

This chemical system is reversible.

PAUSE PAUSE to Ponder Plausibility
  Have you answered the question?

Yes, we have decided that the change is reversible.

Is your answer plausible?

Consider what would happen if the chemical change was NOT reversible, that is, consider what would happen if the change was irreversible.
If the change were irreversible then, once we added the acid to make the solution orange we would not be able to heat it, cool it, or add another substance in order to get the yellow colour back again.
But since we could get the yellow colour back again, the change must have been reversible.

STOP STOP! State the Solution

This is a reversible chemical reaction.

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(1) In this introduction to reversible and irreversible reactions we will only be describing and classifying changes to a chemical system.
If you want to read an introduction to why chemical system are reversible or irreversible go to Spontaneous Reactions: Enthalpy and Entropy Tutorial.

(2) The conditions for this reversible reaction are a little more complicated. Firstly, whether or not the salt dissolves is dependent on the temperature of the system, more salt will dissolve in hot water than cold water.
The presence of other ions in solution also affects the solubiity of the salt.
This is called the "common ion effect".

(3) Well... these "plastics" usually soften rather then melt, but the principle is the same.

(4) thermo from the Greek for heat and chromo for colour, hence thermochromic inks change colour with heat.

(5) See Haber Process for more information.

(6) See esters for more information.

(7) Well... direction of electron flow might be more correct, refer to Electrolysis and Electrolytic Cells for more information.

(8) "Plastic" is the gross material we form into objects, it is usually a mixture of polymers and other substances. It is the polymer that is the major constituent in the plastic, and the production of many polymers is an irreversible chemical reaction.

(9) Carbon dioxide gas is known as a greenhouse gas, see Greenhouse Gases and the Greenhouse Effect for more information.