Le Chatelier’s Principle:
Effect of heat on equilibrium
If the temperature is increased, the equilibrium will shift towards the endothermic side.
A law stating that if changes are made to a system in equilibrium the system adjusts itself to oppose the change.
NOTE: This is NOT in syllabus, you do not NEED to know it, but this coupled with basic physics can help you
understand how things effect equilibrium. Anything highlighted in grey in this chapter CAN BE IGNORED.
Le Chatelier’s Explanation: This is because when the temperature is increased, the equilibrium will shift to REDUCE
THIS CHANGE IN TEMPERATURE ; how can it do this? By promoting the endothermic (heat-absorbing) reaction.
Example:
A2+B2 2AB ΔH=-300kJ (forward reaction)
Since the change in enthalpy is negative this means the forward reaction is exothermic. So to increase rate of forward reaction, decrease temperature.
To increase rate of reverse reaction, increase temperature
Effect of pressure on equilibrium
Liquids and Solids are not really effected by pressure, thus pressure only occurs on gas involving reaction. If the pressure is increased, the equilibrium will shift towards the side producing the least moles of gas.
Le Chatelier’s Explanation: Pressure increases when the volume is squashed/compacted;; so the way to reduce the
pressure is to decrease the volume! It can do this by producing less gas molecules instead of more.
Decreasing pressure will cause the equilibrium to shift towards the side producing most moles of gas.
Example:
N2(g)+3H2(g) 2NH3(g)
There are 2 moles of gas being produced by the forward reaction nd two moles of gas being produced by reverse reaction.
If we increase pressure, it will promote the forward reaction (a.k.a. equilibrium shifts to the right)
If we decrease pressure, it will promote the reverse reaction (a.k.a. equilibrium shifts to the left)
Effect of concentration on equilibrium
By now you will know the effect of concentration (review: increased concentration increases rate of reaction). Therefore the side with the highest concentration will react faster; e.g. if the reactants have a higher concentration the forward reaction will be faster than the reverse reaction.
Thereforce concentration can be used in two ways, by decreasing or increasing. Example:
To increase rate of forward reaction, either increase concentration of reactants or decrease concentration of products To increase rate of reverse reaction, either increase concentration of PRODUCTS or decrease concentration of reactants.
Le Chatelier’s Explanation: Simple;; when you add reactants, the equilibrium will shift to use up these reactants
quickly i.e. promoting forward reaction.
Redox
This page looks at the various definitions of oxidation and reduction (redox) in terms of the transfer of oxygen, hydrogen and electrons. It also explains the terms oxidising agent and reducing agent.
Oxidation and reduction in terms of oxygen transfer
Definitions
For example, in the extraction of iron from its ore:
If the temperature is increased, the equilibrium will shift towards the endothermic side.
A law stating that if changes are made to a system in equilibrium the system adjusts itself to oppose the change.
NOTE: This is NOT in syllabus, you do not NEED to know it, but this coupled with basic physics can help you
understand how things effect equilibrium. Anything highlighted in grey in this chapter CAN BE IGNORED.
Le Chatelier’s Explanation: This is because when the temperature is increased, the equilibrium will shift to REDUCE
THIS CHANGE IN TEMPERATURE ; how can it do this? By promoting the endothermic (heat-absorbing) reaction.
Example:
A2+B2 2AB ΔH=-300kJ (forward reaction)
Since the change in enthalpy is negative this means the forward reaction is exothermic. So to increase rate of forward reaction, decrease temperature.
To increase rate of reverse reaction, increase temperature
Effect of pressure on equilibrium
Liquids and Solids are not really effected by pressure, thus pressure only occurs on gas involving reaction. If the pressure is increased, the equilibrium will shift towards the side producing the least moles of gas.
Le Chatelier’s Explanation: Pressure increases when the volume is squashed/compacted;; so the way to reduce the
pressure is to decrease the volume! It can do this by producing less gas molecules instead of more.
Decreasing pressure will cause the equilibrium to shift towards the side producing most moles of gas.
Example:
N2(g)+3H2(g) 2NH3(g)
There are 2 moles of gas being produced by the forward reaction nd two moles of gas being produced by reverse reaction.
If we increase pressure, it will promote the forward reaction (a.k.a. equilibrium shifts to the right)
If we decrease pressure, it will promote the reverse reaction (a.k.a. equilibrium shifts to the left)
Effect of concentration on equilibrium
By now you will know the effect of concentration (review: increased concentration increases rate of reaction). Therefore the side with the highest concentration will react faster; e.g. if the reactants have a higher concentration the forward reaction will be faster than the reverse reaction.
Thereforce concentration can be used in two ways, by decreasing or increasing. Example:
To increase rate of forward reaction, either increase concentration of reactants or decrease concentration of products To increase rate of reverse reaction, either increase concentration of PRODUCTS or decrease concentration of reactants.
Le Chatelier’s Explanation: Simple;; when you add reactants, the equilibrium will shift to use up these reactants
quickly i.e. promoting forward reaction.
Redox
This page looks at the various definitions of oxidation and reduction (redox) in terms of the transfer of oxygen, hydrogen and electrons. It also explains the terms oxidising agent and reducing agent.
Oxidation and reduction in terms of oxygen transfer
Definitions
- Oxidation is gain of oxygen.
- Reduction is loss of oxygen.
For example, in the extraction of iron from its ore:
Because both reduction and oxidation are going on side-by-side, this is known as a redox reaction. Oxidising and reducing agents
An oxidising agent is substance which oxidises something else. In the above example, the iron(III) oxide is the
oxidising agent.
A reducing agent reduces something else. In the equation, the carbon monoxide is the reducing agent.
An oxidising agent is substance which oxidises something else. In the above example, the iron(III) oxide is the
oxidising agent.
A reducing agent reduces something else. In the equation, the carbon monoxide is the reducing agent.
- Oxidising agents give oxygen to another substance.
- Reducing agents remove oxygen from another substance.
Oxidation and reduction in terms of hydrogen transfer
These are old definitions which aren't used very much nowadays. The most likely place you will come across them is in organic chemistry.
Definitions
Notice that these are exactly the opposite of the oxygen definitions. For example, ethanol can be oxidised to ethanal:
These are old definitions which aren't used very much nowadays. The most likely place you will come across them is in organic chemistry.
Definitions
- Oxidation is loss of hydrogen.
- Reduction is gain of hydrogen.
Notice that these are exactly the opposite of the oxygen definitions. For example, ethanol can be oxidised to ethanal:
You would need to use an oxidising agent to remove the hydrogen from the ethanol. A commonly used oxidising agent is potassium dichromate(VI) solution acidified with dilute sulphuric acid.
Ethanal can also be reduced back to ethanol again by adding hydrogen to it. A possible reducing agent is sodium tetrahydridoborate, NaBH4. Again the equation is too complicated to be worth bothering about at this point.
Ethanal can also be reduced back to ethanol again by adding hydrogen to it. A possible reducing agent is sodium tetrahydridoborate, NaBH4. Again the equation is too complicated to be worth bothering about at this point.
An update on oxidising and reducing agents
Oxidation and reduction in terms of electron transfer
This is easily the most important use of the terms oxidation and reduction.
Definitions
It is essential that you remember these definitions. There is a very easy way to do this. As long as you remember that you are talking about electron transfer:
- Oxidising agents give oxygen to another substance or remove hydrogen from it.
- Reducing agents remove oxygen from another substance or give hydrogen to it.
Oxidation and reduction in terms of electron transfer
This is easily the most important use of the terms oxidation and reduction.
Definitions
- Oxidation is loss of electrons.
- Reduction is gain of electrons.
It is essential that you remember these definitions. There is a very easy way to do this. As long as you remember that you are talking about electron transfer:
An update on oxidising and reducing agents
Oxidation and reduction in terms of electron transfer
This is easily the most important use of the terms oxidation and reduction.
Definitions
It is essential that you remember these definitions. There is a very easy way to do this. As long as you remember that you are talking about electron transfer:
- Oxidising agents give oxygen to another substance or remove hydrogen from it.
- Reducing agents remove oxygen from another substance or give hydrogen to it.
Oxidation and reduction in terms of electron transfer
This is easily the most important use of the terms oxidation and reduction.
Definitions
- Oxidation is loss of electrons.
- Reduction is gain of electrons.
It is essential that you remember these definitions. There is a very easy way to do this. As long as you remember that you are talking about electron transfer:
A last comment on oxidising and reducing agents
If you look at the equation above, the magnesium is reducing the copper(II) ions by giving them electrons to neutralise the charge. Magnesium is a reducing agent.
Looking at it the other way round, the copper(II) ions are removing electrons from the magnesium to create the magnesium ions. The copper(II) ions are acting as an oxidising agent.
Warning!
This is potentially very confusing if you try to learn both what oxidation and reduction mean in terms of electron transfer, and also learn definitions of oxidising and reducing agents in the same terms.
It is recommend that you work it out if you need it. The argument (going on inside your head) would go like this if you wanted to know, for example, what an oxidising agent did in terms of electrons:
If you look at the equation above, the magnesium is reducing the copper(II) ions by giving them electrons to neutralise the charge. Magnesium is a reducing agent.
Looking at it the other way round, the copper(II) ions are removing electrons from the magnesium to create the magnesium ions. The copper(II) ions are acting as an oxidising agent.
Warning!
This is potentially very confusing if you try to learn both what oxidation and reduction mean in terms of electron transfer, and also learn definitions of oxidising and reducing agents in the same terms.
It is recommend that you work it out if you need it. The argument (going on inside your head) would go like this if you wanted to know, for example, what an oxidising agent did in terms of electrons:
- An oxidising agent oxidises something else.
- Oxidation is loss of electrons (OIL RIG).
- That means that an oxidising agent takes electrons from that other substance.
- So an oxidising agent must gain electrons.
- An oxidising agent oxidises something else.
- That means that the oxidising agent must be being reduced.
- Reduction is gain of electrons (OIL RIG).
- So an oxidising agent must gain electrons.