le Chatelier's Principle

When a system is in dynamic equilibrium, the concentration of reactants and products stays the same because the rates of reaction of the forward and reverse reactions are the same.

What if we add an external influence to the system, such as heat, pressure, or more reactants...?

This is where le Chatelier's Principle comes in. It states that the equilibrium system will "shift" to minimise the impact of any external factors (such as those mentioned). What this means is that the rate of one of the reactions will increase (but not the other one!), re-establishing the equilibrium "position" - the concentration of the reactants or products will change until the K_C is correct (for that specific temperature - remember that the K_C for an equilibrium is different at different temperatures).

This is very well explained here (as well as being an overview of everything to do with Equilibrium so far):

We need to be able to explain the effect on an equilibrium of:

1. Increasing the amount (therefore concentration) of a reactant or product
2. Removing some/all (therefore reducing the concentration) of a reactant or product
3. Increasing/decreasing the temperature
4. Increasing/decreasing the pressure
5. Introducing a catalyst

We then need to link these to what we would observe (temperature changes, colour changes etc.)

Changing Concentration of Reactants or Products

When we increase the concentration of a reactant (by adding more of it into the system), the forward reaction is favoured. This means the forward reaction's rate or reaction is increased. This is to "use up" some of the reactants, so re-establishing a dynamic equilibrium system. If the forward reaction is exothermic, the system will also feel warmer. If the forward reaction is endothermic, the system will absorb heat energy from the surroundings, so feel colder.

When we reduced the concentration of a reactant (maybe by reacting some of it with another chemical that we introduce), the reverse reaction is favoured. The rate of reaction of the reverse reaction increases in order to make more reactants, so re-establishing a dynamic equilibrium system. If the forward reaction is endothermic, the system will also feel warmer as the reverse reaction is exothermic. If the forward reaction is exothermic, the system will absorb heat energy from the surroundings, so feel colder, as the reverse reaction must be endothermic.

You can apply both of these to a change in concentration of the products.

Changing the Temperature

If you add heat energy, the system will try to absorb this heat energy, so favour the endothermic reaction until a new dynamic equilibrium position is established.

If you cool the system, the exothermic reaction will be favoured.

Changing Pressure

This only affects gaseous particles. If you increase the pressure, you limit the space for gaseous particles. Therefore, an equilibrium system will favour the reaction which produces fewer gaseous particles until a new dynamic equilibrium is established.

Introducing a Catalyst

This has no effect upon the position of the equilibrium. However, it does mean that dynamic equilibrium is established quicker, as the catalyst increases the rate of both the forward and the reverse reactions.

Equilibrium Constant

The position of a system in dynamic equilibrium can be described mathematically, using the equilibrium constant, K_C.

We need to know how to:

1. Write a K_C expression for an equilibrium
2. Calculate K_C (when given the concentrations of reactants and products at equilibrium).
3. Calculate the concentration of a reactant/product (when given the K_C value and concentrations of all other reactants and products).
4. Explain whether the equilibrium "favours" the forward or reverse reaction, based upon the K_C value.

We can then use this knowledge to apply le Chatelier's Principle to changes in the equilibrium position due to outside influences (our next key concept).

Reversible Reactions and Equilibrium

Reversible Reactions

Some reactions are easily reversed. In class, we saw that blue hydrated copper sulfate crystals can be turned into anhydrous white copper sulfate crystals, by heating (which removes the crystallised water molecules). By adding a drop or two of water, this is reversed, and releases a lot of heat energy!

Equilibrium

When a reversible reaction is contained within a closed system (such as a boiling tube with a rubber/cork bung, or a bottle with a lid on it), the forward and reverse reactions "compete", until the rate of the forward reaction is equal to the rate of reaction of the reverse reaction. The concentrations of the reactants and products do not change, even though both reactions (forward and reverse) are happening. We call this dynamic equilibrium. We will explore this in more detail over the next week.

This video summarises these two concepts well (and is the one we looked at in class on Monday):

Collision Theory and Rate of Reaction

Chemical reactions occur when particles collide with enough energy and in the correct orientation. We can speed up (or slow down) reactions by changing some of the conditions that will cause reactions to occur:

• more often
• with more energy
• in the correct orientation

This part of the topic is focused on how we affect the rate of a reaction, using our knowledge of the Collision Theory.