C2bL8 Equilibrium

Key Words

Activation energy - energy needed to get a reaction going.

Closed System - Nothing can come in or go out.

Decompose - compounds break down to form simpler substances.

Equilibrium - balance.

Reversible reaction - reaction that can go forwards or backwards.

Yield - how much product is made

Grade E

If we burn magnesium to form magnesium oxide, all the magnesium is burned and the reaction goes to completion.  You cannot heat the magnesium oxide and get magnesium back.

Some reactions are reversible.  That means that as well as going forwards, the reaction can go backwards:

reactant 1 + reactant 2  ® product 1 + product 2  (forwards)

product 1 + product 2  ® reactant 1 + reactant 2 (backwards)

For example:

calcium carbonate  ® calcium oxide + carbon dioxide

CaCO_{3 (s)} ® CaO _(s) + CO_{2 (g)}

We could do this reaction in a sealed metal box.  Since this is a closed system, nothing goes in or out.  If there are forward reactions going on at the same time as there are backward reaction, we get a balance, which we call equilibrium. 

At the start, the calcium carbonate decomposes to calcium oxide and carbon dioxide.  As the reaction continues, calcium oxide and carbon dioxide form calcium carbonate.  So there is a reverse reaction.  When the rate of the reverse reaction equals the rate of the forward reaction, we have equilibrium.

If we open the box, the system is no longer closed.  The carbon dioxide escapes, so the calcium carbonate continues to decompose until the reaction goes to completion.

Grade C

When equilibrium is reached in a closed system, it does not mean that there are no reactions; instead the forward reactions happen at the same rate as the backward.  This is called dynamic equilibrium.

In a closed system, we can change the conditions to change the yield by changing the equilibrium point:

• If the reaction is exothermic (gives out heat), the equilibrium would change so that there are more reactants than products.  The yield is reduced.  So there is no point in heating an exothermic reaction to help it along.
• If the reaction is endothermic (takes in heat), the equilibrium would change so that there are more products than reactants.  The yield is increased.  So there is every point in heating an endothermic reaction to help it along.

f we have reactions that involve gases, the pressure has an effect. 

• If we increase the pressure, the equilibrium favours the side with fewer gas molecules;

• If we reduce the pressure, the equilibrium favours the side with more gas molecules.

The in a system that is NOT closed, we can increase the yield by:

• adding more reactants;
• removing the products.

Grade A

We have seen the activation energy. 

Catalysts can reduce the activation energy by a large amount. 

In effect, they enable the reaction to "tunnel" through the energy hill:

The presence of a catalyst reduces the activation for the reverse reaction as well.  The reverse reaction can go back through the "tunnel" in the energy hill.  This makes the reverse reaction happen faster as well.

So both the forward and reverse reaction go at the same faster rate.  In a closed system, the equilibrium point remains the same.