Lab 8 Le Chatlier’s Principle
Lab Manual pp 114-121
Understanding the idea of equilibrium is important to fully appreciate this experiment. Up to this chapter, when we wrote a chemical reaction, we assumed that we grabbed the reactants, mixed them together and the reactants chemically combined to give the products. All of the reactants were used up and the only thing left is the products. Some reactions are just like this, for instance, burning gasoline in our car engine:
2 C8H18 + 25 O2 16 CO2 + 18 H2O
If we were to write the reverse of this reaction, we would expect it would never occur:
16 CO2 + 18 H2O 2 C8H18 + 25 O2
However, there are other reactions that are not like this. We can write the reaction in either direction, start the reactions as written (using only reactants) and we would get product produced:
CH3CO2H + CH3CH2OH CH3CO2CH2CH3 + H2O
or CH3CO2CH2CH3 + H2O CH3CO2H + CH3CH2OH
In this case, the reaction does not seem to go “all the way,” there are some left-over reactants. These reactions are called reversible reactions and they always end when the product to reactant ratio is the same. That point is called the equilibrium point. Often the reactions are written in one direction only, but either a double-headed arrow or two arrows are used to indicate the reaction will go in either direction until that magic equilibrium ratio is achieved.
Le Chatelier was the first to express the idea that once the reaction appears to stop, we can change some part of that reaction (amount of reactant or product, temperature, or pressure) and the reaction starts up again—one direction or the other—until that magic equilibrium ratio is again achieved.
This is a two part lab, you will examine two different reversible reactions. You will set up each reaction four times. Each well will initially be at the equilibrium point. Then you will change some aspect of the reaction (disturb or stress the equilibrium) and observe which direction the reaction proceeds to return to the magic equilibrium ratio (relieve the stress).
Na2EDTA reacts with (chelates) metals (Mg2+) effectively removing them from solution.
Remember since ionic compounds become aquesous,when you add Na2SO4 to the solution you are adding Na+ and SO42- ions to the solution.
The results sheet is on the next two pages.
Results Sheet, Experiment 9
Part 1 Mg(OH)2(s) Mg2+(aq) + 2 OH-(aq)
Step 6, 7, 9 or 10
(depending on the well)
1 drop, removed most pink. 2 drops, pink gone
4 drops started lightening. 10 drops pink nearly gone
Part 1. Questions:
When asked about a shift to an equilibrium, the reaction either shifts toward reactants or toward products (the reaction starts at equilibrium, something is done to the reaction; after the disturbance, the reaction proceeds in the reverse direction as written forming more reactants to relieve the stress or the reaction proceeds in the forward…