EQUILIBRIUM AND LE CHÂTELIER'S PRINCIPLE
Goals
 To observe the effect on equilibrium of adding or removing products and reactants.
 To predict the direction of shift in an equilibrium upon the change in concentration of one of
the components.
Background
Le Châtelier’s Principle states that:
If a stress is applied to a system at equilibrium, the system will respond by shifting in the
direction that reduces the stress and reach a new equilibrium condition.
In this experiment, the equilibrium between iron(III) thiocyanate, FeSCN+2, and it’s ions will be
investigated. This is a particularly easy equilibrium system to study because the dissociated ions (Fe +3
and SCN-1) are essentially colorless while the undissociated iron(III) thiocyanate is a dark red.
Fe3+ +SCN1– ⇋ FeSCN2+
Colorless ⇋ red
Lab Procedure
1. Place a piece of scrap paper under the ceramic spot plate so you can label four of the wells 1 — 4.
In each of these four wells, place 2 drops of 0.05 M NaSCN (sodium thiocyanate), 2 drops of 0.01
M Fe(NO3)3 solution, and 3 drops of water. Make sure you have taken the correct
concentrations of each solution. Mix each with a stirring rod; all of the solutions should appear
red.
2. Individually fill three of the wells at the bottom of the plate ¾ full with 0.10 M Fe(NO3)3, 0.05 M
NaSCN and 1.0 M NaNO3 and take your plate back to your lab station.
3. Add 2 more drops of water to well #1. This well will serve as your color comparison for the following
experiments.
4. Add 2 drops of the 0.10 M Fe(NO3)3 to Well #2 and record your observations.
5. Add 2 drops of 0.05 M NaSCN to Well #3 and record your observations.
6. Add 1 drops of 1.0 M NaNO3 to Well #4 and record your observations.
7. After answering the questions below, rinse all of your lab equipment and leave your lab station
ready for the next class to use.
Question 1. When Fe(NO3)3 was added to the system,
a. Which ion in the equilibrium system caused the "stress"?
b. Which way did the equilibrium shift?
c. What happened to the concentration of SCN1–?
d. What happened to the concentration of FeSCN2+?
Question 2. When NaSCN was added to the system,
a. Which ion in the equilibrium system caused the "stress"?
b. Which way did the equilibrium shift?
c. What happened to the concentration of Fe3+?
d. What happened to the concentration of FeSCN2+?
Question 3. When you added NaNO3, did anything happen? Can you explain this result?
ADDITIONAL EQUILIBRIUM QUESTIONS
For each of the following, indicate the direction the equilibrium would shift AND what would happen to
the concentrations of each substance in equilibrium.
1. The following equilibrium may be established with carbon dioxide and steam.
CO (g) + H2O (g) ⇋ CO2 (g) + H2 (g) + heat
What would be the effect of each of the following on the equilibrium and concentrations?
a.) The addition of more H2O?
b.) The removal of some H2?
c.) Raising the temperature?
d.) Increasing the pressure?
e.) Addition of a catalyst?
2. What would be the effect of each of the following on the equilibrium involving the synthesis of
methanol?
CO (g) + 2 H2 (g) ⇋ CH3OH (g)
a.) The removal of CH3OH?
b.) An increase in pressure?
c.) Lowering the concentration of H2?
d.) The addition of a catalyst?
3. A small percentage of nitrogen gas and oxygen gas in the air combine at high temperatures found
in automobile engines to produce NO gas, which is an air pollutant.
N2 (g) + O2 (g) + heat ⇋ 2 NO (g)
a.) Higher engine temperatures are used to minimize carbon monoxide production. What effect does
higher engine temperatures have on the production of NO gas? Why?
b.) What effect would high pressures have on the production of NO gas? Why?