CH 145- Experiment VI- Fall 2009
Textbook, Chapter 15 (all sections)
Ocean Optics handouts
This laboratory handout
There will be a pre-laboratory quiz!!
No chemical reaction goes to completion: there will always be some amount of reactants left
with the products. For example, although we write
2 CO (g) + O
as though it goes entirely to products, at 2000K only 2% of the CO
when the concentrations of the reactants and products no longer
change over time.
The position of the equilibrium describes the relative amounts of reactants
and products that remain at the end of a chemical reaction.
The position of the equilibrium for
reaction (1) is said to lie with the reactants, or to the left, because at equilibrium very little of the
carbon dioxide has reacted. On the other hand, in the reaction
(g) + O
the equilibrium position lies very far to the right since only very small amounts of H
remain after the reaction reaches equilibrium.
Since chemists often wish to maximize the yield
from a reaction, it is vital to determine how to control the position of the equilibrium.
The equilibrium position of a reaction may shift if an external stress is applied.
may be in the form of a change in temperature, pressure, or the concentration of one of the
reactants or products.
For example, consider a flask with an equilibrium mixture of CO
, as in reaction (1). If a small amount of CO is then injected into the flask, we will find
that the concentration of CO
Here the external stress is the increase in concentration
The system responds by reacting some of the added CO with O
to yield an increased
amount of CO
That is, the position of equilibrium shifts to the left, yielding more reactant and
Reaction (1) will also shift with changes in pressure.
Starting with reaction (1) at
equilibrium, an increase in pressure will cause the position of equilibrium to shift to the side of
the reaction with the smaller number of moles of gas.
That is, by shifting the equilibrium
position to the left, the reaction will decrease the number of moles of gas, thereby decreasing the
pressure in the flask.
In so doing, some of the applied stress is relieved. On the other hand, an
This experiment was modified from
Laboratory Inquiry in Chemistry
by Bauer, Birk, and Sawyer.