The reaction we will look at is :
Al(s) + Fe2O3(s) Al2O3(s) + Fe(l)
This reaction has a very high activation energy, so that the thermite powder is quite stable, we have to add lots of heat to get it started. To do that today we will use the energy generated by a burning Mg ribbon to initiate the thermite reaction. Once started the reaction itself liberatesd an immense amount of energy as heat, which allows the reaction to proceed to completion and give molton iron as the product.
Most chemical reactions take place via a series of sequential steps. A reaction mechanism is a possible description of these steps written by us in an attempt to model the reaction.
A proper mechanism is characterized by:
For example, for the reaction:
we could write
FYI – Fun Examples
Finally, let's look at a couple of example reactions (you don't need to know the chemistry, these are just to demonstrate the possibilities) showing two different reaction orders with the same stoichiometric ratios.
As a general statement can say that all chemical reactions are equilibrium reactions and go toward a state of equilibrium or approach equilibrium. Some reactions reach equilibrium rapidly, some slowly, and some favor products to such an extent that the reactions "go to completion." Regardless of initial concentrations, systems will reach equilibrium given time.
What is an equilibrium - very dynamic situation in chemistry (Great Crabapple War overheads)
Let's look at a classic example of reactions approaching equilibrium, ammonia synthesis. This takes place in the gas phase at high temperature and pressure (Fig 13.5, Zumdahl, p 597; drawn on board):
Vast numbers of chemical reactions operate at equilibrium in the natural world, and it is frequently essential to be able to understand and to predict their behavior.
Quantitatively we can look at the relationship in a reaction at equilibrium represented by
The Equilibrium Expression is then:
Generalizing for the equation (Note that the simple derivation above does not generalize, since rates and stoichiometry are not generally correlated - the relationships between rates at equilibrium and stoichiometry are beyond our study.):
aA + bB + ... cC + dD + ...
Keq = [C]c[D]d/ [A]a[B]b.
A similar expression is the Mass Action Expression:
The mass action expression is algebraically identical to the equilibrium expression, but it applies to a more general case. That is, the equilibrium expression requires that the values in the expression give the equilibrium constant, whereas the mass action expression allows any set of values. Thus the mass action expression is used to describe a system which has not yet reached equilibrium, while the equilibrium expression is a special case of the mass action expression for a system at equilibrium.
Qualitatively we can get an idea of how an equilibrium system will behave by using Le Châtelier's Principle.
If stress is applied to a system at equilibrium, the equilibrium will shift in such a way as to relieve the stress. e.g. if the pressure of carbon dioxide is increased over a solution of carbon dioxide in water, more carbon dioxide will dissolve, reducing the pressure increase. Of course there are other consequences, the system can't just abosorb the carbon dioxide. Note the reaction of carbon dioxide and water,
CO2 + H2O HCO3- + H+
So we should also see a drop in pH, that is it becomes more acidic.
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© R A Paselk
Last modified 22 April 2011