Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 109 - General Chemistry - Spring 2011

Lecture Notes 36: 22 April

Thermite Demonstration

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.

Catalysts

Catalysts make reactions go faster without affecting how far they go (

G is unchanged - the equilibrium is unchanged). They do this by

making it easier to achieve the activation energy (they reduce E_act), or
change the mechanism to one with one or more lower E_act's.

Reaction Mechanisms

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:

a series of elemental steps which when combined give the stoichiometry of the reaction.
the steps are consistent with the rate law for the reaction (for us the rate determining step, rds, or slowest step, gives the order of the reaction).

For example, for the reaction:

A + B

C + D, where r = k[A]²

we could write

2 A

C + X

B + X

A + D

A + B

C + D

Note that the first step, 2 A C + X, will satisfy the rate law, that is there are 2 A's in the collision. Thus the first step is the rate determining step.
And the two steps add up to give the overall stoichiometry, as shown.

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.

Look at the general reaction:

A + B

C + D

specifically

R₃CX + Y^-

R₃CY + X^-

First Order: r = k [A]; & r =-d [A]/dt = d [C]/dt. Example: :S_N1 from organic chemistry, as in the hydroxylation of t-alkylchloride:

Second order: r = k [A][B]. Example: S_N2 from organic chemistry, as in the hydroxylation of primary alkylchlorides (reverse of mechanism shown):

Chemical Equilibrium

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):

N_{2 (g)} + 3 H_{2 (g)}

2 NH_{2 (g)}

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

At equilibrium this system consists of two reactions proceeding in opposite directions at the identical rate:

C characterized by a constant, k₁ to give a rate, r₁ = k₁[A]

C characterized by a constant, k₂ to give a rate, r₂ = k₂[C]

But if r₁ = r₂, then

k₁[A] = k₂[C], and gathering constants

k₁/k₂ = [C]/[A], the ratio of constants is given a new name, the equilibrium constant, K_eq.

Equilibrium Expression

The Equilibrium Expression is then:

K_eq = [C]/ [A].

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 + ...

K_eq = [C]^c[D]^d/ [A]^a[B]^b.

Mass Action Expression

A similar expression is the Mass Action Expression:

Q = [C]^c[D]^d/ [A]^a[B]^b.

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.

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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