Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 107

Fundamentals of Chemistry

Fall 2009

Lecture Notes: 19 November

© R. Paselk 2005
 
     
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Reaction Rates

Collision Theory: We assume that particles must collide in order to react. Thus a first understanding of reaction rates is based on understanding what influences the frequency of collisions.

Transition States and Reaction Progress (Reaction Coordinate) Diagrams.

Reaction with a negative free energy (-deltaG) - products are favored:

reaction progress diagram with negative reaction energy (products favored)

.

How do we interpret this diagram?

Equilibria

Chemical equilibria occur when the rates of the forward and the reverse reactions of a chemical system are identical.

Regardless of initial concentrations, systems will approach and reach equilibrium given time [see Figure 12.15, p 474] (overheads, approach to equilibria by carbon monoxide + water vs. carbon dioxide + hydrogen)

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. 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.

Let's look at an equilibrium system, and try to predict its response using Le Châtelier's Principle and/or the equilibrium expression.

Example: Consider the reaction

CO + NO2 equilibrium double arrow NO + CO2 + heat (226 kJ)

Note that heat appears on the product side - the system is giving up heat, therefore deltaH is negative, deltaH = - 226 kJ

So, what will happen to [CO2] if:

    • CO is added?
    • NO2 is added?
    • NO is added?
    • T is increased?
    • V is increased?
    • Ar is added?
 

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Last modified 19 November 2009