H ) from the bonds broken and formed decreases, resulting in a more stable arrangement
of atoms and electrons. This is a favoured state in nature and can drive these
reactions to occur. | M3 | Lesson #1 | Entropy |
Entropy
Introduction
Chemical reactions can be either exothermic or endothermic. In an exothermic
change the overall enthalpy (
H ) from the bonds broken and formed decreases, resulting in a more stable arrangement
of atoms and electrons. This is a favoured state in nature and can drive these
reactions to occur.
Why then do reactions that absorb energy and result in an overall increase in enthalpy (energy) occur? The answer is that in these system there is always an increase in entropy. What is entropy and how does it increase?
Scientists have found that there is a second drive that causes reaction to
happen. This is the drive towards greater randomness. The degree of randomness
or disorder in a system is called it's entropy and is given the symbol
S. Changes that produce substances with greater randomness (+
S) are favoured in nature and drive the reaction to occur. The faulting list
describe the changes (in general) that result in more random substances.
1) Change in state from solid to liquid.
2) Change in state from liquid to gas.
3) Change in state from solid to gas.
4) Formation of a mixture.
5) Increase in the volume of a system of gases.
Examine a reaction that occurrs when the endothermic reaction is performed. Which of the above changes in state occurred? Notice that two solid reactants formed a liquid product, and a gas was given off.
In all cases if the reaction is endothermic, the entropy of the system must increase.
Often the drives toward greater entropy and decreasing enthalpy oppose each other. In a chemical reaction the balance of increasing entropy and decreasing enthalpy determines if the reaction will occur spontaneously an at what temperature. Let's examine some chemical reactions and try to determine why they occurred.
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Entropy and Enthalpy Click on the test-tube and complete the following laboratory that identifies why each reaction occurred. |