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Entropy

Objectives:

Key Terms:

Notes: (23-3)

In spontaneous reactions there is a tendency for things to become less ordered.  This increasing amount of disorder is referred to as Entropy.  

entropy is the quantitative measure of disorder (randomness) in a system.  
entropy is given the symbol S
DS = Sproducts - Sreactants
when Sproducts > Sreactants the entropy is positive and the amount of disorder increases in the reaction as the products are more disordered than the reactants

(www.saburchill.com/physics/ images/0427.jpg)

Consider the above picture.  Melting ice makes a perfect example of entropy.  As ice the individual molecules are fixed and ordered.  As ice melts the molecules become free to move therefore becoming disordered.  As the water is then heated to become gas, the molecules are then free to move independently through space.  For both of the above reactions the entropy (DS) would be positive.

Here are some good examples of reactions that would have  positive entropy.

  1. Formation of gases
    CO2(s) g CO2(g)
    H2O(l) g H2O(g)
  2. Formation of ions in solution
    NaCl(s) g Na+(aq) + Cl-(aq)
  3. More molecules of product than reactants
    2NH3(g) g N2(g) + 3H2(g)
    NH4Cl(s) g NH3(g) + 3HCl(g)
  4. The temperature of a substance is increased.
    H2O(l, 25oC) g H2O(l, 50oC)

The second law of thermodynamics is that ALL things lead to entropy.  What this means is that with all spontaneous reactions, entropy increases.  

Within you, your community, country, world, solar, system, and universe things are speeding toward increasing disorder (absolute chaos!).

 

 

Last modified: May 21, 2003