entropy help

[zebra]

What is the standard entropy change for the each of the following reactions at 25 degrees C?

 

a. 2 Na(s) + 2H20 (l) ----> 2NaOH (aq) + H2 (g)

b. NaCO3 (s) + @HCl (aq) ----> 2NaCl (aq) + H20 (l) + CO2 (g)

 

Comment on the sign of: delta r S degrees (sorry, don't know how to represent it?)

[hermanntrude]

To calculate the entropy changes you will need to look up the standard entropy for each of the products and reactants. These are usually either provided in the book that contains the question or actually with the question itself. The math you need to use is exactly the same as in Hess's law. It's the equation with the capital sigma signs in it. Do you remember that one? If not, google "standard molar entropy change" and I expect you'll find the equation required. It looks scary but it's dead easy after you've tried it a few times.

 

Commenting on the sign involves a few rules:

 

the entropy of a system increases when:

1) pure liquids or liquid solutions are formed from solids

2) gases are formed from liquids or solids

3) the number of molecules of gas increases as a result of a chemical reaction

4) the temperature increases

 

any further help required, just ask