1. A reaction in which a molecule is broken into two or more smaller molecules.
2. A reaction in which there is an increase in moles of gas.
3. A process in which a solid changes to a liquid or gas or a liquid changes to a gas.
Lets take the following example:
So (NaHCO3) = 102 J/K
So (Na2CO3) = 139 J/K
So (H2O) = 188.7 J/K
So (CO2) = 213.7 J/K
Before we proceed, it is important that we take into account the physical state of the substance for which we want to obtain is value of So In the case of water, there are two values in the table. We must choose the value of H2O (g), which is the physical state of the water in this reaction. If we choose the value for H2O (l) we will be calculating the wrong change in So, since that is not the state of the water in the chemical reaction.
We know that [Delta]So = [Sigma] So (products) - [Sigma] So (reactants); therefore:
[Delta]So = [(So Na2CO3) + (So H2O) + So (CO2)] - 2(So NaHCO3), substituting we obtain:
[Delta]So = ([139+ 188.7 + 213.7] - 2) J/K. When we do the math, we obtain a value of [Delta]So of
+337 J/K, which agrees with our prediction.
Of course, if the reaction is reversed, that is, we write it like:
You should practice problems like this in your book, in order to understand not only how to qualitatively assign the sign, but also to calculate the value of [Delta]So quantitatively.