[ajsammut]

Which of these reactions are expected to show a negative change in entropy:

1) 2SO2 (g) + O2 (g) ----- 2SO3 (g)       Enthalpy change = -196kJ

2) 2S (s) + 3O2 (g) ----- 2SO3 (g)         Enthalpy change = -790kJ

3) S (s) + O2 (g) ----- SO2 (g)         Enthalpy change = -297kJ

10ks for help

[Yggdrasil]

In general, a reaction which decreases the number of molecules in the gas phase decreases the entropy.  For example, take reaction (3).  Here, I would expect you decrease entropy because there are more ways of arranging 2 molecules in space than 1 molecule.

If you really want to check rigorously, however, you can calculate the change in entropy by looking up the standard entropies of the compounds and applying Hess's Law.