Which of the following reactions has a positive entropy change?

When a solid is converted into liquid,entropy

For a sample of a perfect gas,when its pressure is changed isothermally from $p_i$ to $p_f$,the entropy change is given by:

For which one of the following reactions, the entropy change is positive?

For the oxidation of iron:
$4 Fe_{(s)} + 3 O_{2(g)} \rightarrow 2 Fe_2O_{3(s)}$
The entropy change is $-549.4 \ J \ K^{-1} \ mol^{-1}$ at $298 \ K$. Despite the negative entropy change of this reaction,why is the reaction spontaneous?
($\Delta_r H^\Theta$ for this reaction is $-1648 \times 10^3 \ J \ mol^{-1}$)

For the reaction $MgCO_3(s) \rightarrow MgO(s) + CO_2(g)$,the value of $\Delta S$ is: