Which state functions are required to understand the second law of thermodynamics?

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 $-1648 \times 10^3 \ J \ mol^{-1}$)

In which state does matter have the highest entropy?

What is the unit of entropy?

If the enthalpy change for the following reaction at $300 \ K$ is $+7 \ kJ \ mol^{-1}$,find the entropy change of the surrounding (in $J \ K^{-1}$)?
$H_2O_{(s)} \longrightarrow H_2O_{(l)}$

One mole of an ideal gas at $350 \, K$ is in a $2.0 \, L$ vessel with thermally conducting walls,which are in contact with the surroundings. It undergoes isothermal expansion from $2.0 \, L$ to $3.0 \, L$ against a constant external pressure of $4 \, atm$. The change in entropy of the surroundings $(\Delta S_{surr})$ is $...... \, J \, K^{-1}$ (Nearest integer). Given: $R = 8.314 \, J \, K^{-1} \, mol^{-1}$.