What is meant by enthalpy? Can a decrease in enthalpy be the criterion for the spontaneity of all reactions? If not, why?

Explain the entropy concept and discuss the nature of change in entropy (increase or decrease) for: ${\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to {\mathrm{H}}_2\mathrm{O}\left(\mathrm{s}\right)$.

Explain the entropy concept and discuss the nature of change in entropy (increase or decrease) for: ${\mathrm{MgCO}}_3\left(\mathrm{s}\right)\to \mathrm{MgO}\left(\mathrm{s}\right)+{\mathrm{CO}}_2\left(\mathrm{g}\right)$.

Explain the entropy concept and discuss the nature of change in entropy (increase or decrease) for: $2{\mathrm{SO}}_2\left(\mathrm{g}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\to 2{\mathrm{SO}}_3\left(\mathrm{g}\right)$.