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Exothermic reactions always lead to an increase in entropy of the surroundings. In an example like this - the burning of magnesium in oxygen - the entropy of the system decreases because a giant ionic lattice ( Magnesium Oxide) is formed from the magnesium metal and oxygen (gas).
Energy profile for an exothermic reaction.
Endothermic reactions can be spontaneous - so long as the entropy decrease of the surroundings ( due to the reaction's positive enthalpy change is smaller than the entropy increase of the system
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