Question

Why is heat generated when water is added to methanol?

Answer 1

Water molecules associate by H-bonds and methanol molecules also associate by H-bonds but the magnitudes of H-bonds differ between these compounds. When mixed together, they are completely miscible but the total volume of the solution will be less than the sum of the two volumes measured separately because there has been a change in the inter-molecular H-bonding structure. What goes on is that several water-water H-bonds & methanol-methanol H-bonds get broken and several water-methanol H- bonds are formed.

While the first splitting step is endothermic, the second bond making step is exothermic. This indicates that CH3OH favors H-bonding with H2O than H-bonding with its own molecules because with water, methanol will have more chances of H-bonding association taking into account that CH3OH has CH3- part so H2O will approach methanol to the small –OH part much easily.



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