Question

How do we assess the enthalpy change in phase transitions?

Answer 1

You must look at the individual phase transitions.......and decide whether bonds are made or bonds are broken.............

Explanation:

When bonds are made energy is released; when bonds are broken energy is absorbed......

`"Ice melting"`: `H_2O(s) + Delta rarr H_2O(l)` `" ; endothermic"`

`"Steam condensing"`: `H_2O(g) rarrH_2O(l)+ Delta` `" ; exothermic"`

`"Water freezing"`: `H_2O(l) rarrH_2O(s)+ Delta` `" ; exothermic"`

`"Water boiling"`: `H_2O(l) +Delta rarrH_2O(g)` `" ; endothermic"`

In each case the making of bonds entails an exothermic reaction.