Question

For Heat Effects and Calorimetry Lab: For the Heat of Reaction part

Why do salts that have pickedup a little moisture have J/gram values much less than their literature value?

Answer 1

The entalpy of hydration is defined as the change of enthalpy when one mole of an anhyrous salt is hyrated.

Example:

CuSO₄(anhydrous) + 5 H₂O(l) = CuSO₄ 5 H₂O(s) Delta H = -78.2 kJ/mol

The enthapy change accompanying the above process is the enthalpy of hydration of anhydrous CuSO_4.and it correspond to 5 mol of H₂O(l) ..

This is an exothermic process accompanied by the release of 78.2 kJ of heat energy.

This reaction is exothermic because during the hydration process bonds are formed between CuSO₄(anhydrous) and H₂O(l) molecules. Bond formation is always an energy releasing process.

The energy of -78.2 kJ/mol corresponds to to the formation of bonds between 1 mol CuSO₄(anhydrous) and 5 mol H₂O(l)

If the salt has already picked up a little moisture ,the enthalpy change will correspond to less than 5 mol of water.The number bonds formed is less. Hence the energy released is less than the literaure value.