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27 Colligative Properties

# 27 Colligative Properties - Enthalpy of Solution The...

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1 1 Enthalpy of Solution Note: It always costs you the lattice energy and you get back the hydration energy, so figure out the +/- signs from that. The enthalpy of solution , H solution , is the energy that is transferred from the reacting system to the surroundings when a solution is formed at constant temperature and pressure. The enthalpy of solution is the sum of the lattice energy and the hydration energy with the appropriate signs. Consider NaCl(s) Na +( aq) + Cl - (aq) Energy is needed to separate the ions. This energy is called the lattice energy . Then energy is released when the ions are solvated (i.e. interact with water molecules) to form the solution. This energy is called the solvation energy or hydration energy . For NaCl, the lattice energy is 790 kJ/mol and the hydration energy is 780 kJ/mol. What is the enthalpy of solution for sodium chloride? Respond in kJ/mol. 2 What do you think? The vapor pressure of sea water is _____________ the vapor pressure of pure water? A) higher than B) the same as C) lower than

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2 3 Fig. 15-14, p. 745 A really neat measurement of the difference! 4 Measurement shows that the vapor pressure of salty water is linearly proportional to the mole fraction of the water (solvent) in the solution. The vapor pressure of the solvent is lowered by the presence of non-volatile solute particles. A colligative property!