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Heat of Solution for a Salt 2010

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					                                 Enthalpy Changes: Heat of Solution for a Salt

Introduction
The heat of solution for a salt may be endothermic or exothermic, depending upon two factors often
considered in the dissolution of a salt: the energy within the bonds of the salt and the hydration energy of the
ions. The energy within the bonds, or lattice energy, is measured as the energy required when vaporizing one
mole of the solid salt into its gaseous ions (endothermic); the hydration energy, or solvation energy, is the
energy released when the gaseous ions are attracted to water molecules (exothermic). The heat of solution of
a salt is the sum of these two factors.

The specific heat of solution (hsol) can be expressed as the energy per mass of salt in kJ/g, or the molar heat of
solution (Hsol), expressed as the energy per mole of salt in kJ/mol.

Procedure
   1. Clean, rinse, and dry your styrofoam-cup calorimeter. Add 25.0 mL of deionized water to the
      calorimeter and record the initial water temperature.
   2. Obtain about 2.0 g of your assigned salt, accurately recording the amount. Add the salt to the
      calorimeter, replace the lid, and gently swirl. Record the maximum temperature of the water.

Data Table
                                           Initial            Final             Change in
  Volume of         Mass of water
                                        temperature        temperature        temperature,       Mass of salt (g)
  water (mL)             (g)
                                            (oC)              (oC)               T (oC)




Calculations
Given the density of water (1.0 g/mL) and the specific heat capacity of water (4.16 J/goC), find the specific heat
of solution (hsol) and the molar heat of solution for the salt (Hsol).

       hsol = (mass of water heated)(specific heat capacity of water)(temperature change)/(mass of salt)

     Hsol = (mass of water heated)(specific heat capacity of water)(temperature change)/(moles of salt)

Questions
   1. Explain what the value of the specific heat of solution for the salt indicates about the dissolution
       process. What does the value of the molar heat of solution indicate?



   2. If 100 g of the salt were dissolved in water, how much energy would be released?



   3. If 25.0 moles of the salt were dissolved in water, would the temperature change always be the same?
      Explain your answer.
                 Heat of Solution


                    vapourized
                    solute
                    particles


                                  exothermic
                                  energy is
                                  released from
                 endothermic      the system
Energy
                 energy must
                 be put into
                                            dissolved
                 the system                 solute
                                            particles




         solid                   heat of solution
         solut
         e

				
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posted:10/16/2010
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