Hesss Law

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Hesss Law
General Chemistry 101/102Laboratory
ManualUniversity of North Carolina at Wilmington
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Hesss Law

• Purpose

• To use Hesss Law to determine the enthalpy
  change (DH) for a chemical reaction that cannot
  be easily measured by normal experimental
  methods.

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Hesss Law

• Background

• When a chemical reaction is carried out in
  aqueous solution, the heat lost or gained by the
  reaction is equal to the heat gained or lost by
  the solution.

(1) qsolution - qreaction
The minus sign is necessary since heat loss is an
exothermic process (q is negative) while heat
gain is an endothermic process (q is positive).
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Hesss Law

• Background

• The enthalpy change (DH) for a chemical reaction
  taking place in aqueous solution can be
  calculated from the following relationship

specific heat capacity of solution (4.18 J/gºC)
mass of solution (in grams)
temperature change (in ºC)
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Hesss Law
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Hesss Law

• Procedure

• Weigh 0.90 g of CaCO3 into a coffee cup
  calorimeter. Add 5.0 mL of water to the
  calorimeter and stir the mixture.
• Place a temperature probe in the calorimeter and
  start measuring the temperature.
• After 1 minute, pour 20.0 mL of HCl(aq) into the
  calorimeter. Stir the solution and continue
  measuringthe temperature until it finally begins
  to decrease.
• Clean and dry the calorimeter and repeat this
  procedure using 0.60 g of Ca(OH)2.

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Hesss Law

• Procedure

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power connector and serial/USB connector
USB connector on computer
temperature probe connected to the Microlab unit
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graph area
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data table
live data