ENTHALPIES OF REACTION

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ENTHALPIES OF REACTION
Module 20
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ENTHALPIES OF REACTION

• INTRODUCTION
• Many chemical reactions generate energy, some
  absorb energy
• The quantity of energy generated or absorbed in a
  chemical reaction is the difference in the
  chemical energy content (the enthalpy) between
  reactants and products

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ENTHALPIES OF REACTION

• Enthalpy chemical energy content of a substance
  (H)
• The energy absorbed or released during a chemical
  reaction is the change in enthalpy (DH)
• DH (H of products) (H of reactants)

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ENTHALPIES OF REACTION

• Exothermic reaction energy is released to the
  surroundings.
• Endothermic reaction energy is absorbed from
  the surroundings.
• A flow of energy is referred to as heat, so the
  energy change during a reaction is the heat of
  the reaction

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ENTHALPIES OF REACTION

• If the DH is positive, the system (the reaction
  mixture) ends up with more energy than in the
  original reactants Endothermic energy has
  been absorbed
• If the DH is negative, the system ends up with
  less energy than was in the reactants
  Exothermic energy has been released

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ENTHALPIES OF REACTION

• Calorimetry
• If a reaction is carried out in an open insulated
  container, at constant pressure and volume, the
  enthalpy change will equal the energy absorbed or
  released by the system within that container,
• i.e. DH -CDT, where C the heat capacity
  of the system
• Therefore, DH is directly related to DT, and the
  temperature change caused by a reaction can be
  used to determine the DH
• Note DT final T initial T

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ENTHALPIES OF REACTION

• In practice no calorimeter (the insulated
  container) is perfectly insulated and it is
  itself part of the heat loss/gain
• To compensate, we set up a known process to
  measure the calorimeters role
• The measured heat gives us the Heat Capacity of
  the Calorimeter, Ccal, a constant for that
  particular calorimeter

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ENTHALPIES OF REACTION

• Calorimetry
• Doing 3 determinations
• 1) Heat capacity of the calorimeter
• 2) Enthalpy of an Acid-Base Neutralization
• 3) Enthalpy of a Solution Preparation

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ENTHALPIES OF REACTION

• Calorimetry Procedure To Determine the Ccal

larger thermometer
Stir, then measure the temperature change of the
mixture over time
stirring rod
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ENTHALPIES OF REACTION

• Calorimetry Ccal

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ENTHALPIES OF REACTION

• Calorimetry - Calculations
• 1) Heat capacity of
  calorimeter
• Heat lost by hot water Heat gained by cold
  water
• 
  Heat gained by calorimeter
• -(Mass of hot water(m1))(Specific heat of
  water(spH2O))(Temp. change of hot water (DTHOT))
  
• (Mass of cold water(m2))(Specific heat of
  water(spH2O))(Temp. change of cold water(DTCOLD))
• (Heat capacity of calorimeter
  (Ccal) )(Temp. change of calorimeter(DTCOLD))
• that is
• -(m1)(SpH2O)( DTHOT) (m2)(SpH2O)(DTCOLD)
  (CCAL)(DTCOLD)

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ENTHALPIES OF REACTION

• 2) Enthalpy of Acid-Base Neutralization

Acid Solution
Base Solution
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ENTHALPIES OF REACTION

• 2) Enthalpy of Acid-Base Neutralization
• Csoln (Mass of soln.)(Specific heat of
  soln.)
• qneut -(Csoln Ccal) DTneut

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ENTHALPIES OF REACTION

• 3) Enthalpy of Solution

Water, i.e. the solvent
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ENTHALPIES OF REACTION

• 3) Enthalpy of solution
• CH2O (Mass of water)(Specific Heat of water)
• qsol -(CH2O Ccal ) DTsol

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ENTHALPIES OF REACTION

• Calorimetry - Calculations
• Final calculation of Enthalpy (cal / mole)

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ENTHALPIES OF REACTION

• Thermometer Breakage Fee
• 10.00
• BE CAREFUL!!