calorimetry

• Thermochemistry


  The study of the energy changes that accompany physical or chemical changes in matter
• Thermal energy


  Energy available from a substance as a result of the motion of its molecules
• Chemical system


  A set of reactants and products under study, usually represented by a chemical equation
• Surroundings


  All matter around the system that is capable of absorbing or releasing thermal energy
• Heat


  Amount of energy transferred between substances
• Exothermic


  Releasing thermal energy as heat flows out of the system
• Endothermic


  Absorbing thermal energy as heat flows into the system
• Temperature


  Average kinetic energy of the particles in a sample of matter
• Open system


  One in which both matter and energy can move in or out
• Isolated system


  An ideal system in which neither matter nor energy can move in or out
• Closed system


  One in which energy can move in or out, but not matter
• Calorimetry


  The technological process of measuring energy changes in a chemical system
• Specific heat capacity


  The quantity of heat required to raise the temperature of a unit mass (e.g., one gram) of a substance by one degree Celsius or one kelvin
• Specific heat capacities of substances

  • ice: 2.01 J/(g•°C)
  • water: 4.18 J/(g•°C)
  • steam: 2.01 J/(g•°C)
  • aluminum: 0.900 J/(g•°C)
  • iron: 0.444 J/(g•°C)
  • methanol: 2.918 J/(g•°C)
• Calculating quantity of heat


  q = mcΔT
• c


  Specific heat capacity
• Calculating final temperature


  1. q = mcT

  2. T = q/mc
• The final temperature of the methanol is 28.6°C
• Enthalpy change (ΔH)


  The difference in enthalpies of reactants and products during a change
• ΔHsystem = qsurroundings
• Physical changes


  • Energy is used to overcome or allow intermolecular forces to act
  • Fundamental particles remain unchanged at the molecular level
  • Temperature remains constant during changes of state
  • Temperature changes during dissolving of pure solutes
  • Typical enthalpy changes are in the range ΔH ≈ 100 - 102 kJ/mol
• Chemical changes


  • Energy changes overcome the electronic structure and chemical bonds within the particles (atoms or ions)
  • New substances with new chemical bonding are formed
  • Typical enthalpy changes are in the range ΔH ≈ 102 - 104 kJ/mol
• Nuclear changes


  • Energy changes overcome the forces between protons and neutrons in nuclei
  • New atoms, with different numbers of protons or neutrons, are formed
  • Typical enthalpy changes are in the range ΔH ≈ 1010 - 1012 kJ/mol
• The magnitude of the energy change in nuclear changes is a consequence of Einstein's equation E = mc^2
• Physical changes, chemical changes, and nuclear changes have increasingly larger enthalpy changes
• Hsystem and qsurroundings are different in that Hsystem is the change in chemical potential energy of the system, while qsurroundings is the heat transferred to or from the surroundings. They are similar in that they are both measures of the energy change in the overall system.
• Moles


  Numbers of particles represented by chemical symbols in an equation
• Thermochemical equation


  1 mol H2(g) + 1/2 mol O2(g) → 1 mol H2O(g) + 241.8 kJ
• Molar enthalpy


  The enthalpy change per mole of a substance undergoing a change
• Molar enthalpy symbol


  ΔHx, where x is a letter or combination of letters indicating the type of change
• Molar enthalpy of combustion of hydrogen is -241.8 kJ/mol
• Enthalpy changes


  • Can be endothermic or exothermic
  • Negative sign for exothermic, positive sign for endothermic
• Types of molar enthalpy

  • Solution (ΔHsol)
  • Combustion (ΔHcomb)
  • Vaporization (ΔHvap)
  • Freezing (ΔHfr)
  • Neutralization (ΔHneut)
  • Formation (ΔHf)
• Molar enthalpy values are obtained empirically and listed in reference books
• Assumptions in calorimetry


  • No heat transfer between calorimeter and environment
  • Heat absorbed/released by calorimeter materials is negligible
  • Dilute aqueous solution has density and specific heat capacity of pure water
• Calorimetry experiment to find molar enthalpy of solution


  1. Dissolve 7.46 g KCl in 100.0 g water, measure temperature change

  2. Use formula ΔHsol = (m*c*ΔT)/n to calculate molar enthalpy of solution

  3. ΔHsol(KCl) = 17 kJ/mol
• nHsol


  Molar enthalpy of solution
• mcT


  Mass of solvent x Specific heat capacity of solvent x Temperature change
• Assuming the dilute solution has the same physical properties as pure water
• Finding molar enthalpy of solution

  1. Rearrange equation to isolate quantity to solve for

  2. Substitute given information and appropriate constants

  3. Consider mass of water in solution