law 1

• Thermochemistry


  The study of energy or heat flow that accompanies a chemical reaction and/or physical transformation
• Heat


  A form of energy transfer between two objects as a result of their difference in temperature
• System


  The part of the universe being studied
• Surroundings


  Everything else in the universe
• Heat will always flow spontaneously from an area of higher temperature to an area of lower temperature
• Types of systems


  • Open system (can exchange both matter and energy with surroundings)
  • Closed system (can exchange energy but not matter with surroundings)
  • Isolated system (exchanges neither energy nor matter with surroundings)
• Exothermic


  Releases energy (q = negative)
• Endothermic


  Absorbs energy (q = positive)
• Exothermic reactions


  • Melting of ice
  • Reaction of powdered aluminum with iron(III) oxide
  • Photosynthesis
• Endothermic reactions


  • Melting of ice
  • Photosynthesis
• First law of thermodynamics

  Energy is neither created nor destroyed, it can only be transferred between the system and the surroundings
• The first law of thermodynamics states that in any process, the change in energy of a system is equal to the sum of the work done on the system and the heat absorbed by the system
• Sign convention for heat and work

  • System gains heat (+q)
  • System loses heat (-q)
  • Work done on the system (+W)
  • Work done by the system (-W)
  • Pressure-volume work (-PV)
• Units for change in energy

  • J (joules)
  • kJ (kilojoules)
  • 1 kJ = 1000 J
  • 1 calorie = 4.184 J
  • 1 Calorie = 1 kcal = 1000 calories
• Solving sample problem 1

  Given: q = 62 J, W = 48 J

  Calculate ΔU
• Solving sample problem 2

  Given: q = 62 J, W = 474 J

  Calculate ΔU in calories
• Solving sample problem 3

  Given: q = 188 cal, V = 0

  Determine sign of q and calculate ΔU in joules
• Solving sample problem 4

  Given: P = 8 atm, q = -140 J, ΔV

  Calculate ΔU
• Heat capacity


  The amount of energy required to raise the temperature of the sample by 1°
• Specific heat


  The energy required to raise 1 gram of a substance by 1°
• Calorimetry


  The flow of energy (as heat) between the system and its surroundings
• A calorimeter is an insulator apparatus that contains water or any liquid of known heat capacity
• Solving calorimetry sample problem 1

  Given: m = 550 g, ΔT = 70°

  Calculate q
• Solving calorimetry sample problem 2

  Given: m = 76 g, ΔT = -422°

  Calculate q
• Solving calorimetry sample problem 3

  Given: q = 3050 J, ΔT = 27°

  Calculate mass of mercury
• Solving calorimetry sample problem 4

  Given: m = 26.86 g, q = 418.6 J, ΔT = 39.9°

  Calculate specific heat
• Solving calorimetry sample problem 5

  Given: q = 1621 J, m = 38 g, ΔT = -23°

  Calculate final temperature
• Solving calorimetry sample problem 6

  Given: V = 250 mL, ΔT = -14°

  Calculate heat of solution and determine if process is endothermic or exothermic
• Thermochemical equation


  A chemical equation that shows the value and direction of heat involved in a reaction
• Enthalpy (H)


  The amount of heat absorbed or released by a chemical reaction at constant atmospheric pressure in the form of chemical energy
• Enthalpy of a reaction (ΔH)

  The difference between the enthalpy of the products and the reactants
• There is 890.4 kJ of heat released when one mole of methane and two moles of oxygen gas combust with one mole of gaseous carbon dioxide and two moles of liquid water
• Enthalpy is an extensive property


  The magnitude is proportional to the amount of reactants and products in the reaction
• Standard molar enthalpy of formation (ΔHf°)

  The change in enthalpy when the compound is formed from its elements under standard conditions (1 atm, 25°C)
• Standard enthalpy of a reaction (ΔH°)

  Can be calculated from the standard molar enthalpies of formation of the reactants and products
• Solving enthalpy sample problem 1

  Given: ΔHf(ethanol) = -277.7 kJ/mol

  Calculate ΔH° for the combustion of ethanol
• Solving enthalpy sample problem 2


  Given: ΔHf values for reactants and products

  Calculate ΔH° for the combustion of propane
• Hess' law


  The enthalpy of a sum of a series of reactions is equal to the sum of enthalpies of those two reactions
• Solving Hess' law sample problem 1

  Given: Thermochemical equations for two-step reaction

  Calculate ΔH° for the overall reaction
• Solving Hess' law sample problem 2

  Given: Thermochemical data

  Calculate ΔH° for the reaction