THERMOCHEMISTRY

• 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 the energy of the universe is constant
• Sign convention for heat and work

  Heat absorbed by system (+q)

  Heat released by system (-q)

  
  

  Work done on system (+W)

  Work done by system (-W)
• Calorimetry


  The flow of energy (as heat) between the system and its surroundings is measured
• Calorimetry


  Measure the flow of energy (as heat) between the system and its surroundings
• The specific heat of a substance is constant
• The specific heat values of common substances are shown in the table
• Thermochemical equation


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


  Amount of heat absorbed or released by a chemical reaction at constant atmospheric pressure in the form of chemical energy
• Enthalpy of a reaction (∆H)


  Difference between the enthalpy of the products and the reactants
• Endothermic process


  ∆H is positive
• Exothermic process


  ∆H is negative
• Enthalpy is an extensive property, the magnitude is proportional to the amount of reactants and products in the reaction
• Standard molar enthalpy of formation (∆H°f)

  Change in enthalpy when the compound is formed from its elements under standard conditions (1 atm, 25°C)
• Hess' law


  The enthalpy of a sum of a series of reactions is equal to the sum of enthalpies of those two reactions
• Enthalpy of a Chemical Reaction: Thermochemical Equations

  1. Compute for the ∆𝐻 of the reaction

  2. Reverse the 2nd equation since carbon monoxide is in the products side of the desired chemical equation

  3. Apply Hess's law to calculate the ∆𝐻 of the overall reaction from the ∆𝐻 of the two (manipulated) reactions leading to it
• Hess' law


  • The enthalpy of a sum of a series of reactions is equal to the sum of enthalpies of those two reactions
  • Enthalpy is an extensive property; it depends on the amount of reactants and products
  • The ∆𝐻 for a forward reaction is equal in magnitude but opposite in sign to its reverse reaction
  • The ∆𝐻 for a reaction is the same whether it occurs in one step or in a series of steps