5.1 Measuring energy changes

  • Chemical Potential Energy: Energy stored in chemical bonds of reactants and products
  • Heat: Represented by symbol q is energy transferred from warmer body to cooler body. It is a form of energy
  • Kinetic Energy: energy that a body possesses by virtue of being in motion
  • Temperature: Measure of the average kinetic energy of the particles

Chemical Energy

  • When examining energy changes involved in a chemical reaction, we divide the universe into parts:
    • System: The focus, which is the chemical reaction
      • Open System: Transfer of matter and energy is possible across its boundary
      • Closed System: Transfer of matter and is not possible across the boundary
      • Isolated System: Matter can neither enter or exit the system
    • Surroundings: The apparatus in which the reaction is taking place in
  • When a chemical reaction takes place, there are bonds broken and bonds made
    • Energy required to break bond is called bond disassociation energy and is an endothermic process
    • Energy released when new bond is created is an exothermic process

Enthalpy and thermochemistry

  • Enthalpy is an example of a state function. A state function is measurement in which you only record the initial and final, ignoring any changes which occur throughout a measurement
  • Thermochemistry: is the study of heat changes that occur during chemical reactions
    • At constant pressure, the change in enthalpy ΔH (measured in kJ mol-1) is defined as heat transferred by closed system during chemical reactions
  • Calorimeter: Any apparatus used to measure the amount of heat being exchanged with the surroundings

Exothermic and Endothermic reactions

  • Exothermic Reaction: Heat is transferred from system to surroundings, ΔH is negative.
  • Endothermic Reaction: Heat is absorbed from surroundings, ΔH is positive.


  • q=mcΔT is a relationship used to calculate the heat of a group
  • Standard Enthalpy Change of Reaction
    • Change in enthalpy measured at 25°C and pressure of 100 kPa. Denoted by ΔHѳ
  • Enthalpy Change of Formation
    • The energy change upon formation of 1 mol of a substance. Denoted by ΔHѳf
  • Enthalpy Change of Combustion
    • Heat evolved upon the complete combustion of 1 mol of a substance. Denoted by ΔHѳc

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