Lesson 4: Calculating Heat Change

• Heat change

  The amount of heat released or absorbed by either a phase or chemical change

  
• Factors affecting heat change

  • Moles of substance (n)
  • Molar enthalpy of the phase or chemical change (∆H)
• Calculating heat change

  Q = n∆H

  
• The more mass an object contains, the more energy it can absorb or release
• Q universe = Q system + Q surroundings
• Calorimetry

  The science of determining the enthalpy change of a system

  
• Calorimeter


  • A device in which the heat associated with a specific process is measured

  
• Types of calorimeter

  • Constant-pressure calorimeter: also known as simple calorimeters
  • Constant-volume calorimeter: also known as bomb calorimeters
• Constant-pressure calorimeter

  Used to measure the heat absorbed or released by a process or reaction taking place at constant pressure

  e.g. The heat of acid-base neutralizations or the heat associated with dissolving a salt in water can be measured.

  can be constructed using styrofoam cups (coffee-cup calorimeter)
• Standard enthalpy formation values

  
• A coffee-cup calorimeter is considered to be an isolated system
• For an exothermic process

  The heat released will be absorbed by the water in which the reaction took place and by the calorimeter

  
• For an endothermic process

  Heat will be absorbed from the water and from the calorimeter

  
• Qsystem = -Qsurroundings
• Calculating heat of reaction in a simple calorimeter

  1. Qreaction = -(Qcalorimeter+ QH2O)

  2. Qreaction = -QH2O (because Qcalorimeter is very small)

  
• bomb calorimeters try to create an isolated system, so all heat exchange will occur between the bomb and the water, so no heat is transferred outside of the container