Energetics

• What are the Standard Conditions?

  • 100kPa
  • 298K
• What is Endothermic?


  Energy absorbed from surroundings
• What is Exothermic?


  Energy released to the surroundings
• What is Mean Bond Enthalpy?

  The energy required to break a covalent bond in a molecule over a range of compounds
• What is Enthalpy Change?

  The heat energy change at constant pressure
• What is Calorimetry?


  A process that measures the amount of energy absorbed/released during a chemical reaction
• What is the equation for calorimetry?


  q=mc∆T
• Name the units of q=mc∆T

  q -> heat energy - J

  m -> mass of water/substance - g

  c -> specific heat capacity - 4.18Jg-1k-1

  ∆T -> change in temperature - K
• State Hess' Law.

  Enthalpy change of reaction is independent of the route taken
• Why is a polystyrene cup used in colorimetry?

  To prevent heat loss
• Define Enthalpy of Formation

  When 1mol of a compound is formed from its elements under standard states and conditions
• Define Enthalpy of Combustion

  When 1mol of a compound is completely burnt in oxygen under standard states and conditions
• Which is positive/negative? Exo/Endo?

  • Endothermic = positive
  • Exothermic = negative
• Which way does the arrows go for formation data?

  UP
• Which way does the arrows go for combustion data?

  DOWN
• How do you calculate enthalpy change with mean bonds?

  bonds broken - bonds formed
• Explain why the value given for the O=O bond enthalpy is not a mean value.

  Oxygen is the only substance with O=O

  -> not ranged over many compounds
• Design a table to record all the readings necessary to determine an experimental value for the enthalpy of combustion for heptane in this experiment

  a

  
• The student considered using a glass beaker on a tripod and gauze instead of the clamped copper calorimeter.

  Suggest two disadvantages of using a glass beaker on a tripod and gauze

  • Tripod and gauze will reduce heat transfer
  • Glass is a poorer conductor than copper
• reasons why the value of enthalpy of combustion from this experiment is less exothermic than a data book value

  • Heat loss to surroundings
  • Incomplete combustion
• What is another way to calculate q?

  q = -ΔH x mols
• How do you calculate enthalpy?

  enthalpy = q / moles