Enthalpy

• Enthalpy --> The heat content that is stored in a chemical system
• Activation energy --> The minimum energy required to start a reaction by breaking the bonds
• Enthalpy change of formation --> formation of 1 mole of a compound from its element
• Enthalpy change of combustion --> complete combustion of 1 mole of a substance
• Enthalpy change of neutralisation --> formation of 1 mole of water from neutralisation
• Standard conditions:

  • standard pressure is 100KPa. This is very close to a pressure of one atmosphere, 101KPa.
  • Standard temperature is a stated temperature, usually 298K (25 degrees Celsius)
  • standard concentration is 1 mol dm^-3
  • standard state is the physical state of a substance under standard conditions
• Hess's law --> The enthalpy change for a reaction is independent of the route taken
• The enthalpy change to go from A --> B direct is the same as going from A --> C --> B
• The sum of the clockwise arrows = the sum of the anticlockwise arrows in Hess's Law
• Hess's law with combustion data:

  • The enthalpy change when 1 mole of a substance is combusted completely
  • The arrow lead to the products of combustion
• • Energy required = mass x specific heat capacity x temperature change
  • q = m x c x delta T
  • energy change = energy used / number of moles of fuel
  • delta H = q / n
• Specific heat capacity, c --> the energy required to raise the temperature of 1 gram of a substance by 1 kelvin

  • good conductors of heat, such as metals, have small values of c
  • Insulators of heat such as foam plastic have large values of c
• Average bond enthalpy --> the energy required to break one mole of a specified type of bond in a gaseous molecule.

  • energy is required to break bonds
  • bond enthalpies are always endothermic
  • bond enthalpies always have a positive enthalpy value
• • Energy is required to break bonds, bond breaking is endothermic --> delta H is positive
  • Energy is released when bonds from bond making is exothermic --> delta H is negative
• System --> The atoms and bonds involved in the chemical reaction
• Law of conservation --> the amount of energy in an isolated system remains the same. Energy cannot be destroyed or created. It can only be transferred from one form to another.
• Enthalpy change of reaction --> the energy change associated with a given reaction
• Advantages of using a bomb calorimeter:

  • Minimises heat loss
  • Pure oxygen used --> ensures complete combustion
• Experimental methods for enthalpy determination not be accurate:

  • Heat is lost to the surroundings
  • not in standard conditions
  • reaction may not go to completion
• Inaccuracies in mean bond enthalpies:

  • mean bond enthalpies are only valid in gas phase. You might not being doing your reaction in the gas phase
  • Bond enthalpies depend on the particular molecule, whereas mean bond enthalpies are quoted generally.