Enthalpy change

• Enthalpy change is the amount of heat energy taken in or given out during any change in a system provided pressure is constant
• If an enthalpy change occurs then energy is transferred between system and surroundings the system is the chemicals and the surroundings is everything outside the chemicals
• In exothermic change energy is transferred from the system (chemicals) to the surroundings. The products have less energy than the reactants

  
• In an exothermic reaction the enthalpy change is negative
• Common oxidation exothermic processes are the combustion of fuels and the oxidation of carbohydrates such as glucose in respiration
• In an endothermic change energy is transferred from the surroundings to the system (chemicals) they require an input of heat energy e.g thermal decomposition of calcium carbonate
• In an endothermic change the products have more energy then the reactants

  
• In an endothermic reaction the enthalpy change is positive
• What is the standard enthalpy change of formation

  The standard enthalpy change of formation of a compound is the enthalpy change when 1 mole of the compound is formed from its elements under standard conditions (298K and 100kpa) all reactants and products being in their standard states
• Examples of enthalpy change of formation

  Mg(s) + Cl2(g) -> MgCl2(s)

  2Fe(s) + 1.5O2(g) -> Fe2O3(s)
• The enthalpy of formation of an element = 0KJmol-¹
• What is the standard enthalpy of combustion

  The standard enthalpy of combustion of a substance is defined as the enthalpy change that occurs when one mole of a substance is combusted completely in oxygen under standard conditions (298K and 100kpa) all reactants and products being in their standard states
• CH4(g) + 2O2(g) -> Co2(g) + H2O(l)

  Incomplete combustion will lead to soot (carbon), carbon monoxide and water. it will be less exothermic than complete combustion
• When an enthalpy change is measured at standard conditions the symbol is used (circle with a line through it)
• Enthalpy changes are normally quoted at standard conditions which are

  • 100kPa pressure - gas
  • 298K (room temp or 25ºC)
  • Solutions at 1mol dm-³ - solutions
  • All substances should have their normal state at 298K