enthalpy changes

• Standard state

  The physical state of a substance under standard conditions of 100 kPa and a stated temperature (usually 298 K).
• Surroundings

  Everything that is not the chemical system.
• System

  Everything - the atoms, molecules, or ions - making up the chemical compounds being studied.
• Activation energy E

  The minimum energy required to start a reaction by the breaking of bonds
• Endothermic reaction

  A reaction in which the enthalpy of the products is greater than the enthalpy of the reactants, resulting in heat being taken in from the surroundings (ΔH is positive)
• Enthalpy

  The heat content that is stored in a chemical system
• Enthalpy change, ΔH

  Difference in enthalpy between reactants and products in a reaction
• Enthalpy profile diagram

  A diagram for a reaction to compare the enthalpy of the reactants with the enthalpy of the products
• Exothermic reaction

  A reaction in which the enthalpy of the products is smaller than the enthalpy of the reactants, resulting in heat loss to the surroundings (ΔH is negative)
• Endothermic reaction

  A reaction in which the enthalpy of the products is greater than the enthalpy of the reactants, resulting in heat being taken in from the surroundings (ΔH is positive)
• bond enthalpy = bond breaking - bond making
• ΔH = H(products) - H(reactants)
• Law of conservation of energy states energy cannot be created or destroyed
• Standard pressure is 100kPa
• Standard temperature is 298K (25°C)
• Standard concentration is 1 mol dm^-3