Enthalpy Changes

• An exothermic reaction occurs when

  • energy is released from the system into the surroundings, Heat EXITs (leaves the system)
  • products are lower in energy than the reactants (because energy has been released)
• An endothermic reaction occurs when

  • energy is absorbed by the system from the surroundings, Heat ENTERS (goes into the system)
  • products are higher in energy than the reactants
• Delta H is negative for exothermic reactions (since products are lower energy than reactants)
• Delta H is positive for endothermic reactions (since products are higher energy than reactants)
• Delta H is the difference between the energies of the products and reactants
• Activation Energy, is the energy necessary to start the reaction, used to break bonds or intermolecular forces in the reactants
• Catalysts are added to reactions to increase the reaction rate. They are not consumed in the reaction. Some catalysts provide an alternate pathway with lower activation energies. Delta H is NOT changed by a catalyst
• Combustions are exothermic reactions
• All decompositions are endothermic reactions
• Photosynthesis is an endothermic reaction
• Standard Conditions:

  • 101 kPa or 1 atm
  • 298 K or 25 C
• Strong Acids:

  • HCl
  • HBr
  • HI
  • HNO3
  • H2SO4
• Strong Bases:

  • NaOH
  • KOH
  • Ca(OH)2
  • Ba(OH)2
  • Sr(OH)2
• Bond breaking is endothermic
• Bond forming is exothermic
• If more energy is required to break bonds than energy is released when new bonds are formed, the reaction is endothermic
• If more energy is released when new bonds are formed than energy is required to break bonds, the reaction is exothermic.
• The amount of energy required to break one mole of a specific covalent bond in the gas phase is called the bond dissociation energy.
• Neutralizations or acid-base interactions are exothermic
• Enthalpy Change - the energy exchange between a chemical reaction and its surroundings at constant pressure is called the enthalpy change
• Enthalpy - the total energy associated with the materials that react
• Activation Energy - the minimum energy that colliding particles must possess for a reaction to happen
• Standard enthalpy change of reaction - the enthalpy change when the amount of reactants shown in the stoichiometric equation react to give products under standard conditions
• Standard enthalpy change of formation - the enthalpy change when one mole of a compound is formed from its elements under standard conditions
• Standard enthalpy change of combustion - enthalpy change when one mole of a substance is burnt in excess oxygen under standard conditions
• Standard enthalpy change of neutralization - enthalpy change when one mole of water is formed by the reaction of an acid with an alkali under standard conditions
• Specific heat capacity - the energy required to raise the temperature of 1 g of a substance by 1 degree C (1 K)

  • Specific heat capacity of water is 4.18 J g-1 C-1
• Hess's law - the states of the reactants and products must also be the same whichever route is followed