Enthalpy Changes

Cards (28)

  • 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