4. Energetics

    Cards (31)

    • The enthalpy relates to the energy of the bonds broken and made during a chemical reaction
    • Enthalpy changes can be measured and calculated but enthalpy cannot
    • For an exothermic reaction ∆H is negative; for an endothermic reaction ∆H is positive
    • The enthalpy change is the heat energy change at constant pressure
    • Standard enthalpy values are the ΔH values for enthalpy changes of specific reactions measured under standard conditions
    • Standard conditions are 100kPa pressure and a stated temperature
    • Standard enthalpy of reaction (ΔrH ) is the enthalpy change when substances react under standard conditions in quantities given by the equation for the reaction
    • Standard enthalpy of formation (ΔfH ) is the enthalpy change when 1 mole of a compound is formed from its constituent elements with all reactants and products in standard states under standard conditions
    • Standard enthalpy of combustion (ΔcH ) is the enthalpy change when 1 mole of a substance is burned completely in excess oxygen with all reactants and products in their standard states under standard conditions
    • q = change in heat energy in joules
      m = mass in grams of the substance to which the temperature change occurs
      c = specific heat capacity
      ΔT = temperature change in ºC or kelvin (K).
    • Energetics is the study of energy transfers between reacting chemicals and their surroundings
    • Thermochemistry is the study of energy transfers in chemical reactions
    • Bond breaking requires energy & bond making releases energy
    • System: the chemical reaction
    • Surroundings: everything around or outside the chemical reaction
    • Boundary: separates the system from the surroundings
    • fill in the blanks
      A) Boundary
      B) System
      C) Surroundings
    • Dissipated: energy irreversibly lost by the system
    • Enthalpy: the energy content of a system under constant pressure
    • Enthalpy: the energy content of a system under constant pressure
    • Calorimetry is used to physically measure changes in enthalpy
    • Bomb calorimetry:
      • A sample of a compound in a sealed vessel is burned and measured the temperature change
      • Often the calorimeter will just determine the temperature change in the vessel and you will have to calculate the enthalpy change of combustion
      • Bomb calorimetry can be inaccurate due to:
      • Heat lost to the surroundings.
      • Any incomplete combustion that may take place.
      • Loss of some reactant that evaporates before it combusts.
    • Bond enthalpy is the enthalpy change when one mole of bonds is broken in the gas phase
    • There are two main reasons mean bond enthalpies are inaccurate:
      • Mean bond enthalpies are only valid in the gas phase
      • Bond enthalpies depend on the particular molecule, whereas mean bond enthalpies are quoted generally
    • Hess's law:
      The enthalpy change in a reaction is independent of the route taken and depends only on the initial and final states
    • Mean bond enthalpy is the heat change needed to break a covalent bond. It is a mean over different molecules
    • mean bond enthalpy = Σ(bonds broken) − Σ(bonds formed)
    • bond enthalpies are mean values from a range of compounds
    • Water has a known density so therefore, a volume of 50.0 cm3 could be measured out instead of a mass from a balance
    • Hess's law is that enthalpy change is independent of the route taken