Energetics

Cards (27)

  • Enthalpy change is the heat energy measured under conditions of constant pressure
  • The enthalpy change of formation of a compound is the enthalpy change when one mole of a compound is formed from its constituent elements with all products and reactants in their standard states and under standard conditions
  • Enthalpy change of combustion is the enthalpy change when one mole of a substance is burned completely in excess oxygen with all reactants and products in their standard states and under standard conditions
  • Enthalpy change of reaction is the enthalpy change when substances react under standard conditions in quantities given by the equation for the reaction
  • enthalpy of neutralisation is the enthalpy change when one mole of water is formed from its ions in dilute solution. When an acid neutralises an alkali
  • Bond enthalpy is the energy required to break one mole of a covalent bond measured in the gaseous state
  • Mean bond enthalpy is the energy required to break one mole of a covalent bond measured in the gaseous state averaged across many compounds containing the bond
  • Hess's law is that the enthalpy change of a reaction is independent of the route taken
  • In an exothermic reaction, the energy is given out to the surroundings. So the surrounding increase in temperature. The reactants have more energy than the products. Therefore the ΔH is negative.
  • In an endothermic reaction, the energy is taken in to the surroundings. So the surrounding decrease in temperature. The products have more energy than the reactants. Therefore the ΔH is positive.
  • Exothermic reactions include:
    • Combustion of fuels
    • Respiration
    • detonation of explosives
    • Reaction of acids with metals
    • Neutralisation reactions
    • Condensing
    • Freezing
  • Bond forming exothermic
  • Bond breaking is endothermic
  • Endothermic reactions include:
    • Photosynthesis
    • Thermal decomposition of calcium carbonate
    • Dissolving salts in water
    • Boiling
  • Activation energy is the energy required to start a reaction by the breaking of bonds
  • Standard conditions are:
    • 101 Kpa (pressure)
    • 298K (temp)
    • 1 mol dm-3 (conc for solutions)
  • The more negative the standard enthalpy of formation, the more stable the compound
  • Elements in their standard states have a 0 enthalpy of formation
  • Only one mole of a substance is burnt on the left side of the equation
  • The thermal chemical equation for the enthalpy of neutralisation is always:
    H+ + OH- -> H2O
  • How to calculate heat energy change:
    q=mcΔT
  • How to calculate enthalpy change:
    q/moles
  • If the temperature rises, the ΔH is negative
  • If the temperature falls, the ΔH is positive
  • The calculated enthalpy change may be different to the data book because of:
    • Heat loss to environment
    • Incomplete combustion
    • Non- standard conditions
    • Evaporation of alcohol/water
    • Heat capacity of calorimeter/ thermometer not take into account
  • How to improve calorimeter:
    Use a bomb calorimeter, which reduces heat loss as the water is insulated from the surrounding. It is also burnt in 100% oxygen to ensure complete combustion
  • To determine an exact temperature which is not influenced by heat from the surroundings:
    • record temperature of the solution every minute for 4 minutes
    • On the 5th minute, add the reactant
    • Record the temperature every subsequent minute for another 5 to 1 minutes
    • Plot a graph
    • Extrapolate to time of mixing