Energetics

• 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