Energetics

• Chemical energetics


  The study of energy flow in chemical reactions
• Bond energy


  The energy required to break 1 mole of bonds in gaseous covalent molecules under standard conditions
• Reactions can be exothermic or endothermic
• Bond breaking is endothermic and bond making is exothermic
• Breaking bonds


  Energy is required from the surroundings
• Making new bonds


  Energy is released from the reaction to the surroundings
• Transition state


  The point at which all the bonds have been broken and so it is the maximum energy state of the molecule
• Enthalpy change


  Changes in chemical energy that take place during a chemical reaction
• Standard enthalpy changes


  Enthalpy changes that occur under standard conditions of pressure and temperature
• Calorimetry


  The measurement of enthalpy changes in chemical reactions
• Activation energy


  The input of energy required to break all the bonds, putting the molecule in the maximum energy state
• Bonds have to be broken before they can recombine to form products
• Find out more about energy level diagrams
• Enthalpy change


  The heat change that takes place during chemical reactions at constant pressure
• Standard enthalpy changes


  Occur under the standard conditions of a pressure of 100 kPa and a temperature of 298 K (25 °C), with each substance involved in the reaction in its normal physical state (solid, liquid or gas)
• Standard enthalpy changes


  • Enthalpy of formation
  • Enthalpy of combustion
• To calculate the heat transferred in a reaction you need to know three values: the mass of the reactants, the specific heat capacity of the reactants, and the temperature change
• A simple calorimeter can be made from a polystyrene drinking cup, a vacuum flask or a metal can
• Hess' Law


  An application of the first law of thermodynamics: energy can not be created or destroyed. In the case of chemical reactions, the total energy change is the same whichever route is taken.
• You can use Hess' Law to solve energetics problems, by applying a Hess' cycle to given experimental data. This allows you to find the energy change for a reaction that cannot be measured directly.
• Calorimetry is a technique used to measure changes in the enthalpy of chemical reactions, but it doesn't explain why energy is absorbed or released. Models and theories are used to explain energy changes on the basis of bonds breaking and being formed.
• Bond energies are affected by other atoms in the molecule, so average bond enthalpies are listed in data tables
• Keyword definitions to know for energetics

  • Enthalpy change
  • Standard molar enthalpy of formation
  • Standard molar enthalpy of combustion