2.1 thermochemistry

• Exothermic


  Heat is produced
• Fuel contains chemical energy in the form of potential energy
• Enthalpy, H


  The heat energy exchanged between a system and its surroundings during a chemical reaction
• Enthalpy change, ΔH, can be measured
• For an exothermic reaction

  The enthalpy of the reactants is greater than the enthalpy of the products, so ΔH is negative
• For an endothermic reaction

  The enthalpy of the products is greater than the enthalpy of the reactants, so ΔH is positive
• Standard conditions


  • All substances in their standard states
  • Temperature of 298K (25°C)
  • Pressure of 1 atmosphere (101,000 Pa)
• Standard enthalpy change of formation, ΔHf°


  Enthalpy change when one mole of a substance is formed from its elements in their standard states under standard conditions
• Standard enthalpy change of combustion, ΔHc°


  Enthalpy change when one mole of a substance is completely combusted (burnt) in oxygen under standard conditions
• Enthalpy change of reaction, ΔH

  Enthalpy change in any reaction between the number of moles of reactants shown in the equation for the reaction
• The standard enthalpy change for a chemical reaction is calculated from the standard enthalpies of formation of all the reactants and products involved
• Hess's Law


  The total enthalpy change of a reaction is independent of the route taken from the reactants to the products
• Bond enthalpy


  The amount of energy needed to break a covalent bond during a chemical reaction
• Average bond enthalpy


  The average value of the enthalpy required to break a given type of covalent bond in the molecules of a gaseous species
• Breaking bonds requires energy (endothermic, positive), making bonds releases energy (exothermic, negative)
• Heat transferred, q


  q = mcΔT, where m is mass, c is specific heat capacity, and ΔT is change in temperature
• Assumptions: all heat is exchanged with just the solution, solution has same specific heat capacity as water (4.18 J/g°C), density of solution is 1 g/cm³
• The value measured will always be less than the book value due to energy losses to the surroundings