2.1 Thermochemistry

• Enthalpy change


  The amount of heat given out or absorbed in a reaction carried out at constant pressure
• Exothermic reactions


  • Have negative ΔH values
• Endothermic reactions


  • Have positive ΔH values
• Standard conditions


  All substances in their standard states, temperature of 298 K (25°C), pressure of 1 atm (101 000 Pa)
• Standard enthalpy change


  Enthalpy change measured when fixed standard conditions are used
• Enthalpy change of formation, ΔfHθ


  The enthalpy change when 1 mol of a compound forms from its constituent elements in their standard states under standard conditions
• Equation for enthalpy change of formation

  1. Must end up with 1 mol of the compound

  2. May need to use fractions on the left hand side
• The standard enthalpy change of formation of an element in its standard state is zero
• Enthalpy change of combustion, ΔcHθ

  The enthalpy change when 1 mol of a substance is completely combusted in oxygen under standard conditions
• Equation for enthalpy change of combustion

  1. Must start with 1 mol of whatever is being burned

  2. Often need to use fractions in the equation
• Enthalpy change of reaction, ΔrH


  The enthalpy change when the reaction takes place between the reagents as indicated by the stoichiometric equation for the reaction under standard conditions
• Hess's Law


  The total enthalpy change for a reaction is independent of the route taken from the reactants to the products
• Hess's Law equation


  1. Route 1 = Route 2

  2. ΔH1 = ΔH2 + ΔH3
• If ΔfH is given, the direction of the arrows is from the common elements to the reactants and products. Arrow = upwards
• If ΔcH is given, the direction of the arrows is from the reactants and products to the common combustion products. Arrows = downwards
• Bond enthalpy


  The enthalpy needed to break 1 mol of the bond to give separated atoms with everything being in the gaseous state
• 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
• Calculating enthalpy changes using bond enthalpies


  ΔH = Σ(bonds broken) - Σ(bonds formed)
• Calorimetry


  Measuring the heat transferred to the surroundings in a chemical reaction by carrying it out in an insulated container called a calorimeter
• Calculating enthalpy change from calorimetry

  1. q = mcΔT

  2. ΔH = -q/n