THERMOCHEMISTRY

• Exothermic reaction


  Reaction that releases heat to the environment
• Endothermic reaction


  Reaction that absorbs heat from the environment
• Enthalpy (H)


  Thermodynamic term used to describe heat changes taking place at constant pressure
• Enthalpy of reaction (ΔHrxn)

  Difference in enthalpy of the products and reactants
• If reaction is endothermic


  ΔHrxn > 0
• If reaction is exothermic

  ΔHrxn < 0
• Enthalpy can be applied to physical/phase change and chemical change processes
• Physical change example


  • Conversion of ice to liquid water (ΔH = 6.01 kJ/mol)
• Chemical change example


  • Combustion of butane
• Thermochemical equations


  • Equations that show the enthalpy change and mass relationships of reactants and products
• Guidelines for writing thermochemical equations:

  1. Specify the correct state of each reactant and product

  2. Multiplying the reaction by a factor changes the ΔH by the same factor

  3. Reversing a reaction/process does not affect the magnitude of enthalpy but reverses its sign
• Solving a sample problem

  Given: 45 g of ice, find the heat required to melt it completely
• Enthalpy


  The heat energy associated with the conversion of H2O(l) to H2O(g)
• Multiplying the reaction by a certain factor also changes the ΔH by the same factor
• If the ice becomes 2 moles


  The associated enthalpy also doubles
• Reversing a reaction or process does not affect the magnitude of enthalpy but reverses its sign
• If the original reaction is endothermic (ΔHrxn > 0)


  The opposite process must be exothermic (ΔHrxn < 0)
• Solving for the heat required to melt 45 g of ice

  1. Melting a mole of ice has an associated enthalpy of 6.01 kJ/mol

  2. Use stoichiometry to compute ΔH
• Standard enthalpy of formation (ΔH°f)

  The enthalpy indicated is the standard enthalpy, with the reference point being the most stable form of an element at 1 atm pressure
• By convention, the ΔH°f of an element's most stable form is zero
• Standard enthalpy of reaction (ΔH°rxn)

  The enthalpy of a chemical reaction carried out at 1 atm pressure, calculated using the formula: ΔH°rxn = [cΔH°f(C) + dΔH°f(D)] - [aΔH°f(A) + bΔH°f(B)]
• Direct method for solving ΔH°rxn

  1. Applicable only for compounds that can be directly synthesized from their elemental compositions

  2. Calculate ΔH°rxn using the formula: ΔH°rxn = ∑pΔH°f(products) - ∑rΔH°f(reactants)
• Indirect method for solving ΔH°rxn

  1. Use Hess's law of heat summation

  2. Reverse one of the known reactions to obtain the desired reaction

  3. Add the ΔH°rxn of the two reactions algebraically