Enthalpy of a Reaction

• Enthalpy (H)

  • "energy change"
  • quantifies the heat flow into or out of the system in a process the occurs at constant pressure
  • tells us about the "heat content" of a system
• Enthalpy of Reactions (ΔH_rxn)

  • heat transferred in a chemical reaction held at constant pressure
  • the difference between the enthalpies of the products and the enthalpies of the reactants
  • ΔH_rxn = H(products) - H(reactants)
  • ΔH > 0 (+): endothermic
  • ΔH < 0 (-): exothermic
• Enthalpy Changes unit: kJ/mol
• Interpreting Thermochemical Equations

  1.) Physical states of all the reactants and products must be specified.

  2.) Reversing a reaction would result in a change in sign but the magnitude of ΔH remains the same.

  3.) Multiplying both sides of the equation by a factor n, would also change ΔH by the same factor n.

  4.) The stoichiometric coefficients always refer to the number of moles of a substance.
• Standard Molar Enthalpy of Formation (ΔHof)

  Standard states:

  • natural state of a substance at 1 atm
  • an element in its standard state has an ΔHof = 0 kJ/mol
• Standard Molar Enthalpy of Formation (ΔHof)

  Formation Reaction:

  • a reaction where one mole of a substance is formed from elements in their standard states

  It is a formation reaction if it satisfied all of the following:

  • both reactants are in their standard states
  • only 1 mole of the product is produced
• Standard Enthalpy of Reaction (ΔHo_rxn)

  • enthalpy of a reaction carried out at 1 atm
  • ΔHo_rxn = ∑(nΔHof products) - ∑(nΔHof reactants)
• For calculating ΔHo_rxn:

  Direct Method

  • use when only one equation/reaction is given

  Indirect method

  • for multiple step reactions
• Hess Law:

  • the overall enthalpy change in converting reactants to products is the same, regardless if it took place in one step or multiple steps reaction
• Hess law:

  1. Balance the chemical reaction.
  2. Align the reactants and products in the same position as the one-step (main) process.
  3. Manipulate the coefficients align with the main process.
  4. Cancel like substances in reactants and products.
  5. Add the enthalpies.