SCH4U - Chemistry 12 (2024-25) - A

SCH4U-25A

3-3: Summary

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  • Reactions can occur in one step from reactants to products or in multiple steps.
  • Each step in a multiple step process is called an elementary step.
  • Hess's Law states that the change in enthalpy is the sum of the enthalpy changes of each elementary step. 
  • The change in enthalpy of a chemical reaction is the same no matter which path is taken to get from reactants to products.
  • The Standard Enthalpy of Formation (ΔH°f ) of a chemical reaction is the enthalpy that produces one mole of a substance from its elements in their standard states.
  • We can use the values from a table of standard enthalpies of formation to calculate the change in enthalpy for a reaction by piecing the net reaction together in steps.
  • the enthalpy change of any chemical reaction is equal to the sum of the enthalpies of formation of the products minus the sum of the enthalpies of formation of the reactants. In mathematical terms, ΔH°reaction  =  ∑npΔH°f,p   −   ∑ nrΔH°f,r  where np and nr are the number of moles of products and reactants, respectively (even if they are just 1 mol), and ΔHf,p and ΔHf,r are the enthalpies of formation of the product and reactant species, respectively.