Thermochemistry 5 Standard Heats of Formation. Standard heat of Formation Def’n: The enthalpy change ΔH o f for the formation of 1 mole of a substance.

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Presentation transcript:

Thermochemistry 5 Standard Heats of Formation

Standard heat of Formation Def’n: The enthalpy change ΔH o f for the formation of 1 mole of a substance in its standard state from its constituent elements in their standard states. For the most stable forms of elements in their std states, ΔH o f = 0 e.g. Carbon has many different forms, e.g. graphite, diamond, Buckminster fullerene… Graphite is the most stable form of carbon, ΔH o f (graphite) = 0 so we use this, not diamond (, ΔH o f (diamond) = 1.9 kJ/mol) (check out the carbon entry in appendix B, page A-10)

Data tables Most stable form of an element has ΔH o f = 0, so not given in tables More complete list given in appendix B

Finding ΔH o f The standard enthalpy change for any chemical reaction is found by subtracting the sum of the heats of formation from the reactants from the sum of the heats of formation of the products

Uh oh, Problem The first step in producing lead from its ore, galena (PbS) is heating lead sulfide in air Determine ΔH o f for this reaction from the standard heats of formation. Calculate ΔH o when 250 g of PbS reacts with a stoichiometric amount of oxygen Step 1: Determine the ΔH rxn using ΔH o f values from appendix B Step 2: knowing ΔH rxn for the reaction of 2 moles PbS, nest find ΔH rxn for 2.50 g of PbS. (first convert grams PbS into moles PbS)

Nitric Acid is produced industrially by the multi-step Oswald process: The overall process is Determine ΔH rxn for each step in the reaction (Appendix B) Determine ΔH for the overall process Step 1 Determine the ΔH rxn using ΔH o f values from appendix B Step 2 combine individual reactions so their sums are the desired reaction