When comparing enthalpy changes for formation reactions of different compounds, we must choose a reference energy state. It is convenient to set the enthalpies.

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Presentation transcript:

When comparing enthalpy changes for formation reactions of different compounds, we must choose a reference energy state. It is convenient to set the enthalpies of elements in their most stable form at SATP to be zero. As an arbitrary convention, for the sake of simplicity, all other enthalpies of compounds are measured relative to that reference energy state. A formation reaction always begins with elements, so any standard enthalpy of formation reactions are measured from the reference energy state of zero.

Tin(IV) oxide has a greater thermal stability than tin(II) oxide. Thermal stability is the tendency of a compound to resist decomposition when heated. The lower (i.e. more negative) the value of a compound’s standard molar enthalpy of formation, the more stable it is. Δf Hm° = – 280.7 kJ/mol SnO Δf Hm° = – 577.6 kJ/mol SnO2 Tin(IV) oxide has a greater thermal stability than tin(II) oxide.

ΔrH° = ΣnΔfPHm° – ΣnΔfRHm° The standard enthalpy change of a reaction is the sum of the standard enthaplies of formation of the products minus the sum of the standard enthalpies of formation of the reactants. ΔrH° = ΣnΔfPHm° – ΣnΔfRHm°

ΔrH° = – 64.5 kJ – 985.2

= – 802.5 kJ

Homework: Read pgs. 510 – 513 pgs. 514 – 515 Section 11.5 Questions #’s 1 – 9