12.2 Heat and Enthalpy changes

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Presentation transcript:

12.2 Heat and Enthalpy changes

Enthalply Def: (H) heat content of a system at constant pressure Is essentially energy with a small term added that takes pressure and volume into account

Enthalply Change Symbolized as ΔH. Δ is the Greek symbol for “change.” ΔH = H products – H reactants The sign ΔH in an endothermic reaction is positive (+) and for an exothermic reaction is negative (-).

Enthalply Change This calculation does depend on conditions such as temperature, pressure and states. As a result, the standard for comparison is at 1 atmosphere (atm) of pressure and at 25 °C. The substance must also be in its “standard” or purist form at 1 atm For example, carbon at 1 atm can exists as diamond or graphite, but graphite is more stable.

Standard Enthalply Change Is symbolized by ΔH°. All reactants and products should be in their standard states. Examples C3H8 + 5 O2  3 CO2 + 4 H2O where ΔH° = 2043 kJ C + H2O  CO + H2 where ΔH° = +113 kJ

Using Enthalply Changes See sample problem on pg. 386 Complete practice problems 1 and 2 on page 386.