Writing “Thermochemical” EquationsA thermochemical equation is a chemical equation that includes the value of ΔH Example 1: Burning one mole of wax releases 20,000 kJ of heat energy This could be written as: C40H82+ 60.5 O2⎯→40 CO2+ 41 H2O + 20,000 kJ Instead we usually write: C40H82+ 60.5 O2⎯→40 CO2+ 41 H2O ΔH = –20,000 kJ Why is ΔH negative? Recall that, by definition, ΔH = Hproducts– HreactantsIn example 1: Hproducts< Hreactants, therefore ΔH is negative Exothermic Endothermic ΔH (+, –) –(energy decreased/released) + (energy increased/absorbed) Movement of Heat From system, to surroundings From surroundings, to system “Enthalpy diagram” Hinitial(C40H82+60.5 O2) Hfinal(40CO2+ 41H2O) HfinalHinitialNotice in the above equation that the chemicals must be thought of as moles and not atoms/molecules. This is because a mole of molecules will absorb/release far more energy than a single molecule. In fact, if twice
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