Writing “Thermochemical” Equations A thermochemical equation is a chemical equation that includes the value of Δ H Example 1: Burning one mole of wax releases 20,000 kJ of heat energy This could be written as: C 40 H 82 + 60.5 O 2 ⎯→ 40 CO 2 + 41 H 2 O + 20,000 kJ Instead we usually write: C 40 H 82 + 60.5 O 2 ⎯→ 40 CO 2 + 41 H 2 O Δ H = – 20,000 kJ Why is Δ H negative? Recall that, by definition, Δ H = H products – H reactants In example 1: H products < H reactants , therefore Δ H is negative Exothermic Endothermic Δ H (+, – ) – (energy decreased/released) + (energy increased/absorbed) Movement of Heat From system, to surroundings From surroundings, to system “Enthalpy diagram” H initial (C 40 H 82 +60.5 O 2 ) H final (40CO 2 + 41H 2 O) H final H initial Notice 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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This note was uploaded on 04/12/2011 for the course CHEM 1010 taught by Professor Marshall during the Spring '11 term at North Texas.