Enthalpy The Heat Evolved in a Chemical Reaction at Constant Pressure

6 Enthalpy The Heat Evolved In a Chemical Reaction at Constant Pressure 

We define the enthalpy (H) of a system as the sum of its internal energy and the product of its pressure and volume  

Since internal energy, pressure, and volume are all state functions, enthalpy is also a state function. The change in enthalpy (AH) for any process occurring under constant pressure is given by the expression  

To better understand this expression, we can interpret the two terms on the right with the help of relationships already familiar to us. We saw previously that AE = g -E w. If we represent the heat at constant pressure as then the change in internal energy at constant pressure is AE = -E w. In addition, from our definition of pressure-volume work, we know that P AV = -w. Substituting these expressions into the expression for AH gives us the following expression  

Lighters are usually fueled by butane (C4H10). When 1 mole of butane bums at constant pressure, it produces 2658 kJ of heat and does 3 kJ of work. What are the values of AH and AE for the combustion of one mole of butane  

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