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Enthalpy Ch 5.

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Presentation on theme: "Enthalpy Ch 5."— Presentation transcript:

1 Enthalpy Ch 5

2 Enthalpy If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work, we can account for heat flow during the process by measuring the enthalpy of the system. Enthalpy is the internal energy plus the product of pressure and volume: H = E + PV 2

3 Enthalpy When the system changes at constant pressure, the change in enthalpy, H, is H = (E + PV) This can be written H = E + PV 3

4 Enthalpy Since E = q + w and w = −PV, we can substitute these into the enthalpy expression: H = E + PV H = (q+w) − w H = q So, at constant pressure the change in enthalpy is the heat gained or lost. 4

5 Endothermicity and Exothermicity
A process is endothermic, then, when H is positive. 5

6 Endothermicity and Exothermicity
A process is endothermic when H is positive. A process is exothermic when H is negative. 6

7 Enthalpies of Reaction
The change in enthalpy, H, is the enthalpy of the products minus the enthalpy of the reactants: H = Hproducts − Hreactants 7

8 Enthalpies of Reaction
This quantity, H, is called the enthalpy of reaction, or the heat of reaction. 8

9 The Truth about Enthalpy
H for a reaction in the forward direction is equal in size, but opposite in sign, to H for the reverse reaction. H for a reaction depends on the state of the products and the state of the reactants. 9

10 Potential Energy Diagrams
Show the difference in energy (heat) between the reactants and products in a chemical reaction Endothermic Products have more energy than reactants Energy is absorbed to make the product ΔH (enthalpy) is positive

11 Exothermic reaction Reactants have more energy than products
Energy is released to make the product ΔH (enthalpy) is negative

12 Phase Change Diagram Energy must be transferred in order for an object to change phase Freezing & condensing transfer heat to environment Exothermic Melting and vaporization absorb heat from environment endothermic

13 Amount of energy needed to vaporize one mole of a pure substance is the same amount of energy needed to condense one mole of it Same magnitude just opposite signs for enthalpy

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