Energy and Chemical Reactions – Part 2
Enthalpy – continued
Many reactions involve little if any PV work so most or all of the
energy change occurs as a transfer of heat.
Reactions that do not involve gases.
Gases do not appear in
Liquids and solids undergo very small
volume changes so ∆V ≈ 0 and P∆V ≈ 0.
∆H ≈ ∆E
Reactions in which the amount (mol) of gas does not change.
When the total amount of gaseous reactants equals the total
amount of gaseous products, ∆V = 0, P∆V = 0, and ∆H = ∆E.
Reactions in which the amount (mol) of gas does change.
these cases P∆V ≠ 0.
But since q is usually so much larger
than P∆V, ∆H is very close to ∆E.
Most of ∆E occurs as heat
transfer so ∆H ≈ ∆E.
Exo- and Endothermic Processes
Enthalpy is a state function.
The enthalpy change of a reaction is
called heat of reaction,
, which is H
Exothermic process releases heat,
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