Calculate delta H for the following reaction:
Zn(s) + 2H+(aq) ----> Zn2+(aq) + H2(g)
Work is done by Hydrogen gas in expanding against the atmospheric pressure. Since
the performance of work involves the transfer of energy, so the value of ∆U we
measured will not be true value. We now define a new term, the enthalpy change, ∆H,
which takes into account the work done when a system expands or contracts during a
reaction at constant pressure.
∆H = ∆U + p∆V
If work is done by the system, p∆V is positive. If work is done on the system p∆V is
When a system absorbs heat energy, its internal energy increased. If the volume of the
system is fixed (∆V = 0), all heat absorbed will be changed to internal energy. That is
∆H = ∆U.
If the reaction takes place at constant pressure, then the heat absorbed is equal to the
increase in internal energy plus the work done.
The values of ∆H and ∆U only depend on the final and initial states of the system,
they do not matter how the change is made. They are state functions.
For example, the change in potential energy of a block of mass M when taking from
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