This content explains how to calculate the enthalpy change ($\Delta H$) for an exothermic reaction by determining the heat produced ($Q$) and identifying the limiting reactant.
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so here's an example of calculating our
Q for a reaction um and calculating
enthalpies uh for and how to do it this
so what we're doing is we are going to
react zinc with some hydrochloric acid
and we know that we have
3.14 K of heat are produced so as a
reminder when we say that the heat is
produced this is
exothermic of a reaction and then we
want to calculate the change of enthalpy
per mole of zinc for the reaction so we
want to calculate the Delta H for this
reaction and this Delta H is based on
the reaction itself it could be per mole
of zinc it could be per two moles of HCL
so we use our stochiometry to think
about this Delta H so first of all our Q
of the reaction is going to equal - 3.14
k where did I get that because this is
exothermic and whenever we have an
exothermic reaction remember that we
have a goes to B plus heat so heat for
the reaction is leaving the reaction so
that is a negative Q so this is our Q
reaction um and then we have remember
what we're also looking at is we have
two reactants here so when we have two
reactants we have to think limiting
reagents so which one of those is our
limiting reagents so first of all we
have 60.0 milliliters we're going to
liters and then we're going to find how
have so and then when we do this we need
to convert from moles of hydroch acid to
zinc so there's one zinc for every two
hcls and if we calculate that out we end
up getting
0.0225 moles of
zinc so that works out but then we have
to see how many moles of zinc do we
actually have so we can start with 1.34
grams of
zinc we can go ahead and convert that
using our molecular weight so it's one
mole for every 65.3 n g of zinc and we
0.0204 and I'll carry an extra digit
nine of of uh moles of
zinc so the hydrochloric acid requires
more zinc than we have and then the zinc
is there's actually not enough so if we
think about
025 uh
0225 and
0204 we know that we have less zinc than
we need so that will be our limiting
reagent so we can find out that we have
this many uh moles of zinc then we go
back to our problem we're trying to have
our Delta H and that's going to be our Q
of the reaction which we've already got up
up
here and then how many moles of zinc do
we have and then that value is right
here so we're going to have a minus 3.4 K
K
314 divided
0.0204 and I'll carry in an extra Sig
fig moles of zinc and this ends up be minus
minus 153
153
kles uh per
mole of zinc and that is our final
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