Materials and Methods
Refer to the lab manual for
the procedure. One deviation we had was that after the 4 minute mark we did not
open the test tube and stirred the reactants as the test tube was sealed very
tightly and we were afraid of disrupting the volume level.
Results and Discussion
With the use of Hess’s Law we
were able to find the enthalpy of formation of reaction of Zinc Oxide. Since
the direct formation of ZnO is unsafe, we did two different reactions to help
us find the enthalpy of formation of ZnO.
So, we found the enthalpy for
the two reactions and combined it to find the enthalpy of formation of ZnO. We
found the change in volume for both the reactions – it is shown in figures 1
and 2 in the Appendix – and used the change in volume to find the enthalpy from
both the reactions. Moreover, since the pressure is constant, the heat is equal
to the enthalpy change. We found the experimental enthalpy of formation of ZnO
to be -341kJ/mol. Since, the value is negative, it means that the reaction is
favourable so the total energy of the products is lower than that of the
reactants. The literature value of formation of ZnO is -348 ± 23.3 kJ/mol so the
experimental value is off by 2.01%. This could be due to various errors
throughout the experiment.
First off, the experiment was
not completely adiabatic so there was energy transfer within the system which
was not accounted for. Also, we did not stir the reactants as was told in the
lab manual. Moreover, water may have leaked out of the jar when we had to pour
in the HCl or when we had to put the ZnO. Therefore, even if the value final
value was near to perfect, the enthalpy for the reactions separately were
incorrect. We found the enthalpy of zinc and HCl to be -79.76 kJ/mol, when the
literature value is -153.90 kJ/mol. Things that could have been implemented to
make this reaction more successful is to take in the effect of the volume
change due to the reaction as well as the surroundings and thus get a graph
which provides us with a horizontal end point – a point where the graph becomes
Also, the reduction of ferric oxide by zinc is
energetically favourable as kJ/mol, so since it is
exothermic, the reaction can take place.
1. Peter M. Macdonald, David R. McMillen, Richard
Kil JCP221 Lab Manual (2018) – Pages (1-1 1-5) (Experiment 1)
J. Chem. Ed. 37, 634 (1960). ?
D.F., Atkins, P.W. and Langford, C.H. Inorganic Chemistry, W.H. Freeman
& Co., New ?York
(1990) p. 2292232. ?
4. Atkins, P.W. The Elements of Physical
Chemistry, 2nd Edition, W.H. Freeman & Co., New York (1997) p. 49271.
for reaction 1:
1: This is the volume vs time graph for the reaction between zinc and
hydrochloric acid where zinc was used as the limiting reagent. A line of best
fit for the first and last three data points are also plotted to measure the
change in volume. The change in volume was found to be 0.1067ml.
2: This is the volume vs time graph for the reaction between zinc oxide and
hydrochloric acid where hydrochloric acid was used as the limiting reagent. A
line of best fit for the first and last three data points are also plotted to
measure the change in volume. The change of volume was found to be 0.0917ml.
of ice melted in reaction #1:
produced in reaction #1:
Limiting Reagent: Zinc moles =
enthalpy of reaction:
Hess’s Law = (1) – (2) + (3)
Sample Error Calculations
Error in volume (using tolerance as it
is larger than calculated errors)
Error in moles/heat
Error in Enthalpy
in After Hess’ law:
Enthalpy of reduction of ferric oxide: