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Enthalpy Of Chemical Reactions : Hess 's Law

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Experiment 2: Enthalpy of Chemical Reactions: Hess’s Law
The objective of this experiment was to use Hess’s Law to calculate enthalpies of various reactions and compare the results with accepted values. Specifically, the enthalpies under investigation were the enthalpy of formation of magnesium oxide and the enthalpy of reaction between ammonia and hydrochloric acid. In part I, the ΔH of each individual reaction was obtained by performing each reaction inside a calorimeter. Temperature probes were inserted in the calorimeter and ΔT was measured. By using the equation q = Msol’n x Cp x ΔT + Ccal x ΔT, the heat absorbed by the surroundings, q, was obtained for each reaction. The negatives of these values, or heat released by the …show more content…

The second method was using Hess’s law to add the accepted ΔHrxn 1 and flipped ΔHrxn 2 values of the net ionic equations. The same ΔHrxn 3 was obtained from both methods, 52.22 kJ/mol, and was defined as the accepted value. The third method used was performing reaction 3 in a calorimeter, measuring ΔT, and then finding q to obtain ΔHrxn 3. The value obtained from this method was 56.1 kJ/mol. The fourth method was explained in the previous paragraph: by appropriately adding up the measured ΔHrxn 1 and ΔHrxn 2 using Hess’s Law. The ΔHrxn 3 obtained from this method was -54 kJ/mol. The ΔHrxn 3 obtained from the fourth method was more accurate, as it had a 3.4% error from the accepted value, while the ΔHrxn 3 from the third method had an 8.4% error from the accepted value. Both ΔHrxn 3, however, can be considered to be reasonably accurate, as it was within 15% of the accepted value. To improve the results, a more accurate volume measuring instrument, such as a pipette, could have been used to dilute the reactants. This would result in the actual number of moles participating in the reaction becoming closer to the number used in calculation, thus resulting in a more accurate ΔHrxn. In addition, a more insulated calorimeter could have been used to reduce the amount of unaccounted heat escaping, allowing a more confident q measurement.
In part II, the ΔH°f of Mg was determined by a similar procedure used in part I, where Hess’s Law was first used to add the

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