Objective : Hess law states that if a reaction can be carried out in a series of steps, the sum of the Enthalpies of all steps should equal to the Enthalpy change for the total reaction. This statement emphasizes the conservation not only of matter, but also of energy.
If certain reactions are difficult to study in terms of Enthalpy calculations , their enthalpy can be calculated using Hess’ law. The purpose of this experiment is to use a calorimeter with a thermometer to investigate the enthalpy changes in different reactions. After all the data have been collected from the different reactions, the data will be analyzed in order to determine whether or not the data supports Hess law for a reaction that is otherwise difficult to measure.
The heat capacity was first determined for the calorimeter that was to be used.
The enthalpy change was then measured in several separate reactions that could eventually be added up to compare the measured and the calculated ΔH of the third reaction.
Each reaction was performed 3 times and the mean was taken for the graphical data analysis.
The reactions R1a :
HCl + NaOH → NaCl + H2O
NH4Cl + NaOH → NaCl + H2O + NH3 both were exothermic.
The ΔH of the target reaction R3a :
HCl + NH3 → NH4Cl could be measured and calculated.
Similarly, R1b :
NaOH(s) + H2O → Na+(aq) + OH–(aq)
NaOH + HCl → H2O + Na+(aq) + Cl–(aq)
using above R1b and R2b :
NaOH(s) + HCl → Na+(aq) + Cl–(aq) + H2O and compare it with the measured values.
Hess law is an important principle even for things in life today. Hess Law states that if two chemical equations can algebraically be combined to give a third equation, the values of ΔH for the two equations can be combined in the same manner to give ΔH for the third equation.
The experiments that were previously performed show how a reaction that is difficult to perform and measure as an experiment can be calculated based on Hess law.
While the large scale of Hess’ law could be confirmed, there was some discrepancy of the calculated enthalpy change which is thought to be caused by the loss of energy in the relatively simple calorimeter used in the experiments.
Hess law underscores the conservation of energy which is the reason why a perpetual mobile (endless motion) is impossible as energy cannot be generated from nothing.