## Thermodynamics - Enthalpy Of Reaction And Hess's Law

This lab demonstrates the principle of Hess's law. Hess's law states that the change in enthalpy is the same whether a reaction takes place in one step or in a series of steps. This implies that enthalpy is a state function and is independent of the specific path or the series of steps carried out in a chemical reaction.

The following reactions will take place in this lab:

(1) NaOH(aq) + HCl(aq) ---> NaCl(aq) + H2O(l)

(2) NH4Cl(aq) + NaOH(aq) ---> NH3(aq) + NaCl(aq) + H2O(l)

(3) NH3(aq) + HCl(aq) ---> NH4Cl(aq)

According to Hess's law, the enthalpy change, ΔHrxn, for the 3rd reaction equals the sum of the ΔHrxn for reactions 1 and 2, after the 2nd reaction is reversed..

Because each of the reactions are measured for temperature change by calorimetry a Temperature vs Time graph will be drawn to obtain an accurate Tmix, the initial temperature of the mixtures.

Determining The Heat Capacity Of The Calorimeter:

 Time Temp (s) (C) 20 41.3 40 41.0 60 40.8 80 40.5 100 40.3 120 40.1 140 40.0 160 39.8 180 39.5

Determining ΔH for NaOH(aq) + HCl(aq) ---> NaCl(aq) + H2O(l):

 Time Temp (s) (C) 20 34.8 40 35.3 60 35.2 80 35.2 100 35.0 120 34.9 140 34.8 160 34.7 180 34.7

Determining ΔH for NH4Cl(aq) + NaOH(aq) ---> NH3(aq) + NaCl(aq) + H2O(l):

 Time Temp (s) (C) 20 24.1 40 24.1 60 24.1 80 24.1 100 24.1 120 24.1 140 24.1 160 24.1 180 24.1

Determining ΔH for NH3(aq) + HCl(aq) ---> NH4Cl(aq):

 Time Temp (s) (C) 20 33.1 40 33.0 60 32.9 80 32.9 100 32.8 120 32.8 140 32.8 160 32.8 180 32.7

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