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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