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, ΔH_rxn, for the 3rd reaction equals the sum of the ΔH_rxn 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 T_mix, 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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