Grim, Cole
Honors Chemistry 10°
3/31/04
Introduction:
This experiment was done in able to teach us to find the enthalpy change in two reactions, and then use Hess’s Law to find the enthalpy of the third reaction.
Germain Henri Hess is well known for his studies of thermodynamics, specifically for the creation of Hess’s Law, which occurred in 1840. This is a thermodynamic theory which makes enthalpy a state function. (Germain…)
Also known as the law of constant heat summation, Hess’s Law states that if a reaction is carried out in a series of steps, then the enthalpy change for the reaction will be equal to the sum of the enthalpy changes for the individual steps. The total enthalpy change for the reaction does not depend upon the path that you took to get there, so you can get there any method you choose (Blaber).
Results:
HCl + NaOH
Instantaneous Temperature of Mixing: 35°C
Initial Temperature: 23.5°C
Temperature Difference: 11.5°C
Moles: 3M = x mol/.05 L
.15 mol
Enthalpy = (4.18)*(50°C)*(1.09g/mol)*(11.5) = 2619.81 J/mol
2619.81 J/mol * .15mol = 392.97 J
NH4Cl + NaOH:
Instantaneous Temperature of Mixing: 24°C
Initial Temperature: 22.25°C
Temperature Difference: 1.75°C
Moles: 3M = x mol/.05 L
.15 mol
Enthalpy = (4.18)*(50°C)*(1.09g/mol)*(1.75) = 398.67 J/mol
398.67 J/mol * .15mol = 59.8 J
NH3 + HCl:
Instantaneous Temperature of Mixing: 31°C
Initial Temperature: 22.75°C
Temperature Difference: 8.25°C
Moles: 3M=x mol/.05 L
.15 mol
Enthalpy = (4.18)*(50°C)*(1.09g/mol)*(8.25) = 1879.43 J/mol
1879.43 J/mol * .15 mol = 281.91 J
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HCl + NaOH = NaCl + H2O |
392.97 J |
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NH4Cl + NaOH = NaCl + NH3 + H2O |
59.8 J |
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NH3 + HCl = NH4Cl |
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HCl + NaOH = NaCl + H2O |
392.97 J |
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NaCl +
NH3 + H2O = NH4Cl + NaOH |
-59.8 J |
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NH3 + HCl = NH4Cl |
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HCl + |
392.97 J |
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-59.8 J |
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NH3 + HCl = NH4Cl |
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HCl |
392.97 J |
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NH3
= NH4Cl |
-59.8 J |
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NH3 + HCl = NH4Cl |
333.17 J |
Experimental Result: 281.91
Hess’s Law Result: 333.17
Difference: 51.26 ~ 15%
Discussion:
In this experiment, the results from the Hess’s Law method and the experimental method were very different. The experimental result was 51 J apart from the Hess’s Law result. There are many different things that could have triggered this, most likely an error in the lab. There were many potential sources of error in this lab, such as imprecise thermometers that were difficult to obtain an accurate temperature readout, a not sterile lab environment where a contaminant could have easily gotten into the chemical reaction, as well as not sterile lab equipment, as none of it was thoroughly cleaned before proceeding with the lab. All of these different factors could have contributed to the difference in the results for the enthalpy via the two separate methods.
Blaber, Michael. "Hess's Law." Energy Relations in Chemistry: Thermochemistry. 1996. 1 Apr. 2004 <http://wine1.sb.fsu.edu/chm1045/notes/Energy/HessLaw/Energy04.htm>. "Germain Henri Hess (1802-1850): Hess's Law." Kiwi Web. 1 Apr. 2004 <http://www.chemistry.co.nz/hess_law.htm>.