Lab - Reaction Enthalpies and Hess’ Law

Essay add: 29-09-2015, 19:28   /   Views: 363
Reaction Enthalpies and Hess’ Law

Introduction:
This lab will show us how to first determine enthalpy using a simple calorimeter then apply Hess’ law to determine the change in enthalpy for a third reaction.

data:

Part1: The Heat Capacity of the Calorimeter
Ccal = 135kJ (Very off, so we used 30kJ for the rest of our calculations)

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Discussion:
The purpose of this lab was to learn how to determine the change in enthalpy of HCl & Mg and HCl & MgO, and the heat capacity of a calorimeter which should help us better understand Hess’ law and enthalpy in general. To determine the heat of the calorimeter we added hot water to cold water contained inside our calorimeter and observed the change in temperature inside the calorimeter. Knowing that the heat of gained by cold water and the calorimeter should be exactly equal to the heat lost by the hot water, we are able to solve for the heat capacity of the calorimeter itself. We found the heat capacity of our calorimeter to be 135kJ. This result is not accurate for a reason which we were unable to determine. As instructed I used 30J for the heat capacity instead of our experimental value to gain more accurate calculations in the rest of the lab.

Our next task was to determine the change in enthalpy for HCl and magnesium ribbon. We first determined the heat of the reaction, after finding the change in temperature, to be -2220J and -3808J for trials 1 and 2 respectively. From this we could divide by moles of Mg in each trial, and found that the average change in enthalpy was -313kJ/mol  104kJ/mol. The calculated value of 467kJ/mol is 154 kJ/mol higher than the experimental mean. If you account for standard deviation, it may be off by only 50 kJ/mol. Taking this into account, and assuming trial 2 was more accurate, subtracting 50 kJ/mol to our recorded -417kJ/mol gives us our exact calculated value.

Our next task was to determine the change in enthalpy for HCl and MgO. We followed nearly exactly the same procedures as in the reaction of HCl with Mg, only having determine grams of MgO using a different method since it is not possible to put all measured MgO into the calorimeter. We found the change in enthalpy to be -167kJ/mol  31.5kJ/mol which is only off by 15kJ/mol from the calculated change in enthalpy.

One source of error is, once again, in the calorimeter’s themselves. If a hole that cannot be seen develops or if the hole that the thermometer goes through does not seal well, or if the lid does not seal tightly, the results will be adversely affected. The heat capacity calculated for the calorimeter will be too high because heat will be allowed to escape into the outside environment. Also, similar to the previous lab, if either Mg or MgO did not fully react with HCl inside the calorimeter then the solution would not reach its potential and the result would in turn be lower than actual. Two more sources of error are a result of the properties of Mg and MgO. If Mg was allowed to be exposed to air over a period of time and oxidize, a thin MgO coating might occur over the ribbon. This would make the change in enthalpy be less exothermic than it should be since MgO is less exothermic than Mg. Also, MgO can absorb water, making it hydrated. If this were allowed to happen then results would be adversely affected, probably making MgO appear even less exothermic than it is as a result of the specific heat of water.

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