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Student will learn to measure the ∆H values of two reactions by Calorimetry and Hess’s Law
Typology: Lab Reports
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The objectives of this laboratory are as follows:
The combustion of a metal in oxygen produces the corresponding metal oxide as the only product. Such reactions are exothermic and release heat. For example, the combustion of iron releases 1651 kJ of heat energy for every four moles of iron burned:
Since it is difficult to measure the enthalpy of combustion of a metal directly, in this lab it will be determined indirectly by applying Hess’s Law of Heat Summation. Hess’s Law states that the enthalpy change of an overall process is equal to the sum of the enthalpy changes of its individual steps.
given the following information,
Solution : Reactions A and B have to be carefully manipulated before they can be summed to
will they yield the target equation when added together:
2 NO 2 ( g ) + ½ O 2 ( g ) → N 2 O 5 ( g ) Target
In order to use Hess’s Law to find the heat of combustion of a metal, it is first necessary to obtain reaction enthalpies (∆H values) for equations that can be summed together appropriately. To accomplish this, two reactions will be studied in this lab. In one reaction, a given metal will react with hydrochloric acid producing hydrogen and the metal chloride. In the other reaction,
the corresponding metal oxide will react with hydrochloric acid producing water and the metal chloride. For example, the reactions involving iron and iron(III) oxide are as follows:
Since both reactions are exothermic, the heat released ( q ) will be absorbed into the surrounding reaction mixture. As long as the reactions are performed in an insulated container (such as a coffee cup calorimeter) there will be negligible heat exchange with the container walls or outside air. By monitoring the temperature of the reaction mixture when specific quantities of reactants are used, the amount of heat (in J) released by these reactions can be determined by applying the equation:
that occurs during the reaction (in °C), and c is the specific heat capacity of the mixture (in J/g•°C). Note that since the reactions occur in aqueous solution, it is reasonable to substitute the specific heat capacity of water (= 4.184 J/g•°C) for the specific heat capacity of the mixture.
Recall that at constant pressure (the conditions of this experiment), the heat released by the reaction equals the reaction enthalpy:
Since the heat released by each reaction is proportional to the amount of metal/metal oxide
It should be noted that reactions (2) and (3) by themselves still cannot be summed to produce Reaction (1). Another reaction is required:
thermodynamic data in the textbook. Finally, the reactions (2), (3) and (4) and their enthalpies may be summed together according to Hess’s Law to determine the enthalpy of combustion of the given metal (1).
Note that the thermometer must be clamped in place using the slotted stopper and utility clamp/stand. You may also want to place the nested cups in a medium beaker for extra stability.