My chem book gives me the following Hess' Law example:
Mg + 2HCl --> H2 MgCl2 Delta H1
MgO +2HCl --> H2O +MgCl2 Delta H2
H2 + 1/2 O2 --> H2O Delta H3=-68.3kcal
The desired reaction is the formation reaction of MgO as follows:
MgS +1/2 O2 --> MgO Delta H final= function of all the delta H's above.
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I can pretty much understand Hess' Law pretty well, it's just the after quiz questions that have me a little off, such as:
There is a correction factor of about 0.3 kcal/mole for the magnesium reaction because of the evolution of hydrogen gas. Explain the cause of this correction, and should it be added or subtracted from Delta H1?
(My guess would be added, but not completely sure)
The heat capacities of your containers and probe used if you were preforming this experiment, would be ignored in your calculations, causing a small error. What would be one way the heat captivity could be measured experimentally?
(No idea)
And does the error mentioned above cause the calculated heats of the reactions to be less negative or more negative than they should be?
(More negative?)
It is best if MgO is heated to 400 degrees-500 degree Celsius for a few hours before you use it. Why?
(No idea)
55456
To answer these after-quiz questions, let's go through each one step by step:
1. Correction factor for the magnesium reaction:
The correction factor of 0.3 kcal/mole is necessary because the given reaction releases hydrogen gas (H2). However, the reaction as written does not account for the enthalpy change associated with the formation of H2. This correction factor accounts for that missing enthalpy change. Since we need to account for this additional enthalpy change, the correction factor should be added to Delta H1.
2. Measuring heat capacity in containers and probe:
To measure the heat capacity of the containers and probe being used in the experiment, a common experimental method is to perform a calorimetry experiment. Calorimetry involves measuring the amount of heat exchanged during a chemical or physical process. By measuring the heat flow in a controlled experiment, you can determine the heat capacity of the containers and the probe.
3. Effect of ignoring heat capacity on calculated heats of reactions:
Ignoring the heat capacities of the containers and probe causes a small error in the calculated heats of reactions. Specifically, ignoring these heat capacities tends to underestimate the heat changes. Therefore, the calculated heats of the reactions would be less negative (less exothermic) than they should be. This is because some amount of heat is absorbed by the containers and probe, which is not accounted for in the calculations.
4. Heating MgO before usage:
Heating MgO to 400-500 degrees Celsius for a few hours before using it is necessary to remove any impurities or adsorbed moisture from the MgO sample. This ensures that the MgO is in its purest form and does not contain any water or other impurities that could affect the experimental results. Heating also activates the MgO, making it more reactive and ready to participate in the desired reaction.