Thermodynamics Lab

886 words 4 pages
Thermodynamics- Enthalpy of Reaction and Hess’s Law
Objectives:
1. To calculate the heat of reaction of a given reaction using the concepts derived from Hess’s Law.
Pre-lab Questions:
1. Define Heat of Reaction.
The enthalpy change associated with the completion of a chemical reaction.
2. Define Specific Heat.
The energy it takes to raise the temperature of 1 gram of a substance by one degree Celsius.
3. Calculate the heat of reaction assuming no heat is lost to the calorimeter. Use correct significant figures.
Q = c x m x t q = (4.18)(1.02 g/ml x 50ml )(3.9 oC) = -831 J
4. In problem 3 above, the calorimeter has a heat capacity of 8.20 J/goC. If a correction is made to account for heat absorbed by the calorimeter,
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4. Do the lab results support Hess’s Law?
According to our lab results, Hess’s Law was not supported, as there was a 26.6% difference between the measured and calculated values.
5. How could the procedure be changed to achieve greater accuracy?
A cover could be used in order to keep more heat in the system rather than the heat being lost to the surroundings. This would significantly lower the heat capacity value for the calorimeter, giving greater accuracy in the results.
6. Find a table in a reference that lists standard heats of formation for the species included in your net ionic equations. Use them to calculate ∆Hrxn for each of the three net ionic equations. Do these values support Hess’s Law?
Ammonia=-46.19kJ/mol
HCl=-92.3kJ/mol
Sodium Hydroxide=-469.15kJ/mol
Ammonium Choloride=-314.43kJ/mol
These values do support Hess’s Law, as the first two reactions add up to equal the

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