Question

Based on your understanding of the laboratory procedure and setup for Parts B and C of this experiment, which of the following procedural error(s) would result in a lower/smaller than expected enthalpy of reaction, △ H_r* n ? For example, the literature value for △ H_r* n was 50 kJ/mole while the experimentally obtained value was 35 kJ/mole.

I. The lid of the coffee cup calorimeter was not completely sealed.
II. The calorimeter was not completely dried between trials
III. Some salt spilled on the counter prior to adding to the calorimeter.
IV. Less water was added to the calorimeter than was recorded.

I only
Il only
IV only
I, II, and III
I, III, and IV

Answer 1

A lower than expected enthalpy of reaction could be due to heat loss to the surroundings, reactions not carried out under constant-pressure conditions, inaccurate measurements, or the presence of impurities.

Step-by-step explanation:

A common laboratory error that could result in a lower or smaller than expected enthalpy of reaction (ΔHr*n) could be due to heat loss to the surroundings. If the calorimeter is not perfectly insulated, some of the heat produced by the reaction may escape, resulting in a lower temperature rise than expected, and thus a lower calculated ΔHr*n. Additionally, a discrepancy could arise if the reactions are not carried out under the assumed constant-pressure conditions. In a bomb calorimeter, for example, the reaction does not occur at constant pressure, which can lead to differences between the measured heat and the ΔH. Other factors include inaccurate measurements of reactant quantities or the unintentional inclusion of impurities that might influence the reaction. Such errors would affect the stoichiometry and the subsequent enthalpy calculations.