Mg (s) + 2Ag+ (aq) Mg2+ (aq) + 2Ag (s) Solution
Number of moles of Mg = 0.700 ÷ 24.1 = 2.9046 x 10-2 mol
Number of moles of AgNO3 = 75.0/1000 X 0.250 = 1.875 x10-2 mol
Thus AgNO3 is the limiting reagent.
Energy released to surroundings = (75.0)(4.18)(27.2-19.5) = 2413.95 J
Comment: The excess Mg present is ignored as it is present only in small amount, and its specific heat capacity is about 0.25 that of water. Anyway, this is only a crude experiment to understand the principles of ΔH measurement. Some students may feel that the method is inaccurate, especially those studying Physics, because actual methods use better apparatus to reduce heat loss.
Since the reaction produces a rise
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Thus ΔT = final temperature – initial temperature
However, if the question does provide a graph such as the one shown above, extrapolation would be needed to determine a more accurate ΔT.
Hess’s Law of Constant Heat Summation
Not all enthalpy changes of reaction can be measured directly by using calorimetry and hence Hess’s Law can be used to determine the enthalpy changes that cannot be determined by direct measurements.
Hess’s Law states that the enthalpy change of a reaction is independent of the pathways between the initial and final states, i.e. enthalpy change is a state function.
An energy cycle or energy level diagram is used to determine the relationship between all reactions involved before using Hess’s Law to solve for the unknown ΔH.
Consider the example of partial hydrogenation of an alkyne to give an alkene. The partial hydrogenation of 2-butyne is exemplified by the following:
However, it is difficult to conduct the experiment in the laboratory to measure the enthalpy change of hydrogenation of 2-butyne directly. Instead, the enthalpy change of combustion of the alkyne, alkene and hydrogen are determined separately, followed by the application of Hess’s