Magnesium Powder Lab Report

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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…show more content…
Magnesium powder is then added at t = 2.0 min The temperature of the mixture is recorded at equal time intervals. A temperature‐time graph is plotted and extrapolated to the time the reactants were first mixed. The temperature at this point would be the maximum that would have been observed if there was no heat loss at the point of mixing. This method can be employed for both endothermic and exothermic reactions. It is especially useful when the reaction is slow since heat loss to the surroundings would be more significant then. Comment: In Example 1, the question only provides the initial and final temperature readings. 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…show more content…
The reference state is defined to be the most stable state for a particular element at a specified temperature and 1 bar (can often be assumed to be 1 atm). The most common reference state used is 298 K and 1 bar (also known as standard conditions). For example, the standard enthalpy change of formation of oxygen gas at 298 K and 1 bar is zero. (N.B.: standard conditions here are not the same as STP!) ΔH refers to enthalpy change at 1 bar. H⦵ refers to enthalpy change at 298 K and 1 bar. 1) Formation enthalpies can be used to calculate the enthalpies of chemical reactions. Hr⦵ =  nHf⦵ (products)   mHf⦵ (reactants) where m, n = the stoichiometric coefficients of the reactants and products respectively in the given reaction. Comment: just expressed verbally as “products minus reactants” for simplicity! 2) Combustion enthalpies can be used to calculate the reaction enthalpies

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