Calculating a Heat of Formation Using Hess’s Law As discussed in lecture, the simplest and most general method for determining the energy change during a chemical reaction involves the use of the heat of formation (ΔHf) for the reactants and products. Conceptually, the heat of formation is the heat released or absorbed when one mole of the substance is created from its elements at standard temperature and pressure. In some cases, this number can be measured directly (i.e. by reacting the elements at 1.0 atm and 25oC and observing the heat given off). In most cases, however, this is not practical. For instance, the direct calculation of the ΔHf for magnesium oxide would involve burning magnesium metal in a pure oxygen atmosphere; while such a reaction is certainly possible, it would difficult to measure changes in temperature since the reaction is highly exothermic and the product is a solid powder. Instead, in this lab you will be calculating the ΔHf for magnesium oxide by determining the heat of reaction for other reactions whose heats of reaction are either known or easily determined by experiment, and then combining them according to Hess’s Law to generate the heat of formation for MgO. The two equations you will be working with in the lab are: Reaction 1: Mg(s) + 2HCl(aq) --> MgCl2(aq) + H2(g) Reaction 2: MgO(s) + 2HCl(aq) --> MgCl2(aq) + H2O(l) Reaction Mg(s) + O2 (g) --> MgO (s) = heatreaction = -569 kJ/mol (calculated in experiment) Heat of formation for MgO (s) = -601.8 kJ/mol (true value) 1. Look up the true value of the ΔHf for magnesium oxide. Report it, including the source. What is the percent error of your experimental results? (Show your work – not all students calculate this number correctly!) Does this error seem large or small? What, in your estimation, is the most likely cause of error in this experiment? (Do not say “human error” or anything equally vague!)
Calculating a Heat of Formation Using Hess’s Law
As discussed in lecture, the simplest and most general method for determining the
energy change during a
reactants and products. Conceptually, the heat of formation is the heat released or absorbed
when one mole of the substance is created from its elements at standard temperature and
pressure. In some cases, this number can be measured directly (i.e. by reacting the elements at
1.0 atm and 25oC and observing the heat given off). In most cases, however, this is not
practical. For instance, the direct calculation of the ΔHf for magnesium oxide would involve
burning magnesium metal in a pure oxygen atmosphere; while such a reaction is certainly
possible, it would difficult to measure changes in temperature since the reaction is highly
exothermic and the product is a solid powder.
Instead, in this lab you will be calculating the ΔHf for magnesium oxide by determining
the heat of reaction for other reactions whose heats of reaction are either known or easily
determined by experiment, and then combining them according to Hess’s Law to generate the
heat of formation for MgO. The two equations you will be working with in the lab are:
Reaction 1: Mg(s) + 2HCl(aq) --> MgCl2(aq) + H2(g)
Reaction 2: MgO(s) + 2HCl(aq) --> MgCl2(aq) + H2O(l)
Reaction Mg(s) + O2 (g) --> MgO (s) = heatreaction = -569 kJ/mol (calculated in experiment)
Heat of formation for MgO (s) = -601.8 kJ/mol (true value)
1. Look up the true value of the ΔHf for magnesium oxide. Report it, including the source.
What is the percent error of your experimental results? (Show your work – not all
students calculate this number correctly!) Does this error seem large or small? What, in
your estimation, is the most likely cause of error in this experiment? (Do not say
“human error” or anything equally vague!)
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