Hess’ Law in Action Introduction Hess’ Law is a convenient way to calculate the change in enthalpy for reactions that are particularly difficult to carry out in the lab. In this lab, we will be calculating the enthalpy of combustion of magnesium metal: Mg (s) + ½ O2 (g) MgO (s) This reaction is not particularly difficult to carry out, but it is difficult to accurately measure the heat evolved in the reaction without a rather sophisticated calorimeter. But it is possible is to react magnesium and magnesium oxide in hydrochloric acid and accurately measure the change in enthalpy for these reactions, since they take place in solution and very little heat is lost. By rearranging these equations (and looking up the enthalpy of combustion for hydrogen gas), we can calculate the enthalpy of combustion for magnesium using Hess’ Law. Materials Magnesium ribbon water MgO Styrofoam cups 3 M HCl (hydrochloric acid) thermometer probes and lap top Procedure Hints 1. Make a data table and decide what values you need to measure before you start the experiment. 2. A Styrofoam cup will minimize heat loss. Stirring will help the system come to thermal equilibrium. 3. You will need about 60 mL of 1.0 M HCl. In order to dilute the 3 M acid to 1 M, add 20 mL of acid to 40 mL of water. Remember the acid is very corrosive!! Remember to pour the acid into the water (and not vice versa). 4. Try about 0.60 grams of magnesium. This is enough to ensure a reasonable temperature change while still enabling the magnesium to be the limiting reagent. Run at least two trials. Three is better. You may pour the neutralized acid down the drain. 5. Now run the same experiment with MgO instead of magnesium. Try about 1.00 grams of MgO in the same amount of 1 M acid. Run at least two trials. Three is better. You may pour the neutralized acid down the drain. Special Notes ** The acid is corrosive! You MUST use goggles!! ** Do NOT get the balances wet!! ** The specific heat capacity of the acid solution is close to water, but is not exactly the same. A closer value is 3.93 J/g C. Data Analysis 1. Write two balanced chemical equation describing the reaction of hydrochloric acid with magnesium and also with magnesium oxide. 2. Calculate the heat released in each trial above. Then figure out the change in enthalpy (H) per mole of magnesium and per mole of magnesium oxide, and average these values over all trials. Match these values to their corresponding balanced equations above. 3. Use these equations, along with the enthalpy of combustion for hydrogen gas (-285.5 kJ/mole), to calculate the enthalpy of combustion for magnesium metal using Hess’ Law. Show all work.
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