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Hess's Law and the heat of combustion of Mg In this experiment you will use Hess's law to measure the AH of combustion of Mg. To measure the heat of combustion directly would be cumbersome. You will measure the AH of two reactions that can be carried out easily in the lab. These values and the known AH of formation of water will be used to calculate the heat of combustion of Mg: (Reaction 1) Mg(s) + O2(g) This equation can be obtained by combining equations : (Reaction 2) MgO(s) + 2 HCl(aq) (Reaction 3) Mg (s) + 2 HCl(aq) ► O2(g) (Reaction 4) H2(g) + The pre-lab portion of this experiment requires you to combine equations from Reactions 2, 3, and 4 to obtain the equation for Reaction 1 before you do the experiment. Heats of reaction for equations (2) and (3) will be determined in this experiment. As you may already know, AH for Reaction 4 is -285.8 kJ. Prelab: 1) Write the definition of Hess's law. MgO(s) MgCl₂(aq) + H₂O(1) MgCl₂(aq) + H₂ (g) H₂O(1) Select All Smart Lookup 4) Why is AH called a "state function"? Explain. AHRxnl AHRxn2 AHRxn3 2) Illustrate that by combining Equations 2, 3 and 4 you are left with Equation 1 above. AHRxn4 How does Hess's Law allow you to determine the AH for Equation 1 using AH's for Equations 2, 3 and