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Chemical Reactions: Hess

             Germain Henri Hess (1802 - 1850) is important primarily for his thermo-chemical studies, which began in 1839. The contributions of Hess to heat can be summed up in what is called the law of Hess, which is an empirical law. It is explained by thermodynamic theory, which holds that enthalpy is a state function. Chemists have made great use of the law of Hess in establishing the heats of formation of compounds which are not easily formed from their constituent elements.
             Hess's law states "for a chemical equation that can be written as the sum of two or more steps, the enthalpy change for the overall equation equals the sum of the enthalpy changes for the individual steps."" Hess's law is important when you are given an equation with an unknown ΔH and a series of other equations with known ΔH. Hess's law enables you to manipulate the equations in such a way that they can be added together to get the total enthalpy. If a reaction is carried out in a series of steps, ΔH for the reaction will be equal to the sum of the enthalpy changes for the individual steps the overall enthalpy change for the process is independent of the number of steps or the particular nature of the path by which the reaction is carried out. Thus we can use information tabulated for a relatively small number of reactions to calculate ΔH for a large number of different reactions.
             A calorimeter is created using two Styrofoam cups, a piece of cardboard to cover the top, and a precession thermometer that is inserted into the cardboard. The two reactions taking place in the calorimeter is as followed:.
             Mg+1/2O2 →MgO (not done in lab).
             H2+1/2O2 →H2O (not done in lab).
             Mg + 2HCl → MgCl2 + H2.
             • 50.0 ml of 1.0 M HCl is carefully measured into the calorimeter, and its temperature is precisely measured out and noted. .
             • About .2 grams of Mg is measured, noted and mixed into the calorimeter .

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