The determination of the molar enthalpy of the combustion of magnesium using Hesss Law

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The determination of the molar enthalpy of the combustion of magnesium using Hess’s Law

Abstract:

        In order to determine the enthalpy change of the combustion of magnesium, exothermic reactions were done and Hess’s Law was used. The combustion of magnesium is an exothermic process in which heat is released. A calorimeter was used in order to measure the enthalpy change of the magnesium reaction and the magnesium oxide reaction. The calorimeter was built with the use of Styrofoam cups and a thermometer. For the first reaction 0.5 g magnesium metal polished with steel wool was placed in hydrochloric acid. From this reaction there was a 14.0°C increase, as the initial temperature was 33.0 °C and it had increased to 47.0 °C. In the second reaction magnesium oxide was reacted with hydrochloric acid. The initial temperature of 33.0°C , once again had increased to 37.0°C. Although there was not a soaring increase in temperature, there was still an increase which proves that there was the release of heat. The release or production of heat is known as exothermic, and this verifies that the combustion of magnesium is an exothermic process.

Theory:

         Thermochemistry is study of energy changes which is associated with a chemical or physical process. The energy changes are equal to the enthalpy changes (  H ) . In accordance with calorimetry the heat lost is equal to the amount of heat gained. This verifies the law of conservation of energy which states that energy cannot be created nor destroyed. For any reaction the  H is the same regardless of whether it occurs directly or throughout various steps.

Combustion is an exothermic process in which a metal reacts with oxygen gas in order to produce an oxide. This synthesis reaction is also known as the target equation. Magnesium has the tendency to combust rapidly and exothermically in air. Magnesium has the capability to burn rapidly and it occurs at a high temperature.

                                                   Mg(a) + ½ O2(g) → MgO(s)

        Through the usage of Hess’s Law the enthalpy change of a reaction can be found. Enthalpy change is also known as the change in energy . It is the amount of energy that is either consumed or released through a reaction. This value is represented through the symbol of Δ H. In exothermic reactions heat is lost or released, while in endothermic reactions heat is gained or absorbed. Hess’s Law states that the enthalpy change of a chemical of physical process depends on the reactants and its products. Through the usage of manipulation such as addition, multiplication or reversal, the known equations can be manipulated to construct the target equation. The enthalpy change of a reaction can be tested found through the process of using an calorimeter. A calorimeter is a closed system in which matter cannot move throughout the system.

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Calorimetry can be defined as a process in which the changes of energy are measured within a chemical system.  With the use of Styrofoam cups a calorimeter can be easily made. Styrofoam is a great source of insulation. This calorimeter is effective, however it lacks accuracy as a hole is created at the top of a Styrofoam cup which in return allows the exertion of gas. A more effective calorimeter is the bomb calorimeter which is usually used to measure combustion reactions. Through this process the volume of the reaction does not change throughout and stays the same.

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