The Enthalpy of Formation of Magnesium Carbonate
By: Jessica Miller
Lab Partner: Rebekah Melanson
Due Date: Thursday February 3rd 2011
The purpose of this experiment was to determine the enthalpy of formation of magnesium carbonate in a lab setting. This was done using a calorimeter, which was calibrated by adding a known amount of both hot and cold water. Following the calibration of the calorimeter, HCl was placed into the empty calorimeter and then mixed with magnesium and then MgCO3 and the temperature changes were recorded. The heat lost by the hot water is thus equal to the heat gained by the calorimeter. Experimentally, it is difficult to determine the standard enthalpy of formation directly, therefore Hess’ Law is applied and the value is determined indirectly using reactions for the formation of Mg2-, MgCO3, H2O and CO2. These experiments led to the determination that the enthalpy of formation of magnesium carbonate is -1087.53 kJ which led to a percent error of 2.2%.
Magnesium Carbonate, MgCO3, is a white solid that occurs naturally as a mineral, as has a variety uses. MgCO3 is used in flooring, fireproofing, fire extinguishing compositions, cosmetics, dusting powder, and even toothpaste. It can also be used as a smoke suppressant in plastics, a reinforcing agent in neoprene rubber, a drying agent, and an additive in table salt to keep it free flowing (Magnesium Carbonate). The purpose of this experiment is for one to be able to determine the standard enthalpy of formation of Magnesium Carbonate. Every chemical and physical change is accompanied by a change in energy, which usually occurs in the form of heat. The heat of a reaction, also referred to as the change in enthalpy, is denoted by the symbol H. The enthalpy of reaction is expressed as the heat quantity that is evolved or absorbed when one mole of the reactant is converted to product. This is considered to be exothermic (-H) when heat is lost to the surroundings, and endothermic (+H) when heat is gained from the surroundings. In order to determine the enthalpy of formation of Magnesium Carbonate, one must first determine the value of H for Magnesium using calorimetry. The next step would be to determine the value of H for MgCO3, however, since this cannot be determined directly, we must measure the H for the reverse reaction and then change the sign. Since the H values for H2O and CO2 are taken from reference values, Hf (MgCO3) can now be determined using the following four equations: Mg(s) + 2H+(aq) Mg2+(aq) + H2(g)
Mg2+(aq) + H2O(l) +CO2 (g) MgCO3(s) +2H+(aq) H2(g) + 1/2O2(g) H2O(l) C + O2(g) CO2(g) Using Hess’ Law, these equations can be added to give the experimental value for Hf(MgCO3) with the following equation: Mg(s) + C + 3/2O2(g) MgCO3(s)
Hess’ Law, a term coined from the Russian chemist and physician Germain Hess, is a law that states, “the enthalpy change for a reaction that is carried out in a series of steps is equal to the sum of the enthalpy changes for the individual steps” (Hess’ Law). The law states that the energy change for any chemical or physical process is independent of the path taken from the initial to final state. Hess’ Law allows for the enthalpy change for a reaction to be calculated even when it can’t be measure directly. This is done by doing basic algebraic operations based on the chemical equation of reactions using previously determined values for the enthalpies of formation (Hess’ Law). As previously mentioned, determining H1 and H2 are done using a process known as calorimetry. Derived from the Latin word calor (meaning heat) and the Greek work metry (meaning to measure), calorimetry is the science of measuring the heat of chemical...
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