Using Hess’s Law to Calculate Heat of Formation of of Magnesium Oxide

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Heats of Reaction and Hess’s Law Lab

Background: Thermochemistry is the branch of chemistry that deals with the amounts of heat evolved or absorbed during chemical reactions. When the pressure is constant, the amount of heat gained or lost in a chemical or physical change is the enthalpy change for the process, or heat of reaction. Heat of reaction can be calculated by using Hess’s Law which states that the heat released or absorbed in a chemical process is the same whether the process takes place in one or in several steps, also known the law of constant heat summation. By using Hess’s Law, the heat of reaction can be determined without actually measuring the enthalpy change. This is extremely helpful because some reactions are very difficult to find the heat of reactions for directly. This experiment will find the heat of reaction for two chemical reactions and require Hess’s Law to determine the heat of reaction for the final desired product.

Calorimetry is the science that determines the changes in energy of a system by measuring the heat exchanged with the surroundings. It often requires the use of a calorimeter which is an instrument for measuring the amount of heat involved in a chemical reaction. In this experiment, the first two reactions are exothermic so the temperature of the solution and container will rise and some heat will be lost to the surroundings.

Purpose: The purpose of this experiment is to calculate the heat of formation of magnesium oxide by studying a series of reactions involving magnesium and magnesium oxide and using Hess’s Law.

Hypothesis: If the heat of reaction is found for two equations, then Hess’s Law will be able to determine the heat of reaction of the desired product.


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...actual heat of reaction for Part II was -146kJ/mole. The heat of formation found for the formation of liquid water was -286kJ/mole. By using Hess’s Law, the heat of reaction for the formation of magnesium oxide was found to be -496kJ/mole which yielded a -17.6% error.

A possible source of error could be loss of heat during the reaction that could have affected the final temperature results. Since a real calorimeter wasn’t used and a plastic cup calorimeter was used, there is a good chance that the system lost more heat to the surroundings than it would have if a real calorimeter had been used. That said, using the plastic cup calorimeter is a good substitute for a real calorimeter in classroom laboratory experiment because it still acts as an insulator to keep the heat inside and makes a better calorimeter than other materials because of its high specific heat.
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