LAB - Using Hess’ Law and calorimetery to determine the molar enthalpy of combustion of magnesium
Thermochemistry is the study of energy changes accompanying chemical transformations and the change in enthalpy is a useful figure used by thermo-chemists. Enthalpy is defined as the total internal energy of a system plus the product of pressure and volume. When pressure and volume are constant, Enthalpy can be defined as the total heat content or sum of all thermodynamic energies within a system. These energies include potential bond energies, potential nuclear energies as well as the sum of the kinetic energy of electrons. Chemists are unable to measure enthalpy because it is ...view middle of the document...
The following equation equates heat transfer in/out of the surrounding to the enthalpy change of the system being observed.
Hsystem= -Q= -(mass)(specific heat capacity)( in temperature)
These relationships make it possible to measure the energy change of a chemical transformation and thus, the change in enthalpy. This is useful for the study of thermochemistry because it can be used to determine the net energy that will be absorbed or released during a chemical transformation. This is even more valuable when expressed as a molar enthalpy (Joules per mole). This can easily be calculated by dividing H by the number of moles (n).
n=mass(m)molar mass (M)=mM
H°= Hn or H=n× H°
In the process of applying these relationships, a scientist may encounter a reaction that cannot be safely conducted within a calorimeter because of large energy releases or difficulty in accurately measuring the heat transfer. However, in 1840, Dr. Henri Germain Hess published the following law that now carries his name:
“the heat evolved or absorbed in a chemical process is the same whether the process takes place in one or in several steps”
Hess’ Law can also be referred to as the law of heat summation. So in accordance to this law, one can calculate the change in enthalpy of a reaction by splitting it up into multiple steps, and then summing up the enthalpy changes of each reaction to find the overall enthalpy change for the entire process.
The equations are manipulated by reversing the equations and multiplying the stoichiometric co-efficients. In order to reverse multiply the equations enthalpy change by -1 and switch the reactants with the products. Also, when multiplying the co-efficients of an equation by a constant k, one must also multiply the enthalpy change by the same constant. After manipulating the equations in this way, one can add all the products and all the reactant substances. Then if a substance is found on both the reactant and product side of the net equation, it can be removed from the net equation. The enthalpy changes are also summed, and in this way, the total enthalpy change can be determined.
By breaking up the combustion of magnesium into 3 equations, finding the enthalpy of each reaction using calorimetry for 2 equations and using the given enthalpy of formation for the last reaction, dividing by moles to find the molar enthalpy and finally using Hess’ Law to sum up the molar enthalpies (for Mg and MgO respectively), the molar enthalpy of combustion of Magnesium derived with less than 15% error (due to lack of extremely precise equipment and controlled lab environment).
The 4 equations are as follows:
(1) Mg(s) + 12O2g MgO(s) Hcomb° = ?
(2) Mg(s)+2HClaq H2(g)+MgCl2(aq) Hr°= ?
(3) MgO(s)+ 2HCl(aq) H2O(l)+ MgCl2(aq) Hr°= ?
(4) H2(g)+ 12O2(g) H2O(l) Hf° = -285.8kJ
One can then manipulate Equations 2-4 to create equation 1. First once can reverse equation 2 and then cancel out all the redundant...