A comparison between the Enthalpy of formation of MgO acquired via a series of reactions and Hess's Law (will they be equivalent?)

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Enthalpy of Formation


The enthalpy of formation (ΔH°f) is defined as the heat change associated with the formation of one mole of a compound from its elements in their standard states. The major obstacle here, though, is that it is very difficult to measure most enthalpies of formations in a standard laboratory due to the amount of heat or toxic products that are formed. Thus, in order to acquire the desired results, one must use Hess's Law in which the enthalpy of a series of reactions are measured and added together to give the final answer of the enthalpy of formation.

In this situation, to determine the enthalpy of formation for MgO is very difficult in the laboratory (Mg(s) + ½ O2 (g) ( MgO(s)). But, through a series of reactions and Hess's Law, the determination of the enthalpy of formation is made possible. Thus, in this investigation, we will see how accurate and precise Hess's Law can be.

MgO(s) + 2 H+(aq) ( Mg2+(aq) + H2O(l) ΔH°2 = ?

Mg(s) + 2 H+(aq) ( Mg2+(aq) + H2(g) ΔH°3 = ?

+ H2(g) + ½ O2(g) ( H2O(l) ΔH°4 = -286 kJ/mol

Mg(s) + ½ O2 (g) ( MgO(s) ΔH°f = ?

The series of reactions that will be used and whose enthalpies will be combined to give the enthalpy of formation of MgO are listed above. The first step, though, has MgO in the reactant side, but in the equation above that, MgO is supposed to be in the product side (enthalpy of formation). Thus, the sign for the enthalpy of the first reaction must be switched; if it's positive, then it becomes negative and vice versa.


If a series of reactions are used to determine the ΔH°f of MgO, then the experimental value acquired in this investigation should be very close to the accepted value due to Hess's Law.


Independent Variables:

The Molarity of the Acid used

The type and amount of the elements used

Dependent Variables:

The enthalpy of formation of the MgO

Control of Variables:

Several reactions that have been careful constructed are performed under special conditions to acquire the desired results.

Materials and Method


200ml of 3.0M HCl Graduated Cylinder

Magnesium Strips Thermometer

Styrofoam Cup w/Lid Weighing Scale

MgO powder

Method and Procedure

Part 1: MgO(s) + 2 H+(aq) ( Mg2+(aq) + H2O(l)

Take the Styrofoam cup and put 100 ml of 3.0 M HCl in it.

Weigh out about one gram of powdered MgO.

Make a very small hole in the lid of the Styrofoam cup just big enough for the thermometer to fit through so no heat escapes.

Record the initial temperature of the HCl

Then, place the MgO into the cup and quickly place the lid on it to prevent the heat from escaping

Pick up the cup and stir it and record the largest temperature reached by the reaction.

Part 2: Mg(s) + 2 H+(aq) ( Mg2+(aq) + H2(g)

Repeat every step from Part 1 except use about 0.5 g of Mg metal (strips) instead of the MgO and record the initial and final temperatures in the cup.