To determine the standard enthalpy of formation of Magnesium Oxide using Hess Law.

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International Baccalaureate Diploma Program (IBDP)

Session: May 2015

Chemistry HL Lab Report

Lab Report Title: To determine the standard enthalpy of formation of Magnesium Oxide using Hess’ Law.  

Criteria Assessed:

  • Data Collection and Processing (DCP)
  • Conclusion and Evaluation (CE)

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International School, Singapore

AIM: To determine the standard enthalpy of formation of Magnesium Oxide using Hess’s law.


The objective of this experiment was to determine the change in enthalpy when one mole of Magnesium (Mg) reacts with half a mole of Oxygen (O2) to give one mole of Magnesium Oxide (MgO). The balanced chemical equation is as follows:                                                       Mg (s) + O2 (g)  MgO (s) ---- ΔHMgO f

The reaction between Magnesium and Oxygen to form Magnesium Oxide is essentially the combustion of Magnesium and since every combustion reaction is an exothermic reaction, this reaction too is an exothermic reaction, i.e. it too will produce heat to the surroundings. In fact, the combustion of Magnesium is highly exothermic as it produces flames whose temperatures reach almost 2500oC ( At such high temperatures, a very bright white light is produced and if directly looked upon for long periods of time, the high content of ultra-violet radiation has the potential to damage unprotected eyes. Moreover, such high temperatures cannot be measured using a common thermocouple (K-type) so they need much more sophisticated setup of Ir-Rh thermocouples in an inert atmosphere. All these factors together make it extremely difficult to calculate the enthalpy of formation of MgO directly.

Swiss-born-Russian scientist, Germain Henri Hess had come up with the idea of calculating the enthalpy of reaction of a certain reaction using an alternate set of stepwise reactions which would add together in such a way that it would give the same reaction. The enthalpy of reaction for each of these reactions can then be added together to give the enthalpy of reaction of the primary reaction (formation of MgO, in this case). This is because he stated that according to the law of conservation of energy, the total enthalpy change of a reaction will depend only on the difference between the enthalpy of the product and the enthalpy of the reactants and not on what path it follows. This is known as Hess’s law.

The alternate set of stepwise reactions that were followed in this experiment to arrive at the value for the enthalpy of formation of MgO(s) are as follows:

  1. Mg (s) + 2HCl (aq.)   MgCl2 (aq.) + H2 (g)  ---------- ΔHX 
  2. MgO (s) + 2HCl (aq.)   MgCl2 (aq.) + H2O (g) ---------- ΔHY 
  3. H2 (g)  + O2  (g)  H2O (l) ---------- ΔHH2O = -285 kJ.mol-1 

Adding all the three reactions, we obtained the following reaction:

Mg (s) + 2HCl (aq.) + MgCl2 (aq.) + H2O (g) + H2 (g) + O2 (g)               MgCl2 (aq.) + H2 (g) + MgO (s) + 2HCl (aq.) + H2O (g)

  • Mg (s) + O2 (g)               MgO (s)

Since reaction 2 was reversed, the sign on the value of ΔHY was also reversed because the amount of heat required to form the reactants back from the products will be the same as the amount of heat given out when the reactants formed the product (in this case, the exothermic neutralization reaction of MgO and HCl). Therefore, to arithmetically calculate the value for the enthalpy of formation of MgO the following equation was then followed:                             ΔHMgO = ΔHX - ΔHY + ΔHH2O.

Experiment 1 and 2 could be safely and efficiently carried out under the school lab conditions with the objective to fulfill the aim.

To carry out the arithmetic calculations required, values of the reaction enthalpy of the all the three reactions were needed but since the value for ΔHH2O had already been provided to us by the lab in-charge, all that was needed to be determined were the values for ΔHX and ΔHY. These were determined separately by dividing the experiment into 2 parts called part X and part Y where part X was used to determine the value for ΔHX by carrying out reaction 1 and part Y was used to determine the value for ΔHY by carrying out reaction 2.  


Independent Variables:

  1. Length of Magnesium (For part X): For all the trials, a 3cm strip was cut away from the same roll of Magnesium ribbon. It was ensured that the strip cut was as straight as possible and also, the strip was clean of any contaminants. The strips were weighed each time on the same electronic balance and each time, the mass of Magnesium was approximately the same.  
  2. Mass of Magnesium Oxide (For part Y): For all trials, 0.05g of MgO was weighed out using the same electronic balance. The container claimed to contain MgO with only trace amounts of impurities.

Dependent Variables:

  1. Maximum temperature reached: Temperature of the solution was recorded using the same thermometer each time so that no additional systematic errors are introduced.
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Table 1: Controlled Variables


Table 2: Apparatus required for the experiment.

Table 3: Chemicals required for the experiment.


  1. Setting up the calorimeter – Two Styrofoam coffee cups were taken and one was placed inside the other with a rubber band in between the cups to create an air gap. A lid was placed on top and through the hole, a thermometer was placed through it. Once the calorimeter was setup, it was let aside.

Part X – With Magnesium Strip

  1. Using the ...

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