Calorimetry can be defined as a process in which the changes of energy are measured within a chemical system. With the use of Styrofoam cups a calorimeter can be easily made. Styrofoam is a great source of insulation. This calorimeter is effective, however it lacks accuracy as a hole is created at the top of a Styrofoam cup which in return allows the exertion of gas. A more effective calorimeter is the bomb calorimeter which is usually used to measure combustion reactions. Through this process the volume of the reaction does not change throughout and stays the same.
The first reaction which is a part of the process of finding the enthalpy change of the combustion of magnesium is :
Mg(s) + 2HCl(aq) → H2(g) + MgCl2(aq)
The solid magnesium metal in the form of a ribbon reacts with hydrochloric acid to form a gas and salt. Magnesium Chloride and Hydrogen Gas are produced. This reaction is an exothermic reaction as heat is released. This chemical reaction is a heat transfer process. In this process the magnesium is the limiting reactant and will completely dissolve. The formation of water and magnesium chloride assists in the finding of the molar enthalpy of the combustion of magnesium. Through the chemical reaction of the magnesium oxide powder and the hydrochloric gas, heat will also be released.
MgO(s) + 2HCl → H2O(l) + MgCl2(aq)
Through the addition of both these equations and the use of Hess’s Law, the enthalpy of the combustion of magnesium can be found.
Apparatus
- Goggles
- Steel Wool
- Centigram Balance
- Thermometer
- Styrofoam Cups (2)
- 100 mL graduated cylinder
- Scoopula
- 10 – 15 cm strip of magnesium ribbon
- Magnesium Oxide Powder
- 1.00 mol /L hydrochloric acid
Method
- A hundred mL of 1.00mol/L of hydrochloric acid was measured and placed into a polystyrene cup (Styrofoam cup).
- The initial temperature of the acid solution was measured to the nearest 0.2°C.
- The magnesium ribbon was polished with steel wool and the mass of about 0.5 grams of magnesium metal was measured.
- The magnesium solid was then placed in the solution, stirred and maximum temperature was recorded when the solution reached its highest point.
- The products were disposed as directed by the teacher and the equipment was dried.
- The first five steps where then repeated again using approximately one gram of magnesium powder.
Observations
Calculations
Mass of Magnesium Oxide Powder
Beaker Mass: 28.41 g
Beaker Mass + MgO : 29.39 g
Mass of MgO : 29.39 g – 28.41 g = 0.98 g
Enthalpy change per mole of Magnesium metal
-
Moles of Magnesium
Mg= 0.515g
Mg= 24.31g/mol
Mg=
=
= 0.0212 moles of Magnesium
Enthalpy change of Magnesium Oxide powder
-
Moles of Magnesium Oxide
MgO= 0.98 g
MgO= 24.31g/mol + 16.00g/mol
= 40.31 g/mol
MgO =
=
= 0. 024 moles of MgO
The thermochemical equation for Magnesium is :
The thermochemical equation for Magnesium Oxide
The molar enthalpy of the combustion of magnesium:
The target equation is :
-
Mg (s) + 2HCl(aq) → H2(g) + MgCl2(aq) ΔH1= -276.0kJ
-
MgO(s) +2HCl→ H2O (l)+MgCl2(aq) ΔH2= -69.7kJ ( This equation must be flipped )
-
H2 + ½ O2(g) → H2O (l) ΔH3= -285.8kJ
Mg (s) + 2HCl(aq) → H2(g) + MgCl2(aq) ΔH1= -276.0kJ
H2O (l)+MgCl2(aq) → MgO(s) +2HCl ΔH2= +69.7kJ
H2(g) + ½ O2(g) → H2O (l) ΔH3= -285.8kJ
In order to obtain the target equation ,the second equation is flipped and this allows the MgO to stay, as it is needed for the target equation . The sign of the enthalpy change for the equation will also become positive.
In order to figure out the enthalpy change of the combustion of magnesium, the terms are crossed out and the ΔH from all three equations is added together.
ΔH= -276.0 + (+69.7) + (-285.8)
= -492.1kJ
Percentage Error
% error= measured – accepted x 100%
accepted
= -492.1kJ/mol – (-601.6kJ/mol) x 100%
-601.6kJ/mol
= -18.20%
∴ the percentage error of the enthalpy of combustion of magnesium is -18.20%.
The percentage error value of this is about 1/5 the actual value and this is due to the fact that
the equipment used in the lab was inaccurate (the calorimeter). The actual value of the molar
enthalpy of the combustion of magnesium was probably tested using a more accurate piece of
equipment and this could have been the bomb calorimeter. The bomb calorimeter would have
produced accurate values, whereas the Styrofoam calorimeter used was inaccurate and had
many deficiencies.
Extension Questions
- The changes were exothermic. Exothermic is the release of loss of heat. When the reactions occurred. In both reactions, the temperature had increased from the initial value. For example, for the magnesium strip there was a visible increase of about 14°C. Furthermore for the magnesium oxide an increase of 4°C was present as well. It is evident that exothermic reactions have taken place because, as stated before the state of exothermic refers to a release of heat. The system in this case released energy into the surroundings and as a result there was a increase in the temperature.
- The measured value that limited the precision of the value is the magnesium metal that was present in the first reaction. This is evident, due to the fact after the reaction occurred all the magnesium metal was gone.
- a) The calculated enthalpies of the reactions would be inaccurate if some heat were transferred to the air of the Styrofoam cup because there would be a loss of heat. There will be a decrease in temperature as gas would have left from the chemical reaction. Overall there would be a lower enthalpy value.
b) The calculated enthalpies of the reactions would be inaccurate if the surface of the magnesium ribbon had a coat of MgO because the reaction would occur slowly. The reaction would not provide an answer that is accurate as the magnesium would not be able to combust properly. The MgO present will reduce the magnesium’s to produce a bright flame. The magnesium oxide will react to give out less energy compared to pure magnesium reacting. As a result there would be a decrease in the temperature change and this would cause the enthalpy to be lower.
- Based on the evaluation of the experimental design and evidence, Hess’s Law is an acceptable way to calculate enthalpies of reaction. This is due to the face that when the molar enthalpy of the combustion of magnesium was calculated through Hess’s Law it was found that is was -492.1 kJ/mol. However the accepted value for the combustion of magnesium is actually a -601.6 KJ/mol. This is a huge difference, however through the calculations for the percentage error, it was evident that there was about an eighteen percent error and as a result it can be stated that Hess’s Law in actually acceptable.
- An experimental technique that could be used to determine the enthalpy of combustion of magnesium directly is through the usage of a bomb calorimeter. A bomb calorimeter is often used for combustion reactions. In this type of calorimeter, the volume present is not altered when the reaction occurs. Another technique that can be used is to replace the hydrochloric acid used in the experiment with water. When water is used instead of the acid, a change in temperature may occur and this will definitely provide a more accurate answer.
Experimental Uncertainty
-
There is a potential instrumental uncertainty that occurs from the centigram balance. The centigram balance was used to measure the magnesium and the beaker possesses an uncertainty of 0.0001g. This may provide inaccurate data and alter the mass, resulting in an inaccurate calculation of the enthalpy change. As well, there is also an opening present on the balance, and the mass of air may have been added to the other masses that were calculated.
- There is another potential instrumental uncertainty present in the graduated cylinder. The cylinder possesses an uncertainty of ±0.5mL and as a result the measured value of the hydrochloric acid can be inaccurate. As a result there would not be an exact amount of 100 mL of HCl present in reaction.
- A possible experimental error could have been present in the magnesium metal used. If all the MgO was not polished off using steel wool, it would result in an enthalpy that is lower than what would have been if the magnesium used was pure.
Conclusion
Exothermic reactions were produced when there was an increase in the temperature for the magnesium reactions. Using a calorimeter, for the first reaction with the magnesium metal, the molar enthalpy was -276kJ/mol. For the second reaction with the magnesium oxide, the molar enthalpy was a value of -69.7 KJ/mol. Through calculations using Hess’s Law, the enthalpy change was found to be a value of -492.1kJ.