Introduction
This lab is an experiment involving thermodynamics that aims to determine the enthalpy change of a chemical reaction through calorimetry. Enthalpy is a thermodynamic quantity that is related to the amount of heat released or absorbed during a chemical reaction. Hess’s law is based on the principle that the enthalpy change of a chemical reaction is independent of the pathway taken, and is only dependent on the initial and final states of the system. In this experiment, the objective is to determine the enthalpy change of a reaction by indirectly measuring the heat released or absorbed during the reaction using calorimetry. The enthalpy change of a reaction can be calculated using Hess’s law and the enthalpy changes of other reactions. This experiment involves finding the enthalpy of two reactions and utilizing one known enthalpy to determine the enthalpy change of a fourth unknown reaction. Calorimetry is a technique used to measure the heat of a chemical reaction by measuring the temperature change in a system. In this experiment, the temperature change is measured using a calorimeter, a device designed to insulate the system and measure the temperature change. The calorimeter used in this experiment is a coffee cup calorimeter, which consists of a styrofoam cup with a lid and a thermometer. The enthalpy change of a reaction can be calculated using the equation:
ΔH = q / n
Where:
→ ΔH is the enthalpy change of the reaction,
→ q is the heat absorbed or released by the reaction
→ n is the amount of substance in moles.
Hess’s law can be used to calculate a reaction’s enthalpy change if other reactions’ enthalpy changes are known. Hess’s law states that the enthalpy change of a reaction is equal to the sum of the enthalpy changes of the reactions that make up the overall reaction, where each reaction is multiplied by a coefficient to balance the equation. Overall, the Hess’ law experiment is an important experiment in the field of thermodynamics, as it allows for the determination of the enthalpy change of a chemical reaction by measuring the temperature change in a system.
This information can be used to understand the energetics of chemical reactions better and to make predictions about the behavior of chemical systems.
Purpose
The purpose of this investigation is to use Hess’s law to determine the molar enthalpy of combustion of magnesium, using calorimetry.
Hypothesis
If the change in enthalpy is observed in the reaction of Mg and MgO with HCl, then the resulting reaction will be an exothermic reaction with an enthalpy change of -618.9 kJ
Question
What is the molar enthalpy of combustion, ΔHC of magnesium?
Materials
- Lab apron
- Eye protection
- Steel wool
- Centigram or milligram balance
- Thermometer
- Polystyrene calorimeter
- 100-mL graduated cylinder
- Scoopula
- 10- to 15-cm strip of magnesium ribbon
- Magnesium oxide powder
- 1.00 mol/L hydrochloric acid
Experimental Design
Measured masses of magnesium and magnesium oxide will be added to measured volumes of known concentration hydrochloric acid solution. The temperature changes will be determined. Calculated enthalpies of the reaction will be combined, using Hess’s law, to determine the enthalpy of combustion of magnesium.
Procedure
- Measured 100.0 mL of 1.00 mol/L hydrochloric acid into a polystyrene cup
- Measured the initial temperature of the acid solution to the nearest 0.2°C
- Polished a length of magnesium ribbon with steel woolDetermined the mass (± 0.01g) of approximately 0.5 g of magnesium metal and added it to the solution containing 1.00 mol/L hydrochloric acid
- Stirred the solution and recorded the maximum temperature that the solution attains
- Disposed of the products as directed by the instructor, rinsed and dried the equipment
- Repeated steps 1 to 6 using approximately 1 g of magnesium oxide powder (± 0.01g)
Pre-Lab Question
* Show how the three known equations and their enthalpies of reaction can be combined, using Hess’s law, to yield the target equation and its enthalpy of combustion
* Therefore, the calculated ΔH of magnesium is -618.9 kJ
Observations
| Item | Temperature (°C) |
| 100 mL of 1.00 mol/L HCl (aq) | 21.9 |
| with ≈ 0.5 g of Mg (s) | 40.6 |
| 100 mL of 1.00 mol/L HCl (aq) | 24.5 |
| with ≈ 1 g of MgO (s) | 29.1 |
Analysis
* Was the enthalpy changes exothermic or endothermic?
During the experiment, it was observed that both chemical reactions release heat into their surroundings, indicating they were exothermic reactions. The overall enthalpy change, which reflects the amount of heat absorbed or released during a reaction, showed a negative value, further confirming that the reactions were exothermic. When magnesium metal (Mg (s)) and magnesium oxide (MgO (s)) were added to the system, the temperature of the solution and the surrounding environment increased. This was due to the release of thermal energy by the exothermic reactions, which was absorbed by the surrounding solution. The release of this energy to the surroundings, in addition to the heat released by the reactions, is indicative of an exothermic change.
The energy absorbed by the surrounding solution was transferred from the system reaction.
The reaction’s change in enthalpy is:
* Which of the measured values limited the precision of your value?
In the process of conducting the experiment, it became clear that the majority of the values, including substance mass and volume, were under control, especially the mass of the hydrochloric acid, magnesium, and magnesium oxide, as well as the enthalpy change of the water and its specific heat capacity. Most of the numbers used in the calculations had nothing to do with the experimental procedures and depended on how long after the reaction had finished before the temperature was recorded. Temperature could change at any time due to its environment or the thermometer’s inherent inaccuracy. Furthermore, every time the calorimeter was opened, heat could escape, changing the reaction’s temperature. Thus, the one set of
measured values in this experiment that was responsible for errors in the final enthalpy value was the temperature.
Evaluation
* Explain how and why your calculated enthalpies of reaction would be inaccurate if…
i) Some heat were transferred to the air or Styrofoam cup;
If some heat were transferred to the air, the heat leaving the closed system would not be accounted for when measuring temperature. The continuous heat loss to the air would decrease temperature, making it difficult for experimenters to determine the accurate amount of energy required for the reaction to occur initially. The calculations for enthalpies would show lower temperature values as the HCl (aq) would absorb less heat, ultimately resulting in a lower overall value for the enthalpies of the reaction. During the experiment, the heat of solution was used to determine the heat of reaction, and if some heat was lost to the air, the enthalpies of the reactions would be less exothermic than the theoretical value.
ii) The surface of the magnesium ribbon had a coating of MgO;
If the magnesium ribbon had a coating of magnesium oxide (MgO (s)), it would be oxidized during the reaction with hydrochloric acid, and this would impact the overall enthalpy of the reaction. The reaction between MgO and HCl would result in oxygen reacting with the acid before the magnesium could, increasing the temperature of the solution. As the reaction lengthens, more energy would be released to the surroundings, resulting in a decrease in the final enthalpy of combustion of magnesium. In summary, the presence of a coating of MgO on the surface of a magnesium ribbon would decrease the enthalpy of the reaction, making it inaccurate to measure the actual enthalpy change. It is, therefore, essential to ensure that the magnesium ribbon used in the experiment is clean and free of any impurities that may impact the reaction.
Suggest some other possible sources of experimental error in this investigation. There are several sources of experimental error that could have affected the results of this investigation. One source of error is related to the equipment used in the experiment. For example, if the thermometer was not calibrated properly, it could have produced inaccurate temperature readings, leading to errors in the calculation of the enthalpy change. Additionally, if the calorimeter used to hold the HCl was not completely dry and contained some water, it would have introduced extra water into the system, leading to reactions which were unaccounted for.
Another source of error is related to the assumptions made during the experiment. The assumption that the specific heat capacity of the metal was constant may not have been accurate, as the specific heat capacity can vary with temperature and other factors. There may also have been errors introduced during the experiment’s data collection and analysis phases. For example, if the temperature readings were not taken at the same time intervals, it could have affected the accuracy of the calculated results.
Additionally, if the measurements were not recorded accurately, or if there were errors in the calculations performed to determine the enthalpy change, this could have led to inaccuracies in the final results. Finally, errors may have been introduced due to human error or bias. Errors such as misreading measurements or incorrectly recording data could have affected the accuracy of the calculated enthalpy change.
* The accepted value for the molar enthalpy of combustion of magnesium is -601.6 kJ/mol. Calculate a percentage difference by comparing your experimental values and the accepted values.
The percentage difference between the experimental value and the accepted value is 4.8 %
Is Hess’s law an acceptable method to calculate enthalpies of reaction?
During the lab experiment, a comparison was made between the theoretical and experimental values using Hess’s Law to calculate enthalpies of reaction. It was observed that the percentage difference between the theoretical and experimental values was approximately 4.8%, which is below the conservative acceptable percent difference of 5%. Other potential sources of error, such as heat loss through the calorimeter and other human errors, were also taken into account. After considering these factors, it can be concluded that Hess’s Law is an acceptable method to calculate enthalpies of reaction. The results obtained from the experiment demonstrate that Hess’s Law provides reliable values for enthalpies of reaction, indicating its usefulness in practical applications.
Synthesis
* Suggest an experimental technique that could be used to determine the enthalpy of magnesium combustion directly.
Bomb calorimetry is an experimental technique that is highly effective in determining the enthalpy of combustion of a substance, including magnesium. This method is based on the principle that when a substance is burned in an oxygen-rich environment, the heat generated during the combustion reaction can be used to calculate the enthalpy of combustion. The process of bomb calorimetry involves placing a sample of the substance inside a bomb, which is then filled with pure oxygen. The bomb is placed in a calorimeter containing water and the initial temperature of the water is recorded before the combustion reaction is initiated.
When the substance in the bomb is ignited, the heat released by the reaction is transferred to the surrounding water in the calorimeter, causing its temperature to rise. The temperature change is then monitored until it reaches a stable value, indicating that the reaction is complete. By measuring the mass of the substance burned and the temperature change of the water, it is possible to calculate the enthalpy of combustion of the substance using the formula q = m x c x ΔT, where q is the heat released, m is the mass of water, c is the specific heat capacity of water, and ΔT is the temperature change of water. Bomb calorimetry is a highly precise technique, providing accurate and reliable measurements, which could be used to determine the enthalpy of combustion of magnesium.
Conclusion
This lab aimed to determine the molar enthalpy change for the combustion of Mg through Hess’s Law. The final molar enthalpy change in the reaction was observed to be -631.21 kJ. This value was acquired using Hess’s law to determine the total enthalpy change through the reaction of Mg, HCl, MgO, and HCl. Using the enthalpy changes from these two reactions, we were able to calculate the total enthalpy change for the combustion of Mg.
This lab proved useful in demonstrating the use of a calorimeter and Hess’s Law to determine the change in enthalpy of a reaction. The lab also allowed us to observe the change in temperature and energy produced due to an exothermic reaction and how different types of reactions can have an impact on the energy produced. A few things that could be improved in a future experiment could be to ensure that no energy is lost to the surroundings which could have had an effect on the temperature readings. In conclusion, this lab successfully demonstrated calculating the enthalpy change of a reaction using Hess’s Law.
