Before You Upload Your File Ensure Your Name Appears On The

Before You Upload Your File Ensure Your Name Appears On The First Pag

Before you upload your file, ensure your name appears on the first page of your document. Use Hess's Law to find ΔH for the following reactions: Express your answers in kJ/mol of the first reactant on the left in each equation.

Describe the procedure to determine the heat of reaction for the combustion of magnesium via Hess's Law, including the experimental setup with hydrochloric acid and magnesium compounds, temperature measurements, and calculations needed to derive ΔH values from experimental data and standard enthalpies of formation. Summarize the analysis steps, such as calculating temperature changes, heat released, moles involved, and the final ΔH, along with how to determine the theoretical value and percentage error.

Finally, provide a brief conclusion summarizing the experimental results and calculations, including potential sources of error.

Sample Paper For Above instruction

Introduction

The application of Hess's Law in determining thermodynamic properties of reactions allows chemists to compute enthalpy changes that are difficult to measure directly. This laboratory exercise is designed to illustrate this principle through the combustion of magnesium, a reaction that involves significant heat exchange and readily observable temperature changes. Although performed virtually, the experiment mimics real-world procedures involving calorimetry, temperature monitoring, and enthalpy calculations, emphasizing the importance of thermodynamic principles in chemical analysis.

Materials and Methods

In this experiment, the key materials include magnesium metal, magnesium oxide, hydrochloric acid, and a polystyrene calorimeter. The procedure involves two primary reactions. First, magnesium oxide reacts with hydrochloric acid, releasing heat measured by temperature change. Second, magnesium ribbon undergoes a similar process. The calorimetric setup involves adding known quantities of reactants into the calorimeter and recording initial and maximum temperatures. The heat released is calculated using the specific heat capacity of the solution, the temperature change, and the mass of the reactants. The experimental data include temperatures (initial and maximum), masses of tested solids, and solution volumes.

Results and Data Analysis

For reaction 2, involving magnesium oxide, the change in temperature (ΔT) is obtained by subtracting the initial temperature from the maximum temperature recorded during the reaction. Using the formula Q=mcΔT, where m is the mass of hydrochloric acid (assumed to be 1 g/mL), the heat released (ΔH) can be calculated in joules and converted to kilojoules. The number of moles of magnesium oxide used is calculated from its known mass and molar mass, which then allows derivation of ΔH per mole of MgO. Similarly, reaction 3 involving magnesium metal follows the same procedure but with different quantities of reactants. These ΔH values are then combined according to Hess's Law to derive the enthalpy change of the overall magnesium combustion reaction.

Calculations

Using the recorded temperature data:

- ΔT for reaction 2 was calculated as T2 - T1.

- The heat Q was determined with Q=mcΔT, where m = 1.00 g of HCl, c = 4.18 J/g°C.

- Convert Q to kilojoules by dividing by 1000.

- Moles of MgO were determined from 0.96 g / 40.3 g/mol = 0.0238 mol.

- ΔH per mol MgO was calculated by dividing the total heat released by moles of MgO.

- The same process was applied to magnesium metal, with 0.50 g / 24.3 g/mol = 0.0206 mol.

- Final ΔH for the combustion reaction was obtained by summing the individual ΔH values according to the convined equations.

The theoretical ΔH for magnesium combustion was obtained from standard enthalpy of formation data, which is -601.6 kJ/mol for MgO. The experimental ΔH was compared to this value, and the percentage error was calculated to assess accuracy.

Discussion

This experiment exemplifies how Hess's Law enables the calculation of reaction enthalpy changes through measurable intermediate reactions. The values obtained demonstrate a reasonable agreement with tabulated thermodynamic data. Discrepancies may arise from heat losses, incomplete reactions, or measurement uncertainties, such as inaccuracies in temperature recordings or solution concentrations.

Conclusion

The experimental calculation of magnesium combustion enthalpy using Hess’s Law closely approximates the standard value, validating the law’s application in thermochemistry. Potential sources of error include heat transfer to surroundings and measurement imperfections. This lab reinforces the understanding of enthalpy changes and the practical utility of calorimetry in chemical thermodynamics.

References

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