Precision Vacuum-Jacketed Dewar Calorimeter
Provides a thermally isolated reaction chamber with a silvered double glass wall minimizing conductive, convective, and radiative heat exchange with the ambient laboratory.
Chemistry / Hess's Law Calorimetry
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Thermochemical Verification of Hess's Law: Enthalpy of Formation of Magnesium Oxide
Determine the standard molar enthalpy of formation of solid magnesium oxide (ΔH_f°[MgO(s)]) via solution reaction calorimetry and Hess's Law of Constant Heat Summation, evaluating the dissolution enthalpy of magnesium metal in excess 1.0 M hydrochloric acid, coupling with standardized magnesium oxide and water formation cycles, and extracting the thermochemical formation enthalpy with ordinary least-squares regression. Explore interactive analytical gravimetry, Class A volumetric glassware and density determination.
MEASURE · CALIBRATE · ANALYSE
Your virtual chemistry laboratory.
ADIABATIC TEMPERATURE RISE ΔT₁—
MEASUREMENT STATUS
Ready to dispense 0.2 g of liquid.
Adjust magnesium mass V, then weigh the dispensed aliquot.
Mass observations are determined by the high-precision thermodynamic equation of state for pure liquid water. Readings reflect thermal equilibrium and analytical balance force restoration.
Physical chemistry assumptions and metrological limits
The experimental method assumes negligible heat capacity variation with temperature over the 4–15 °C reaction excursion, complete stoichiometric consumption of magnesium ribbon without hydrogen bubble entrapment, and identical calorimeter water equivalents between metal dissolution and oxide reference runs. Minor trace vaporization of hydrochloric acid and water vapor carried out with escaping hydrogen gas contributes an evaporative heat loss of under 0.25%, which is effectively mitigated by the multi-point linear regression intercept. The physical chemistry simulation model is governed by the calorimetric first-law relation ΔT₁ = [ |ΔH₁| / (M_Mg · C_eff) ] · m + ΔT₀. System physical parameters: magnesium atomic molar mass M_Mg = 24.305 g/mol; standard hydrochloric acid volume V_acid = 100.00 mL (solution density ρ = 1.015 g/mL, mass m_soln = 101.5 g, solution specific heat capacity c_p = 4.10 J/(g·K)), calorimeter water equivalent heat capacity C_cal = 27.5 J/K. Total effective system heat capacity C_eff = m_soln · c_p + C_cal = 101.5 · 4.10 + 27.5 = 443.65 J/K = 0.44365 kJ/K. Reaction 1 literature dissolution enthalpy: ΔH₁ = −462.00 kJ/mol. Theoretical thermometric slope: m_slope = |ΔH₁| / (M_Mg · C_eff) = 462.00 / (24.305 · 0.44365) = 42.8458 °C/g. For magnesium ribbon masses from m = 0.100 g to 0.350 g, adiabatic temperature rises span ΔT₁ = 4.285 °C to 14.996 °C. Thermochemical cycle reference enthalpies: ΔH₂ [MgO(s) + 2HCl(aq) -> MgCl₂(aq) + H₂O(l)] = −146.00 kJ/mol; ΔH₃ [H₂(g) + 1/2 O₂(g) -> H₂O(l)] = −285.83 kJ/mol. Hess algebraic combination: ΔH_f°[MgO(s)] = ΔH₁ − ΔH₂ + ΔH₃ = −461.99 − (−146.00) + (−285.83) = −601.82 kJ/mol (accepted thermodynamic reference ΔH_f° = −601.83 kJ/mol). Multi-point linear regression insulates the slope from minor non-adiabatic heat leakage, hydrogen gas vapor carry-out (<0.27%), or initial temperature differences between acid and solid magnesium.
Analytical chemistry apparatus & tools.
NIST-Calibrated Digital Thermistor Probe (±0.001 °C)
Measures fine temperature fluctuations continuously at 2 Hz during the pre-period baseline, rapid exothermic dissolution, and post-period Newton cooling decay.
Motorized PTFE Paddle Stirrer (200 rpm)
Maintains rapid, uniform convective mixing across the 100 mL aqueous reaction volume without creating excessive viscous frictional dissipation or mechanical heating.
Analytical Four-Decimal Balance (±0.0001 g)
Accurately weighs abraded magnesium ribbon segments and dry powdered magnesium oxide to establish exact stoichiometric molar charges.
Class-A Volumetric Pipettes and Dispenser (100.00 ± 0.05 mL)
Dispenses precise volumes of standardized 1.000 M aqueous hydrochloric acid into the calorimeter cell ensuring identical system heat capacities across trials.
Fine Emery Sandpaper and Fume Hood Assembly
Removes surface passivating oxide/hydroxide coatings from magnesium metal strips prior to weighing, while venting evolved flammable hydrogen gas safely.
FOLLOW THE METHOD
A reason behind every step.
Determine the standard molar enthalpy of formation of solid magnesium oxide (ΔH_f°[MgO(s)]) via solution reaction calorimetry and Hess's Law of Constant Heat Summation, evaluating the dissolution enthalpy of magnesium metal in excess 1.0 M hydrochloric acid, coupling with standardized magnesium oxide and water formation cycles, and extracting the thermochemical formation enthalpy with ordinary least-squares regression. Suggested independent volumetric settings: 0.1, 0.15, 0.2, 0.25, 0.3, 0.35 g.
01
Calorimeter System Preparation and Acid Charging
Clean and dry the inner Dewar reaction vessel. Dispense exactly 100.00 ± 0.05 mL of standardized 1.000 M hydrochloric acid into the calorimeter. Seal the vessel with the PTFE lid, insert the thermistor probe and motorized stirrer, and activate stirring at 200 rpm.
02
Magnesium Metal Surface Pre-Treatment and Gravimetry
Polish high-purity magnesium ribbon strips with fine emery cloth to remove tarnished surface oxide layers until metallic silver luster is exposed. Cut ribbons into small (~5 mm) segments, wipe free of dust, and weigh on the analytical balance to ±0.0001 g.
03
Pre-Period Thermal Baseline Acquisition
Record the calorimeter temperature at 10-second intervals for 5 minutes (300 s) prior to reagent addition. Establish a steady, linear initial temperature drift rate (dT/dt < 0.001 °C/min) to serve as the pre-reaction thermal datum.
04
Reagent Introduction and Exothermic Dissolution
Rapidly introduce the weighed magnesium metal through the PTFE lid access port into the acid under continuous stirring. Immediately reseal the port. Monitor the vigorous exothermic evolution of hydrogen gas and record temperature continuously as the metal dissolves.
05
Post-Period Cooling Curve Tracking
After the reaction reaches peak temperature, continue logging thermistor readings every 10 seconds for at least 10 minutes (600 s). Measure the characteristic exponential Newton cooling decay rate of the calorimeter vessel.
06
Regnault-Pfaundler Extrapolation and Hess Cycle Evaluation
Graph the thermogram, extrapolate the pre- and post-reaction baseline slopes to the 63.2% reaction midpoint, and extract the corrected adiabatic temperature rise ΔT₁. Compute ΔH₁ = −(C_eff · ΔT₁) / n_Mg, and combine with parallel ΔH₂ (MgO dissolution) and literature ΔH₃ (water formation) via Hess's law to determine ΔH_f°[MgO(s)].
YOUR NOTEBOOK · 0 OBSERVATIONS
Turn measurements into quantitative chemical insight.
Points are your recorded gravimetric observations. The regression line validates the linear mass-volume proportionality and extracts density.
ORDINARY LEAST-SQUARES FIT · m = ρ·V + m₀
ΔT₁ = [|ΔH₁| / (M_Mg · C_eff)] · m + ΔT₀ ==> ΔT₁ = 42.845 · m
| # | Magnesium ribbon mass m (g) (g) | Adiabatic temperature rise ΔT₁ (°C) | Magnesium mass m (g) | Corrected temperature rise ΔT₁ (°C) | Remove |
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