Thermochemistry: An Ice Calorimeter Determination of Reaction Enthalpy

Thermochemistry: An Ice Calorimeter Determination of Reaction Enthalpy

September 12, 2011

Abstract: We determined the enthalpy of reaction for 1.00M sulfuric acid and solid magnesium by measuring heat transfer from the system to surroundings using an ice calorimeter. The equation for the reaction is〖 H〗_2 SO_(4(aq))+Mg_((s)) □(→┴( ) ) MgSO_(4(aq))+H_2. Our experimental enthalpy of reaction was -324 kJ/mol, compared with the theoretical value of -470.9 kJ/mol, showing a 31.2% error. We attribute this difference to halting data collection before the reaction came to completion.

Introduction

Chemical reactions either require an input of energy or they evolve energy, releasing it to the surroundings. We can measure energy transfer using calorimetry, which allows us to monitor the energy transfer based on a temperature change or some other physical change. We can use this data to calculate the enthalpy of a reaction, or the heat of a reaction, when there is a measurable heat transfer in the case of temperature change or when there is a measurable change in the volume of a substance. We can also measure exothermic heat transfer from the system to the surroundings with the surroundings held at a constant temperature, or at phase equilibrium, using calorimetry techniques. In the case of isothermal reactions, where heat energy is transferred at constant temperature, we can measure volume changes in the surroundings, or the calorimeter chamber, consisting of an ice and water mixture, in our experiment.1 When calculating the experimental enthalpy of a reaction, we are really finding the energy required to break bonds compared to the energy released while making bonds. When the energy released from making bonds exceeds the energy needed to break bonds, or when the product substances have stronger bonds than the reactants, the reaction is exothermic. Measuring heat transfer and calculating the enthalpy of a reaction using Thermochemical data rather than by using bond energies is a more accurate method of finding the heat of a reaction, which was the purpose of this experiment. 2

Methods

We measured 5.00 mL of 1.00 M H2SO4 using a 5.00 mL TD pipet and cut 0.2273 grams of Magnesium ribbon that we weighed on the scale in the lab into 0.5 cm pieces after first removing as much existing oxide coating as possible with steel wool. We used the stopwatch function on a cell phone for tracking time.

The apparatus we used for our procedure was an ice calorimeter intended to measure an isothermal process, or a transfer of heat energy, at constant temperatures. The ice calorimeter stopper was modified to hold a reaction test tube and a pipet for reading volume changes as well as a fill tube, with the stopper fitting into the opening of a beaker acting as the calorimeter chamber. Instead of measuring a temperature change in the surroundings, we measured the heat transfer by monitoring the volume change in the surroundings, or the water and ice mixture in the calorimeter chamber. The heat energy from the reaction in the test tube transferred into the surrounding ice and water mixture in the beaker, melting the ice and allowing us to read a volume change in the pipet, which we recorded at 30 second intervals.

We insulated the entire apparatus in an ice bath in order to maintain the environment at a constant temperature, but there was no way to completely prevent heat leaks. The reaction itself could not be completely isolated. With Hydrogen as a product of this reaction, there was an opportunity for heat loss when the gas escaped from the reaction test tube. However, we also removed air from the system, in order to ensure that the volume change was only caused by the ice changing phase into liquid water. The apparatus was insulated and the sulfuric acid was cooled with the apparatus for 10 minutes as we watched the water levels in the pipet, and finally recorded the pre-reaction volume change at 30 second intervals for five minutes. Heat from the reaction of sulfuric acid and magnesium was slowly absorbed by the surroundings, or the water and ice in the calorimeter chamber, which allowed the system and surroundings to stay in equilibrium while the ice melted.3 We were able to record the volume change that occurred during the reaction and use those values to calculate our heat of reaction.

Data

Table 1. Ice Calorimeter Data

Before Reaction

Time (s) Pipet (mL)

0 .869

30 .865

60 .865

90 .862

120 .861

150 .860

180 .859

210 .858

240 .857

270 .856

300 .855

During Reaction

Time (s) Pipet (mL)

330 .812

360 .735

390 .665

420 .608

450 .560

480 .527

510 .500

540 .489

570 .460

600 .450

630 .439

660 .430

690 .419

720 .412

750 .408

780 .400

810 .399

After Reaction

Time (s) Pipet (mL)

840 .392

870 .389

900 .385

930 .382

960 .379

990 .377

1020 .374

1050 .372

Figure 1. Calorimeter Data

Calculations

The reaction of Sulfuric Acid and Magnesium in the Ice Calorimeter evolved heat causing a quantity of ice to melt. This lowered the volume of