HEATS OF REACTION AND HESS’S LAW

Introduction

One sign that a chemical reaction is taking place in a mixture is the observation of a temperature change. The temperature change is detected by the emission of heat to, or the absorption of heat from the

surroundings. These are referred to as exothermic and endothermic processes, respectively. The measurement of the heat given off or absorbed is important because these changes reflect changes in the energy states of the molecules involved in the chemical reaction. In a chemical reaction, chemical bonds are broken, which requires energy, and new chemical bonds are formed, which is accompanied by the release of energy. The heat given off or absorbed in a chemical reaction is called the heat of reaction (or enthalpy of reaction, ΔH).

Sometimes a heat of reaction for a particular reaction is difficult to measure directly. Hess’s Law (also called the Law of Constant Heat Summation) provides a method for obtaining heats of reaction for reactions that cannot be easily measured.

Hess’s Law: If a process can be considered to occur in stages or steps (either actually or hypothetically) the enthalpy change for the overall process can be obtained by summing the enthalpy changes for the individual steps.

In this experiment you will measure the heat of reaction for the reaction between magnesium and hydrochloric acid. You will then measure the heat of reaction for the reaction between magnesium oxide and hydrochloric acid. Finally, these two heats of reaction along with the heat of formation for water and Hess’s Law will be used to determine the heat of formation of magnesium oxide.

Procedure

PartI. Determination of the Heat of Reaction of Magnesium with Hydrochloric Acid

In this part of the lab experiment, you will measure the molar heat of reaction for the reaction of magnesium with hydrochloric acid. The equation for the reaction is:

Mg(s) + 2HCl (aq)  MgCl2(aq) + H2(g)

The heat transferred in the reaction is measured in a “styrofoam cup” calorimeter (see figure below).

  1. Transfer 50.0 mL of 1.00 M HCl to the Styrofoam calorimeter.
  2. Put your thermometer through the Styrofoam lid and measure the temperature of the hydrochloric acid solution. Record this temperature on the data sheet as the initial temperature.
  3. Weigh out on the balance enough strips of magnesium to give a mass of about 0.12 gram. Record the mass of the magnesium to the nearest .001 gram on the data sheet.
  4. Lift the lid and the thermometer slightly and transfer the magnesium strips to the calorimeter. Put the lid and thermometer back in place. The reaction between the magnesium and hydrochloric acid will proceed.
  5. Gently rotate the calorimeter and stir the thermometer in order to get some stirring action. The temperature will increase.
  6. Record the highest temperature you see as the final temperature on the data sheet. It will probably take a couple of minutes to reach this highest temperature.
  7. Take the difference between the final temperature and the initial temperature, and record this as the temperature change.
  8. Pour the contents of the calorimeter down the drain. Rinse the calorimeter out with distilled water and dry it out with a paper towel to prepare it for the next reaction. Also, rinse the thermometer with distilled water and wipe it dry. Do a second trial.

Part II. Determination of the Heat of Reaction of Magnesium Oxide with Hydrochloric Acid

This part of the experiment involves measurement of the molar heat of reaction for the reaction of magnesium oxide with hydrochloric acid. The equation for the reaction is:

MgO(s) + 2HCl (aq)  MgCl2(aq) + H2O(l)

  1. Transfer 50.0 mL of 1.00 M HCl to the calorimeter.
  2. Measure the temperature of the hydrochloric acid solution and record this initial temperature.
  3. Determine the mass of a weight boat. Add enough magnesium oxide to the weigh boat to increase the mass by about 0.35 gram and record this as the initial mass of the weigh boat plus MgO.
  4. Weigh the weigh boat plus the MgO that was left and record this mass as the second mass of the weigh boat plus MgO. This difference is the mass of MgO reacted with the HCl solution.
  5. Transfer the MgO to the HCl solution; put the lid and thermometer in place. The reaction will proceed.
  6. Rotate the calorimeter and stir with the thermometer.
  7. Record the highest temperature you see as the final temperature. It will probably take about three minutes to reach this highest temperature. There will be some MgO powder left in weigh boat after it is transferred to the calorimeter.
  8. Pour the contents of the calorimeter down the drain. Rinse the calorimeter out with distilled water and dry it out with a paper towel to prepare it for the next reaction. Also, rinse the thermometer with distilled water and wipe it dry. Do a second trial.

Part III. Use of Hess’s Law to Determine the Heat of Formation of Magnesium Oxide

You will use the heat of reaction of magnesium with hydrochloric acid solution, the heat of reaction of magnesium oxide with hydrochloric acid solution and the heat of formation of water to make a Hess’s Law calculation of the heat of formation of magnesium oxide. The molar heats of formation of magnesium and magnesium oxide with hydrochloric acid are measured in parts A and B of this experiment. Use your measured values of the heats of reaction for these two reactions in your Hess’s Law calculation. The value of the heat of formation of water is well established in the chemical literature. For the chemical reaction: H2 (g) + 1/2 O2(g)  H2O (l) ΔH = -286 kJ

Use this value for the heat of formation of water in your Hess’s Law calculation. You will use the heats of reaction for the three preceding reactions to determine the heat of reaction for the reaction:

Mg(s) + 1/2 O2(g)  MgO(s)

Heats of Reaction and Hess’s Law Data Sheet

Name______Date______Section______

Part I. Determination of the Heat of Reaction of Magnesium with Hydrochloric Acid

Trial lTrial 2

Final temperature______

Initial temperature______

Temperature change______

Mass of magnesium______

Data Analysis

The heat of reaction will equal the heat (q) released by the solution in the reaction vessel. Since the solution is mostly water, we assume that its density is that of water 1.00g/mL. A 50.0 mL solution would have a mass of 50.0 g. Also, the specific heat of the solution is taken to be the specific heat of water, 4.184 J/gC. Record the heat of reaction in kJ. Calculate the number of moles of magnesium, the molar heat of reaction, and the average molar heat of reaction.

Trial 1Trial 2

Heat of reaction (kJ)______

Moles of Mg (mol)______

Molar heat of reaction (kJ/mol)______

Average molar heat of reaction (kJ/mol) ______

SHOW CALCULATIONS BELOW SO PARTIAL CREDIT CAN BE AWARDED!

Part II.Determination of the Heat of Reaction of Magnesium Oxide with Hydrochloric Acid

Trial 1 Trial 2

Final temperature______

Initial temperature______

Temperature change______

Initial Mass of the weigh boat+MgO______

Final Mass of the weigh boat + MgO______

Mass of MgO______

Data Analysis

The heat of reaction will equal the heat (q) released by the solution in the reaction vessel. Since the solution is mostly water, we assume that its density is that of water 1.00g/mL. A 50.0 mL solution would have a mass of 50.0 g. Also, the specific heat of the solution is taken to be the specific heat of water, 4.184 J/gC. Record the heat of reaction in kJ. Calculate the number of moles of magnesium oxide, the molar heat of reaction, and the average molar heat of reaction.

Trial 1Trial 2

Heat of reaction (kJ)______

Moles of MgO (mol)______

Molar heat of reaction (kJ/mol)______

Average molar heat of reaction (kJ/mol) ______

SHOW CALCULATIONS BELOW SO PARTIAL CREDIT CAN BE AWARDED!

Part III.Use of Hess’s Law to Determine the Heat of Formation of Magnesium Oxide

Write in the average molar heats of reaction for the reaction of magnesium with hydrochloric acid and the reaction of magnesium oxide with hydrochloric acid. Make a Hess’s Law calculation in the space provided below, using the heats of reaction of magnesium and magnesium oxide and the heat of formation of water, to calculate the heat of reaction for the reaction

Mg(s) + 1/2 O2 (g)  MgO(s)

Mg(s) + 2HCl(aq)  MgCl2 (aq) + H2 (g)ΔH = _____kJ (from Part I)

MgO(s) + 2HCl(aq)  MgCl2(aq) + H2O(1)ΔH = _____kJ (from Part II)

H2(g) + 1/2 O2 (g)  H2O (1)ΔH = -286kJ

Show Hess’s Law Calculationhere:

Compare your results to the literature value (-601.8 kJ) by determining the percent deviation.

% deviation______

Provide a plausible explanation of the observed difference in your value compared to the accepted value for the heat of formation of magnesium oxide.