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Chemistry 1°

Lab 67: Hess’s Law

OBJECTIVES

INTRODUCTION

Magnesium oxide (MgO) is produced when magnesium (Mg) burns at high temperatures, producing a bright, hot flame. It is difficult to measure the heat of reaction, or enthalpy, because the reaction is so rapid. Hess’s Law states that the enthalpy of a reaction is the same, despite the order the steps of the reaction occurred.

Magnesium oxide reacts exothermically with hydrochloric acid and produces magnesium chloride, liquid water, and heat. When solid Mg reacts with HCl (hydrochloric acid), magnesium chloride (MgCl2), hydrogen gas, and heat are produced. Heat is produced because the reaction is exothermic. The two previous reactions and the enthalpy for water, the change in enthalpy for magnesium burning in oxygen can be used to determined.

Pre-Lab Discussion

  1. What safety procedures need to be observed when working with each of the following compounds?
    1. Magnesium oxide:

      2. Since MgO dust is toxic if inhaled, all steps involving MgO should be executed under a fume hood.

    2. Magnesium chloride:

      3. Since MgCl2 is moderately toxic if ingested, fingers and hands should be kept away from the mouth.

    3. Hydrochloric acid:

    HCl is corrosive and should be handled carefully. If a splash or spill occurs, immediately wash with water and notify the instructor.

  2. State Hess’s Law in your own words.

    3. Hess’s Law states that the enthalpy change for a reaction is the same as the sum of the enthalpy changes of a series of reactions that give you the same product.

  3. Write the balanced chemical equations for the three reactions described in the investigation.

    4. 2Mg (s) + O2 (g) à 2MgO (s)

    MgO (s) + 2HCl (aq)à MgCl2 (aq) + H2O (l) + heat

    Mg (s) + HCl (aq)à MgCl2 (aq) + H2 (l) + heat

  4. What is the sign of the change in enthalpy for each of the three reactions used in this investigation. Why?

Since heat is being released in all three reactions, that means that the reaction is exothermic, making the enthalpy change negative.

MATERIALS

Chemical splash goggles

Laboratory apron

Magnesium oxide (MgO)

Laboratory balance

Graduated cylinder, 100-mL

Hydrochloric acid (HCl), 1.0 M

2 plastic foam cups

thermometer

cardboard cover for cup

piece of Mg ribbon

SET-UP

Take safety precautions and put on goggles and lab apron.

PROCEDURES

Part A
  1. Obtain a sample of MgO and measure the mass of the MgO to the nearest 0.0001 g. Record the data in Data Table 1: Observations Part A.
  2. Use a graduated cylinder to place 100.0 mL of 1.0 M HCl into a plastic foam cup.
  3. Measure the initial temperature of of HCl to the nearest 0.5 degrees Celsius and also measure the volume. Record both the temperature and volume in Data Table 1: Observations Part A.
  4. Place the cup in another cup.
  5. Taking safety precautions, work under a fume hood and add MgO to the HCl. Immediately cover the inner cup with a lid and insert a thermometer into the hole in the lid. Gently swirl the cup to mix the contents.
  6. Record the highest temperature reached by the MgO and HCl mixture in Data Table 1: Observations Part A.
  7. Dispose the MgCl solution as directed, rinse the cups and dry them with a paper towel.

Part B
  1. Obtain a 0.5000 g piece of Mg ribbon (about 25 cm). Measure and record its mass to the nearest 0.0001 g in Data Table 1: Observations Part B.
  2. Use a graduated cylinder to place 100.0 mL of 1.0 M HCl into a plastic foam cup.
  3. Measure the initial temperature of of HCl to the nearest 0.5 degrees Celsius and also measure the volume. Record both the temperature and volume in Data Table 1: Observations Part B.
  4. Place the cup in another cup.
  5. Add Mg to the HCl. Immediately cover the inner cup with a lid and insert a thermometer into the hole in the lid. Gently swirl the cup to mix the contents.
  6. Record the highest temperature reached by the MgO and HCl mixture in Data Table 1: Observations Part B.
  7. Dispose the MgCl2 solution as directed, rinse the cups and dry them with a paper towel.

RESULTS

Data Table A: Observations

Mass of MgO and Bottle: _ 8.1623g

Mass of Bottle: 7.1295g

Mass of MgO: 1.0328g

Part A

Mass of MgO (g)

1.0328

Volume of HCl (mL)

100.

Initial temperature of HCl (° C)

23.0

Final temperature of MgO/HCl (° C)

28.5

Part B

Mass of Mg (g)

.5071

Volume of HCl (mL)

100.

Initial temperature of HCl (° C)

24.0

Final temperature of Mg/HCl (° C)

47.0

CONCLUSIONS

Calculations

Part A
  1. Calculate the number of moles of MgO used.

    2. Mg: 24.31g 1.0328g MgO 1 mol MgO .02562 mol MgO

    O: +16.00g 40.31g MgO

    MgO: 40.31g

  2. Calculate the mass of the HCl solution. Assume the density of the HCl solution is the same as water (1.00 g/mL).

    3. 100.mL HCl 1.0g

    1 mL HCl

  3. Calculate the change in temperature

    4. Final Temperature: - 28.5ºC

    Initial Temperature: 23.0ºC

    5.5ºC

  4. Calculate the amount of heat released by the reaction. Ignore the heat capacity of the MgCl2, and assume the specific heat of the HCl solution is the same as water (0.00418 kJ/g-° C).

    5. q= mCpD T

    q=(100.g)(.00418kJ/g-ºC)(+ 5.5ºC)

    q= 2.299kJ

    = + 2.3 kJ

  5. Calculate the heat of reaction in kilojoules per mole of MgO.

2HCl(g) + MgO(s) à MgCl2(aq) + H2O(l)

.02562 mol MgO 2 mol HCl 2.3 kJ = .117852 kJ = +.12 kJ

1 mol MgO 1 mol HCl

Part B

  1. Calculate the number of moles of Mg used.

    2. .5071g Mg 1 mol Mg .0208597 mol Mg = .02086 mol Mg

    24.31g Mg

  2. Calculate the mass of the HCl solution. Assume the density is the same as water (1.0 g/mL).

    3. 100.mL HCl 1.0g

    1 mL HCl

  3. Calculate the change in temperature.

    4. Final Temperature: - 47.0ºC

    Initial Temperature: 24.0ºC

    23.0ºC

     

     

     

     

  4. Calculate the amount of heat released by the reaction.

    5. q= mCpD T

    q=(100.g)(.00418kJ/g-ºC)(+ 23.0ºC)

    q= 2.299kJ

    = + 2.3 kJ

  5. Calculate the heat of the reaction in kilojoules per mole of Mg.

 

 

 

Critical Thinking: Analysis and Conclusions

  1. Write the balanced thermochemical equation for the formation of one mole of liquid water from gaseous hydrogen and oxygen.

    2.  

  2. Based on your data, write a balanced thermochemical equation for the reaction of one mole of magnesium oxide with hydrochloric acid.

    3.  

  3. Based on your data, write a balanced thermochemical equation for the reaction of one mole of magnesium with hydrochloric acid.

    4.  

  4. Combine the three equations from Questions 1-3 so they will add to make a balanced thermochemical equation for the burning of one mole of magnesium in oxygen. You may have to reverse one or more of the equations.

    5.  

  5. Based on your data, calculate the change in enthalpy for the burning of magnesium in oxygen. Use the appropriate sign on your answer.

    6.  

  6. Calculate the percent error for this investigation given the known heat of reaction is –601.8 kJ/mol MG.

 

Critical Thinking: Applications
  1. Given the following information:

    2. NH3(g) + HCl(g) à NH4Cl(s) D H = -176.0 kJ/mol

    N2(g) + 3H2 (g) à 2NH3(g) D H = -92.2 kJ/mol

    N2(g) + 4H2(g) + Cl2(g) à 2NH4Cl(s) D H = -628.9 kJ/mol

    Calculate the D H for the synthesis of hydrogen chloride gas from hydrogen and chlorine gas. The equation is

    H2(g) + Cl2(g) à 2HCl(g)

    2NH4Cl à 2NH3 + 2HCl +176.0 kJ/mol

    2HN3 à N2 + 3H2 +92.2 kJ/mol

    N2 + 4H2 + Cl2 à 2NH2Cl -628.9 kJ/mol

    2NH2Cl + 2NH2 + N2 + 4H2 + Cl2 à 2NH3 + 2HCl + N2 + 3H2 + 2NH4Cl

    4H2 + Cl2 à 2HCl + 3H2

    +176.0 kJ/mol

    +92.2 kJ/mol

    -628.9 kJ/mol

    -360.7 kJ/mol

  2. The Calorie (note the capital C) mentioned in connection with foods is actually a kilocalorie (1000 calories). If 4.18 joules are equal to 1 calorie and a cup of ice cream releases 200 kilocalories, how many cups of ice cream release the same amount of energy as the reaction producing one mole of liquid water from its constituent gases?

    3.  

     

     

    Going Further:

        1. Investigate the life of Germain Hess, a Swiss chemist who in 1840 proposed the law of heat additivity that came to bear his name. Write a report and present it to the class.

Germain Henri Hess was born in Geneva in1802 and three years later he was brought to Russia. As a Swiss chemist and doctor, Hess was a pioneer in the field of thermo chemistry. Hess studied at the University of Dorpat (present day Tartu, Estonia) where he took part in a geological expedition to the Urals. Soon after, he set up a medical practice in Irkutsk. He settled in St. Petersburg, Russia in 1830 and became a professor at the Technological Institute. In 1840, Hess’s law was published and in 1842, Hess proposed his law of thermo neutrality, stating that "in exchange reactions of neutral salts, no heat effect is observed" (Hess). Hess died in 1850 at the age of 48 but managed to make a significant contribution to the world of chemistry before doing so.

 

REFERENCES

Hess, Germain Henri. Central Array of Relayed Transaction for the Advance of General Education. Retrieved October 28, 2003, from http://www.cartage.org.lb/en/themes/Biographies/MainBiographies/H/Hess/1.hmtl.