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Heat of Comustion of Magnesium

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Heat of Comustion of Magnesium

Name _____________________ Class ______________ Date _________

    Activity C23: Heat of Combustion - Magnesium

    (Temperature Sensor)

Concept DataStudio ScienceWorkshop (Mac) ScienceWorkshop (Win)

    Reactions & energy C23 Combustion.ds C22 Heat of Combustion C22_COMB.SWS

Equipment Needed Qty Equipment Needed Qty

    1 Stirring rod 1 Temperature Sensor (CI-6505A)

    1 Protective gear PS Balance (SE-8723)

    1 Base and Support Rod (ME-9355) Chemicals and Consumables Qty

    Beaker, 250 ml 1 Hydrochloric acid (HCl), 1.00 Molar 200 ml

    1 Magnesium oxide (MgO) 1 g Clamp, Buret (SE-9446)

    Graduated cylinder, 100 ml 1 Magnesium ribbon (Mg) 0.5 g

    Slit stopper 1 Styrofoam cup 1

     Weighing paper 2

    Purpose

    The Heat of Combustion of a substance is a piece of information that chemists and chemistry students need to know when conducting chemical reactions. The amount of heat given off by one substance may be different than the amount of heat given off by a similar substance. In this activity you will calculate the Heat of Combustion of magnesium ribbon. Background - Calorimetry

    Calorimetry is the measure and study of heat transfer. In calorimetry, a substance is burned, usually in air or in pure oxygen. For example, you could “burn” a strip of magnesium metal and use the energy given off to heat a known quantity of water in a calorimeter cup. (This is a

    traditional method for measuring the heat of a reaction in many

    chemistry labs.)

    What Do You Think?

    Based on your experiences, what do you think are some possible sources of error in the calorimeter method? How might these sources of error effect the measurement of the Heat of Combustion of a substance?

    Take time to answer the ‘What Do You Think?’ question(s) in the Lab Report section.

    (Note: See the „Optional‟ section of this activity for more information about the calorimetry

    method.)

    Background Heat of Reaction

    A second method to determine the Heat of Combustion is also a traditional method, but it does not directly “burn” the substance. Instead, this method uses the energy released or absorbed during simple chemical reactions to calculate indirectly the Heat of Combustion.

    For example, when magnesium combines with oxygen during combustion, the reaction is represented by the equation:

    IV. Mg (s) + 1/2 O (g) -> MgO (s) 2

    C23 ? 1999 PASCO scientific p. 169

Chemistry Labs with Computers Student Workbook

    C23: Heat of Combustion - Magnesium 012-07005A

    (Magnesium and oxygen form magnesium oxide.)

    If other reactions have the same overall outcome, then you can combine the energy released or absorbed by each of the other reactions to calculate the energy produced by the first reaction.

    Combining equations is a valid method for calculating the Heat of Combustion. Here are three reactions that are equivalent to the combustion of magnesium. Note: You will be asked to confirm that equations I, II, and III are equivalent to equation IV.

    I. MgO (s) + 2 HCl (aq) -> MgCl (aq) + HO (l) 22

    (Magnesium oxide and hydrochloric acid form magnesium chloride and water.) II. Mg (s) + 2 HCl (aq) -> MgCl (aq) + H (g) 22

    (Magnesium and hydrochloric acid form magnesium chloride and hydrogen gas.) III. H (g) + 1/2 O (g) -> HO (l) 222

    (Hydrogen and oxygen form water.)

    Pre-Lab

    In the space provided in the Lab Report section, combine equations I, II, and III to obtain

    equation IV.

SAFETY REMINDERS

     Wear protective gear while handling chemicals.

     Follow directions for using the equipment.

     Dispose of all chemicals and solutions properly.

    For You To Do

    Use the Temperature Sensor to measure the change in temperature during two chemical reactions of magnesium and hydrochloric acid. Use ScienceWorkshop or DataStuidio to record, display,

    and analyze the data.

    Use the data to calculate the heat of reaction for magnesium oxide and hydrochloric acid (equation I) and the heat of reaction for magnesium and hydrochloric acid (equation II).

    Use the heats of reaction for I, II, and III to determine the heat of reaction for IV the Heat of

    Combustion for magnesium.

    The heat of reaction for equation III above is ?H = -285.8 kJ/mol.

    p. 170 ? 1999 PASCO scientific C23

Name _____________________ Class ______________ Date _________

PART I: Computer Setup

    1. Connect the ScienceWorkshop interface to

    the computer, turn on the interface, and

    turn on the computer.

    2. Connect the DIN plug of the Temperature

    Sensor to Analog Channel A on the

    interface.

    3. Open the file titled as shown:

    DataStudio ScienceWorkshop (Mac) ScienceWorkshop (Win)

    C23 Combustion.ds C22 Heat of Combustion C22_COMB.SWS

     The DataStudio file has a Workbook display. Read the instructions in the Workbook The ScienceWorkshop document has a Digits display and a Table display of Temperature. Data recording is set for one measurement per second.

    PART II: Sensor Calibration and Equipment Setup

    You do not need to calibrate the sensor.

    1. Use a base and support rod, a clamp, and a slit stopper to support a Temperature Sensor as

    shown.

    2. Place a Styrofoam cup into a 250-mL beaker as shown in the diagram. Measure out 100.0

    ml of 1.00 Molar HCl into the Styrofoam cup.

    3. Lower the Temperature Sensor into the solution.

    Clamp

    Slit stopper

    TemperatureSensor

    Styrofoam cup

    Beaker

    4. Tare your balance to the weight of the weighing paper. Weigh out about 1.00 g of

    magnesium oxide, MgO, on a piece of weighing paper. Record the exact mass used in your

    data table.

    Safety Alert!

    C23 ? 1999 PASCO scientific p. 171

Chemistry Labs with Computers Student Workbook

    C23: Heat of Combustion - Magnesium 012-07005A

    Magnesium oxide dust is mildly toxic by ingestion. DO NOT INHALE the dust! p. 172 ? 1999 PASCO scientific C23

Name _____________________ Class ______________ Date _________

PART IIIA: Data Recording - Magnesium Oxide and Hydrochloric Acid

    1. When everything is ready, start recording data.

    2. After about five seconds, add the white magnesium oxide powder to the solution. Observe the change in temperature on the Digits display.

    3. Use the stirring rod to stir the contents of the cup until a maximum temperature has been

    reached and the temperature starts to drop. Then stop recording data. 4. Remove the Temperature Sensor from the cup and rinse the end of the sensor. 5. Discard of the solution as directed and rinse the cup.

    PART IIIB: Data Recording - Magnesium Ribbon and Hydrochloric Acid

    6. Repeat the procedure with 0.50 g of magnesium ribbon rather than magnesium oxide

    powder. Be sure to measure and record the mass of the magnesium ribbon. 7. When everything is ready, start recording data.

    8. After about five seconds, add the white magnesium oxide powder to the solution. Observe the change in temperature on the Digits display.

    9. Use the stirring rod to stir the contents of the cup until a maximum temperature has been

    reached and the temperature starts to drop. Then stop recording data. 10. Remove the Temperature Sensor from the cup and rinse the end of the sensor. 11. Discard of the solution as directed and rinse the cup.

     You will have two runs of data at the end of the data recording. Analyzing the Data

    1. Set up the Table display so it has two columns: one for the first run of data (magnesium

    oxide and hydrochloric acid) and one for the second run of data (magnesium ribbon and

    hydrochloric acid).

    2. Use the Table data analysis tools to find the initial temperature T and maximum 1

    temperature T for Run #1 (magnesium oxide and hydrochloric acid). Record these values. 2

    3. Find the initial temperature T and maximum temperature T for Run #2 (magnesium 12

    ribbon and hydrochloric acid). Record these values.

    Calculations

    1. Calculate the change in temperature, ?T, for both reactions.

    2. Calculate the energy released by both reactions, q, using the formula

    q = Cm?T p

     Cp = 4.18 J/g?C, and m = 100.0 g of HCl solution. Convert joules to kJ in your final answer. 3. Determine the heat of reaction, ?H, for both magnesium oxide and magnesium ribbon.

    (Remember, ?H = -q.)

    C23 ? 1999 PASCO scientific p. 173

Chemistry Labs with Computers Student Workbook

    C23: Heat of Combustion - Magnesium 012-07005A

    4. Determine the moles of magnesium oxide (MgO) and magnesium ribbon (Mg) used. 5. Use your calculation of the heats of reaction and the number of moles to calculate ?H/mol

    for magnesium oxide (MgO) and magnesium ribbon (Mg).

    6. Determine ?H/mol for magnesium (Mg) based on the heats of reaction for magnesium

    oxide and hydrochloric acid (Reaction I), magnesium ribbon and hydrochloric acid

    (Reaction II), and hydrogen and oxygen. (Reaction III.)

     Remember, ?H for Reaction III is 285.8 kJ/mol

    7. Determine the percent difference between your calculation and the accepted value for the

    Heat of Combustion. The accepted value for this reaction can be found in a table of

    standard heats of formation.

    Formulas

    TT;Tfi

    q(4.18J/gC)100.0g,?T

    H;;q

    massMgO#molesMgO 40.3g/mol

    massMg#molesMg24.3g/mol

    HH/mol#moles

    Actual;Experimental%difference100Actual

    Record your results in the Lab Report section.

    p. 174 ? 1999 PASCO scientific C23

Name _____________________ Class ______________ Date _________

C23 ? 1999 PASCO scientific p. 175

Chemistry Labs with Computers Student Workbook

    C23: Heat of Combustion - Magnesium 012-07005A Lab Report - Activity C23: Heat of Combustion - Magnesium

    What Do You Think?

    Based on your experiences, what do you think are some possible sources of error in the

    calorimeter method? How might these sources of error effect the measurement of the Heat of

    Combustion of a substance?

Data Table

    Data Item Reaction I (MgO) Reaction II (Mg)

    1 Volume of 1.00 M HCl mL mL

    ?C ?C 2 Final temperature, T 2

    ?C ?C 3 Initial temperature, T 1

    ?C ?C 4 Change in temperature, ?T

    5 Mass of solid g g

    6 Heat, q kJ kJ

    7 ?H = -q kJ kJ

    8 Moles mol mol

    9 ?H/mol kJ/mol kJ/mol Determine the Heat of Combustion (Reaction IV) based on the heats of reaction for I, II, and III: Eqn. Reactants Results Heat of Reaction

    MgCl (aq) + HO (l) -> MgO (s) + 2 HCl (aq) I. ?H = 221

    Mg (s) + 2 HCl (aq) -> MgCl (aq) + H (g) II. ?H = 222

    H (g) + 1/2 O (g) -> HO (l) III. ?H = 2223

    Mg (s) + 1/2 O (g) -> MgO (s) IV. ?H = 24

    Determine the percent difference between your calculation and the accepted value (-602 kJ/mol).

    Percent difference =

    p. 176 ? 1999 PASCO scientific C23

Name _____________________ Class ______________ Date _________

Optional Calorimeter Method

    In this method, use the heat given off by a piece of burning magnesium to heat up a known volume of water. Use the mass of water heated, its specific heat (the specific heat for water is c = 1.0 cal/g ?C or 4.18 J/g?C, and the change in the temperature of the water to calculate the heat of combustion of the magnesium.

    This method is often used to calculate the caloric content of many foods such as peanuts or sugar. (Refer to the “Food Energy Content” activity.)

    Follow the procedure in the “Food Energy Content” activity. Modify the equipment set-up so that

    a magnesium ribbon can be burned instead of a food sample. Use a crucible to hold the burning magnesium. Use extreme caution when working with burning magnesium.

    Safety Alert!

    Never look into the flame produced by the burning magnesium or directly at the light given off from the combustion. The light energy is very intense and contains UV radiation that could possibly damage your eyes and vision.

    Follow the same calculation method as in the “Food Energy Content” activity. Note any and all possible sources of error that may result using this method.

    Questions

    1. What is the Heat of Combustion of magnesium based on your data from the calorimeter

    method?

    2. What is the percent difference between your calculation and the accepted value?

3. Which method do you think was more reliable?

C23 ? 1999 PASCO scientific p. 177

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