Measuring the Heat Energy of a Chemical Change

Patricia Riley                 Lincoln Park High School
                               2001 N. Orchard St. Mall
                               Chicago IL 60614
                               (312) 534-8130

Objectives:

To compare the heat conducting properties of water and paper by boiling water.
To study the Law of Conservation of Energy.
To study the differences between heat and temperature.
To calculate the heat produced by a burning candle.
To calculate the heat produced by a burning nut.
To compare the heat produced by equal masses of different types of nuts.

Materials needed:

Teacher's demonstration:        Equipment per group of 4:
  1 ring stand                    thermometer           test tube
  1 small ring clamp              candle                test tube holder
  paper cups                      petri dish            large cork
  tongs                           graduated cylinder    dissecting pin
  goggles                         matches               disposable lighter
  apron                           water                 various nuts
                                  balance

Strategy:

1.  Fit a paper nut cup into the ring clamp, attach the clamp to the ring stand, 
fill the cup to the brim with water, and place a candle under the cup.  Ask the 
students to make predictions as to what will happen when the candle is lighted.  
Light the candle.  On the chalk board record the observations.  In a separate 
column record the questions this experiment raises among the students.  In a 
third column list possible answers (hypotheses) suggested by the students for 
their questions.  Where ever possible test the hypotheses with a brief 
experiment.  Have the students draw a conclusion:  the cup does not burn because 
the water removes the heat from the paper before the paper becomes hot enough 
to burn.
2.  Now investigate how much heat is produced by the candle.  Explain that a 
calorie is a unit of heat.  Ask students what they know about calories; most 
will say that foods contain calories, exercise burns calories, and calories can 
be counted.  Emphasize that a candle burns wax and air to release calories of 
energy (heat and light), just as our body burns food, though without a flame, to 
release energy (heat, muscle energy, etc.).  Have the students work in groups 
and:
     a.  First attach the candle to a petri dish.  Then weigh the dish and 
candle on a balance.
     b.  Measure 10mL of water into a graduated cylinder and then pour into a 
test tube.
     c.  Take the temperature of the water.
     d.  Use a test tube holder to hold the test tube.  Light the candle.  Hold 
the test tube in the flame, while gently moving the test tube in small circles.
     e.  Continue heating for two or three minutes.  Take the temperature of the 
water just before removing the test tube from the flame.  Be sure the 
thermometer does not touch the glass as its read.
     f.  Blow out the candle.  Weigh the candle and the petri dish.
     g.  Record all observations:  water temperature before heating, the water 
temperature after heating, volume of water heated, mass of candle and dish 
before heating, mass of candle and dish after burning, mass of candle burned.
     h.  Record also any questions raised by the group during the experiment.
     i.  Calculate the heat produced by the burning candle:
          Heat produced = Heat absorbed =  Mass of X  Temperature X  1 cal   
          by candle       by water         water      change of       g x oC
                                                      water
     j.  Calculate the heat produced by one gram of candle:
          Heat produced by one gram = Heat produced by burning candle = cal/g
                                       Mass of candle burned
     k.  Pool the class data on the board.  Discuss the questions the students 
raised in their groups.  Students should draw conclusions:  even though the 
groups all burned different masses of candle, they all came up with the same 
number of calories per gram (cal/g).
3. Now consider whether all substances that burn produce the same amount of heat 
per gram burned.  Students may say that foods do not all contain the same number 
of calories.  Have the students again work in their groups to explore this by 
repeating step 2 for three varieties of nuts (suggested:  Brazil, peanuts, 
walnuts; be sure they are raw, not roasted).  A nut is stuck onto a dissecting 
pin (already inserted into a cork).  Burn the nut instead of the candle.  Ignite 
the nut with a disposable lighter.  Heat the test tube of water until the nut 
stops burning.  Record all data and questions raised.  Pool class data.  Draw 
conclusions:  all varieties of nuts do not produce equal amounts of energy.

References:

Schug, Ken.  Demonstration given during S.M.I.L.E., Fall, 1992.
Summerlin, Lee R. and Ealy, James L., Jr.  Chemical Demonstrations:  a Source 
    Book for Teachers, Volume I.  American Chemical Society, Washington, D.C., 
    1988.