Heat of Combustion of Candle Wax

CONTENT:

            This lab deals with heat transfer, combustion of candle wax, heat loss within a system, and physical/chemical changes that occur as a result of energy transformations.  Transfer of heat is seen in weather studies (i.e., within thunderstorms the release of heat of condensation helps to power the cell), in absorption of solar radiation at the surface of the earth and re-release of that energy to the atmosphere (Greenhouse heating), and in biological systems when food is burned to provide energy for the organisms use.

  Energy is an important consideration in all physical and chemical changes.  This experiment has you look at the energy involved in the chemical change of candle wax.  We will be using joules instead of calories (1 cal = 4.185 joules or 1 joule = 0.2390 calories.)

OBJECTIVE:

            In this lab heat energy from a burning candle is directed to a Coke can filled with water.  The elevation in the temperature of the water as the candle burns indicates heat is being transferred.  Students should readily observe the principle of heat exchange.  Students must attempt to design the procedure and equipment such that heat is neither gained nor lost to the environment.  The student then must calculate the heat of combustion of candle wax (i.e., the amount of heat energy released per gram of wax consumed.)

   Remember the principle of heat exchange:  the heat lost by some object should be equal to the heat gained by the object to which the heat is transferred.  Look over the general set-up.  An aluminum Coke can filled with water is suspended over the burning candle.  How can we determine the amount of heat lost by the candle as it burns and how can be eliminate heat loss to the environment or gain from the environment?

  Make a drawing of the experimental setup.  Try to incorporate into your design and experimental procedure factors that will eliminate heat loss/gain from the environment. (hint: Simple insulation of the system is not enough.  Use your knowledge of heat transfer mechanisms, specific heats, temperature differences, etc. to design a system that will not product a high percent error.)  After the lab group decides on the factors they wish to control and decides how to build their apparatus, the group should confirm what information they wish to collect and in what order.

PROCEDURE:

I.         Attach candle to notecard base (with heated wax) and record mass of candle/card.  Record.

II.        Record mass of empty can (with rod inserted).  Add 150 mL of very cold water.  Record mass of can and water.  Calculate amount of water used.

III.      Record room temperature.

IV.       Design insulating system.  Remember that oxygen must get to the flame.

V.         Prior to lighting candle, record temperature of water in can.

VI.       With constant stirring as candle burns, heat water to a temperature as many degrees above room temperature as it was below room temperature prior to heating.

VII.     Put out candle flame as gently as possible.   Record final temperature (realize that the temperature may  in fact continue to go up after the flame is put out.  Calculate change in temperature.

VIII.   Mass candle/card.  Record.  Calculate change in mass of candle.

Sample Data Table:
mass of candle and base before burning:
mass of candle and base after burning:
mass of empty can:
mass of can and water:
room temperature:
temperature of water before heating:
temperature of water after heating:

CALCULATIONS:

1.         Using the specific heat of water, calculate the amount of heat transferred in both calories and joules.

2.         Calculate the heat of combustion of candle wax in both cal/g and J/g.

3.         Actual values for paraffin wax is about 42 kJ/g.  Calculate percent error with your value.

 Questions:

1. In what way did your insulation help control heat loss or gain?

2. Assuming some heat is always going to be gained from the room and that some heat will probably be lost, how can you have the two changes cancel each other out?

3. What purpose would be served by starting with cold water an equal number of degrees below room temperature as the number of degrees above room temperature finally reached?

4. To do the calculations, it was assumed that all the joules released by the burning was went totally into the water and that no extraneous joules of energy were absorbed by the water.  What factors in this experiment could have validated those assumptions.  What factors might have led to errors in the final results?

5. What further modifications in your procedure and experimental design would you now make to reduce error.

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 Problem:

A given birthday candle has a mass of 0.84 g.  The mass remaining after 1 minute of burning = 0.77 g.  What would be the burnout time for that candle.  Explain why your answer might not be exactly the same as the actual burnout time.

 Problem:

A 5.00 gram sample of motor oil is burned in a calorimeter.  The calorimeter contained 720. mL of water initially at 21.4° C.  After the oil was burned, the water temperature was measured at 33.9° C.  What was the heat of combustion of the motor oil in J/g?

Extended Problem:

You are given a block of wax that has a mass of 150. grams (molar mass = 450).  The melting point is 95.0° C, the specific heat of the solid is 0.80 J/g° C, the specific heat of the liquid is 1.2 J/g° C, the heat of fusion is

60. J/g.  The was is heated on a stove that provides 6.0 E 3 J of heat per minute.  The room temperature is 25 ° C.

a)  how many joules of heat are required to heat the wax to melting?

b)  how long will this take?

c) how many joules of heat are required to melt the wax and how long will this take?

d) how many joules of heat are required to heat the liquid wax to 245° C and how long will it take?

e) make a graph of the data.    What do the slopes of the solid heating and liquid heating indicate about the relative specific heats?

f) at 245° C the wax will burst into flame and completely react with the air to produce CO₂ and H₂O.  The heat of combustion is 1200 kJ/mole.  How many joules of heat are produced with the wax burns?  Is this enough to melt and heat to burning another block of wax of the same size?

 Refer to the Data Table below to answer questions:

1.  How much heat is required to melt a sample of 550 grams of copper?

 2.  How much heat is absorbed as a 95.0 gram sample of water is heated from 10.5° C to 48.2° C?

 3.  How much heat is absorbed by 25 grams of ethanol as it evaporates?

 4.  A 950. kg granite wall inside a solar house is used to absorb heat during the day.  During the day it reaches a temperature of 24.0° C .  At night, it will cool off to 17.0° C.  How much heat will be released by the wall?  How much heat would be released if the wall was made of 20. kg of aluminum filled with 180 kg of water and had the same temperature change?