Heat of Combustion of Paraffin Wax



Heat of Combustion of Paraffin Wax

Introduction

In this activity, you will calculate the heat of combustion of a candle (paraffin wax) and compare this quantity with the known value.

Materials:

Candle, index card, matches, ring stand, ring, stir rod, empty soda can, 100 mL graduated cylinder, digital scale, thermometer.

Procedure (check off each step as you finish it):

___1. Hold a lighted match near the base of a candle so that some

melted wax falls onto a 3 x 5 index card. Immediately push

the base of the candle into the melted wax. Hold the candle

there for a moment to fasten it to the card.

___2. Determine the combined mass of the candle and index card.

Record the value in your data table.

___3. Carefully measure out 100 mL of chilled water. (The chilled

water, provided by your teacher, should be 10 to 15°C colder

than room temperature). Pour the 100-mL sample of chilled

water into an empty soft-drink can.

___4. Set up the apparatus as shown in the diagram, but do not light the candle yet! Adjust the can so the

top of the candlewick is about 2 cm from the bottom of the can.

___5. Measure the water temperature to the nearest 0.1 °C. Record this value in your data table.

___6. Place the candle under the can of water. Light the candle. As the water heats, stir it gently.

___7. As the candle burns and becomes shorter, you may need to lower the can so the flame remains just

below the bottom of the can. CAUTION: Lower the can with great care.

___8. Continue heating until the temperature increases by at least 25ºC. (For example, if your water

temperature is 15ºC before heating, you should heat it to at least 40ºC).

___9. When the desired temperature is reached, extinguish the candle flame. Continue stirring the water

until its temperature stops rising. Record the highest temperature reached.

___10. Determine the mass of the cooled candle and index card, including all wax drippings, and

record the value in your data table.

Name ___________________________________ Due Date ___________

Lab Partner(s) __________________________________________________________

Heat of Combustion of Paraffin Wax

|Measurement |Value (with units) |

|Initial mass of candle plus index card | |

|Initial water temperature | |

|Final water temperature | |

|Final mass of candle plus index card | |

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Questions & Calculations:

1. If the density of water = 1.00 g/mL, calculate the mass of water heated (in grams).

Recall that density = mass/volume.

2. Calculate the temperature change of the water (∆T). The temperature change is equal

to the final temperature minus the initial temperature.

3. Calculate the heat (in joules) absorbed by the water using the formula below. The

specific heat capacity of water equals 4.18 J/g ºC.

Heat absorbed = (mass of H2O) x (Specific Heat of H2O) x (Temperature Change)

q = m x C x ∆T

4. Calculate the mass of paraffin burned in the experiment. This is the difference

between the initial mass of the candle and the final mass of the candle.

5. Using the formula below, calculate the paraffin’s heat of combustion in J/gram.

6. Convert the heat of combustion of paraffin into kJ/gram. 1 kilojoule = 1000 joules.

7. The actual heat of combustion of paraffin is approximately 42 kJ/gram. Using this

information and your answer to the previous problem, calculate your % error in this

experiment.

8. In calculating the heat of combustion, you assumed that all of the heat released from

the burning candle was absorbed by the water.

a. Is this a good assumption?

b. What else may have absorbed some of the heat? Be specific.



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