Lab Heat of Combustion.pdf

LAB: COMBUSTION!! Name Group Period Purpose: To determine the heat of combustion for candle wax; to compare this heat of combustion to those of other hydrocarbons. Background: When you strike a match, a hot yellow flame is produced as the match combines with oxygen, which can be used to light a candle. When you burn the match, or the candle, or any other hydrocarbon, energy is released: the process of combustion is exothermic. The chemical reaction for this process is: C 25 H 52 + 38O 2 à 25CO 2 + 26H 2 O. In order to determine the amount of heat released by this process, we need to capture the heat; normally, we do this by collecting it in a sample of water of known mass. In this lab, you will measure the amount of heat released by burning a bit of wax, and calculate the heat of combustion of the wax. Helpful Equations: 1. 1. E = mcΔT; E is energy in Joules (or calories), m is mass in grams, c is specific heat capacity in J/g• o C (or cal/g• o C), ΔT is the change in temperature in degrees Celsius. 2. specific heat of water: c = 4.2 J/g• o C = 1.0 cal/g• o C Procedure: 1. GOGGLES AND APRONS…. 2. Melt a bit of wax from the bottom of the candle and use it to stick the candle securely to an index card. 3. Determine the mass of the candle and index card, and record this in your data table. 4. Carefully measure ABOUT 50 mL of water in the graduated cylinder, and pour it into the soda can. Record the exact volume you used (to the nearest tenth of a mL), and also the exact temperature (to the nearest tenth of a degree). 5. Set up the apparatus as shown on p. 204 (or as demonstrated by your teacher). Adjust so that the bottom of the can is just above the candlewick. 6. Light the candle, and place it directly beneath the can. Stir the water gently as it heats. Continue heating until the water reaches ABOUT 35 o C. 7. Extinguish the candle, but continue stirring the water and monitoring the temperature until it stops rising. Record the highest temperature reached. 8. Once they have cooled, measure and record the mass of the candle and index card. 9. Repeat the entire experiment (trial 2). Data: Heat of Combustion of Wax Volume of water (mL) Initial mass of candle/index card (g) Final mass of candle/index card (g) Initial temperature of water ( o C) Highest temperature of water ( o C) TRIAL 1 TRIAL 2

Prelab: 1. What is the purpose of this experiment? 2. What is the chemical equation for the burning of paraffin (wax)? 3. Define combustion: 4. Define exothermic: 5. What is the purpose of the card? 6. You will be using a soda can for a calorimeter, how will you support the can? 7. How are we measuring the heat released by the burning candle? 8. What is the specific heat of water or c water ? What does this number mean? 9. What is the formula (equation) for calculating the heat absorbed by the water? 10. If we had 100 grams of water in the can, and by heating the water with the candle raised the temperature 32°C, then how much heat (in Joules) was absorbed by the water? Use the formula from Question #9 and the value c water from question #8. 11. If the candle and card had a pre-­‐burn mass of 15.27 g and a post-­‐burn mass of 14.89 g how much paraffin (wax) was converted to products? 12. Heat of combustion is the amount of heat released per gram of burning substance. Using your answers from Questions #10 and Question #11, determine a heat of combustion for paraffin. (heat of combustion = heat released / grams burned)

Calculations: (Page 204-­‐205): Show your work for each calculation, including units. 1. Calculate the mass of water heated. (Hint: the density of liquid water is 1.0 g/mL. Thus each milliliter of water has a mass of 1.0 g) Trial 1: Trial 2: 2. Calculate the total rise in temperature of the water. Trial 1: Trial 2: 3. Calculate how much thermal energy was used to heat the water sample. Use values from the two preceding steps to reason out he answer, as illustrated in the sample problem on p.204. Trial 1: Trial 2: 4. Calculate the total mass of paraffin wax burned. Trial 1: Trial 2: 5. Calculate the heat of combustion of paraffin, expressed… a. In units of joules per gram (J/g) of paraffin. Hint: Assume that all the energy released by the burning paraffin wax is absorbed by the water. Divide the total thermal energy by the mass of paraffin burned to get the answer. Heat of combustion = Thermal energy released Mass of paraffin burned Trial 1: Trial 2: b. In units of kJ/g. Trial 1: Trial 2:

Questions: Answer in complete sentences below. 1. In calculating the heats of combustion, you assumed that all thermal energy from the burning fuel went to heating the water. a. Is this a good assumption? Explain. b. What other laboratory conditions or assumptions might cause errors in your calculated values? 2. Do you think an experiment using a more refined calorimeter would give a higher or lower value than the one you determined in your calculations? Explain. 3. Assume some black soot formed on the bottom of your soda can during your experiment. Would this contribute to a high or low value for your heat of combustion? Explain. Conclusion: What did you do? What was the purpose of the experiment? What did you determine and learn from conducting this experiment? What errors did you encounter while conducting this experiment?