The aim of this experiment is to calculate the enthalpy change in various alcohols.

pentanol        C5H11OH

hexanol        C6H13OH

octanol        C8H17OH

The reaction that is involved in burning alcohols is exothermic, as heat energy is lost. The energy is given off when the alcohol reacts with the oxygen in the air to form water and carbon dioxide. The longer alcohol molecules need more energy to break their bonds compared to smaller molecules, and so will be less energy efficient than the smaller alcohol molecules.

METHOD

The following equipment is needed:

Seven different alcohols (see above)                        Copper calorie meter

Spirit lamp                                                        Cork lid        

Clamp stand                                                        Aluminum foil

Distilled water                                                Measuring cylinder

Thermometer                                                Weighing scales

Matches                                                        Stop watch

PROCEDURE

• Carefully place required alcohol into spirit lamp
• Measure and record the weight of the spirit lamp and alcohol contents
• Place spirit lamp with alcohol content onto base of stand
• Place calorie meter onto clamp and lower so that the base of the calorie meter is approximately 2cm above the spirit lamp. DO NOT adjust this height when repeating the procedure
• Measure 50cm³ of distilled water
• Pour into calorie meter
• Put cork lid onto calorie meter, and place thermometer through the central hole in the lid. Take care to make sure that the thermometer is not touching the base of the calorie meter
• Wrap a loose layer of aluminum foil around the clamp stand to cover the calorie meter and spirit lamp, leaving a small gap at the base
• Record the start temperature of the water
• Light spirit lamp and record temperature of water at 30 second intervals for three minutes
• After three minutes, extinguish flame on spirit lamp
• Measure and record the new weight of the spirit lamp and its alcohol contents
• Allow thermometer and calorie meter to cool
• Repeat procedure for remaining alcohols

RESULTS

CALCULATIONS AND OBSERVATIONS

Methanol: CH3OH

∆H=mc∆t

∆T=49-22        =27°C

∆H=50x4.2x27

∆H=5,670j/g

5670/1000        =5.67

∆H=5.674j/kg

Propanol: C3H7OH

∆H=mc∆t

∆T=50-20        =30°C

∆H=50x4.2x30

∆H=6,300j/g

6300/1000        =6.3

∆H=6.3j/kg

Pentanol: C5H11OH

∆H=mc∆t

∆T=48.5-23        =25.5°C

∆H=50x4.2x25.5

∆H=5,355j/g

5355/1000        =5.355

∆H=5.355j/kg

Octanol: C8H17OH

∆H=mc∆t

∆T=52-21                =31°C

∆H=50x4.2x31

∆H=6,510j/g

6510/1000        =6.51

∆H=6.51j/kg

Butanol: C4H9OH

∆H=mc∆t

∆T=45.5-22.5        =23°C

∆H=50x4.2x23

∆H=4,830j/g

4830/1000        =4.83

∆H=4.83j/kg

Hexanol: C6H13OH

∆H=mc∆t

∆T=47-22.5        =24.5°C

∆H=50x4.2x24.5

∆H=5,145j/g

5145/1000        =5.145

∆H=5.145j/kg

Ethanol: C2H5OH

∆H=mc∆t

∆T=57-24                =33°C

∆H=50x4.2x33

∆H=6,930j/g

6930/1000        =6.93

∆H=6.93j/kg

CONCLUSION

The principle is that the more carbon atoms that are present in the alcohol molecule, the more energy is given out. As shown in this experiment, Butanol is the worst performer as it was the least energy efficient. Ethanol is the best for combustion, as it is the most energy efficient. To improve this experiment, an increased range of alcohol could be used in order to support this conclusion.