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# Comparing the Energy Released By combustion of Different Alcohols

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Introduction

COMPARING THE ENERGY RELEASED BYCOMBUSTION OF DIFFERENT ALCOHOLS

## AIM

To investigate and compare the relationships between the amounts of energy released from different alcohols in the alcohol series.

## BACKGROUND INFORMATION

Alcohols are a series of organic homologous compounds with the general formula of C(n) H(2n+1) OH. They gain their properties, which are different to those of other compounds in the homologous series due to the OH bonded to the Carbon. The 4 simplest alcohols are Methanol, Ethanol, Propanol and Butanol. Each alcohol has one more carbon and two more hydrogens than the previous one.

When alcohols are heated to react in a copious supply of oxygen they undergo complete combustion to form carbon dioxide and water. The reaction is an exothermic one and produces a flame emitting light and thermal energy. This is because the energy put into the alcohol to break the bonds is less than the energy given out by new bonds forming. Therefore, different alcohols release different amounts of energy. The diagrams and calculations on the following sheets show the theoretical energy release of each alcohol using the bond energy values below.

 BOND TYPE BOND ENERGY(KJ/mol) 0-H 463 C-H 412 C-O 360 C=O 805 (Carbon dioxide) O=O 496 C-C 348

The calculations show that: (the negative values mean energy is lost from the compounds and is therefore given out as heat or light)

 ALCOHOL ENERGY RELEASED (KJ/mol)) Methanol -659 Ethanol -1279 Propan-1-ol -1899 Butan-1-ol -2519

Middle

## PRELIMINARY INVESTIGATION

• How high do I place the calorimeter above the burner?

I found that a decent sized flame was 4cm and therefore I clamped the calorimeter 7cm above the top of the burner. This allowed for the flame to burn freely without the bottom of the calorimeter interfering with the combustion and making it unfair.

• How large should the flame be?

Although the size of the flame will not effct the results diretly because the rate at which the alcohol burns is irrelevant due to the fat that the change in mass is also measured. However, the size of the flame can make the results inaccurate in other ways. I found that the small flames flickered to much and did not produce a constant enough source of heat. However, I also found that the large flames were blown around a lot by the slightest of drafts and therefore the heat was not always being transferred into the calorimeter. I decided that 4cm was a good height to use as it was not affected by the drafts too much and did not flicker.

• How much water do I fill the calorimeter with?

The amount of water in the calorimeter needs to be a sensible amount as too

Conclusion

The results support my prediction in the way that I predicted that as the length of the chain increased so to would the energy released. However, I also predicted that the graph would show directly proportional results, which it does not. My prediction also stated that the results would be lower than the theoretical values due to heat loss which proved to be correct.

## EVALUATION

I believe the method I used was as accurate as possible with the equipment provided, although the results do not show this. The energy released per mol values are a lot lower than the theoretical values due to heat loss. Because of the large amount of heat loss the results are quite varied and unreliable. I only encountered one anomalous result, which is highlighted in red on the results table. I believe this is because of less heat being lost during this test than the was lost during the other tests for propan-1-ol. To overcome this problem barrier could be placed around the burner to shield it from drafts and guide the heat towards the calorimeter.

I believe that the results show a curve because more heat was lost for the small-chained molecules due to the lower temperatures of flame being blown away more easily than those flames with higher temperatures.

This student written piece of work is one of many that can be found in our GCSE Electricity and Magnetism section.

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