Comparing the Energy Released By combustion of Different Alcohols

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.

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

The negative values mean energy is lost from the compounds and is therefore given out as heat or light as an exothermic reaction.

The graph shows that as the length of alcohol chains increases the energy released by combustion will also increase directly proportionally. This is shown by the straight line passing through the origin on the graph on page 6. This is because the longer chains have give out more energy from the bonds formed in the products than the energy that went in to break the extra carbon and 2 hydrogen bonds.

During this investigation I will be using propan-1-ol and butan-1-ol. The 1 refers to the structure of the compound, as Propanol and Butanol are capable of having more than one structure but share the same molecular formula. This is important as other isomers will release different levels of energy due to the different arrangement of bonds.

PREDICTION

The evidence given previously states that the longer the chain of the molecules the more energy it will release. Therefore I expect Methanol to give of the least energy and Butan-1ol to release the most amount of energy. This is due to the enrrgy of the products in Butanol being higher than the energy in Methanol.

This is a preview of the whole essay

PREDICTION

My graph should look the same as the one on the previous page for the theoretical energy release values. However, I will expect my results to show a lower level of energy release because not all of the energy will go into heating the water. Some of the heat will be lost by being blown away and some energy will take the form of light.

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 much will not heat up enough to be recorded accurately and too little will heat up so much it boils and the results are useless. Therefore I decided to use 70cm³ (70g) of distilled water as it was heated up enough to be recorded acuratly but did not boil.

How often do I stir the calorimeter?

The calorimeter needs to be stirred so that the water is moved around and uniform heating occurs. Uniform heating means that all the water is the same temperature and not just the bottom of the water is heated. Uniform heating make results more accurate. I decided to plunge the stirring wire down and up once every 5 seconds in order to mix the water.

How long do I heat the calorimeter for?

The length of time the water is heated for is important as a short period of time will not heat the water sufficiently enough to take accurate results. However, heating the water for too long will boil the water to 100°C and therefore the results will not show any trend, as this is the maximum temperature for water. I chose to heat the water for 150 seconds (2.5min) as this was a suitable time.

METHOD

Set-up apparatus as shown in diagram.

Fill calorimeter with 100cm³ of distilled water and record temperature in °C with thermometer for 30 seconds to allow an accurate reading. Read thermometer at eye level in order to reduce parallax error due the reflection of light through the glass.

Select burner and check the height of the flame is 4cm using a ruler. Change length of wick using tweezers to change the height of the flame.
Weigh the burner filled with the desired alcohol with the lid on it using a top pan balance. Ensure the balance is on a flat stable surface and place burner on pan gently in order to keep reading accurate. I shall use the first four alcohols in the series: methanol, ethanol, propan-1-ol and butan-1-ol.
Record mass in grams to the nearest hundredth.
Clamp the calorimeter 7cm above the top of the burner and place lid on calorimeter properly. Measure the height of calorimeter above burner using a ruler.
Place burner on a heat-proof mat, under calorimeter; remove burner lid and light immediately with a match. Withdraw match immediately as the heat of the wood combusting could effect the temperature of the water in the calorimeter. Begin timer immediately when wick is lit. Do not remove lid before match is lit as some alcohol may evaporate and alter the burners weight before it is lit.
Plunge stirrer up and down once every 5 seconds to ensure uniform heating. During heating keep lid on calorimeter to prevent the water evaporating and heat loss.
Record waters temperature every 30 seconds by using thermometer at eye-level. Do not remove thermometer from water to take reading, as this will change its temperature.
After 150 seconds of heating extinguish flame by replacing burner’s lid.

Record the water’s final temperature in °C.

Re-weigh burner and record mass in grams to the nearest hundredth.
Repeat 3 times and take an average of results.
Repeat for all four alcohols.
Calculate energy released by each alcohol per gram using the formula below:

Energy Released per gram= C x Mw x /\T

(J/g) Mf

Where, C= specific heat capacity of water (4.17 J/g°C)

Mw=mass of water (g)

/\T=change in temperature (°C)

Mf=Mass of fuel used (g)

Calculate energy released per mol using average J/g and the following formula:

Energy released per mol= Mr x J/g

(J/mol)

Where, Mr=molecular mass

J/g=Energy released per gram (average)

Safety Precautions

Because this investigation involves the use of open flamed burners, I will need to be very careful in the laboratory. I will remove all loose pieces of clothing that may fall into flame and set light.
I will wear safety goggles to protect my eyes as I am heating a liquid, which could spit, into my eyes when hot.
I will use a heat-proof mat to protect the workbench from the heat of the burner.
I will abide by all standard laboratory rules such as: Do not run.

Results

PTO

CONCLUSION

The graph shows that as the length of the molecule chain in an alcohol increases so to does the amount of energy it gives of when completely combusted. The curved ‘line of best fit’ shows that the relationship is not proportional.

The energy released increases because as the length of molecule chain increases because the larger chains only need a small input of energy to break the original bonds yet gives out a larger amount of energy when the new bonds are formed. The energy of the products is lower compared to that of the reactants in longer chain molecules than smaller ones.

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.