methanol, propanol, pentanol, heptanol and octanol.
Below are the different chemical bonds. Each requires a different amount of energy to break itself:
Below is a table with the information I am likely to find to find after doing my practical. If we know the number of bonds broken and formed, the alcohols formula, and the amount of energy we can work out the theoretical Enthalpy of combustion /kJ mol.
Hypothesis
For this investigation I will be looking at how the number of carbon atoms effects the number of kJ per mole. The trend is quite clear that the Theoretical Enthalpy of combustion /kJ increases as the number of Carbon and Hydrogen atoms increases. Therefore I can say that the kJ per Mole will also increase as the number of carbon and hydrogen atoms increases. Take for example Methanol and Octanol. Methanol has less carbon and hydrogen atoms therefore will have a lower number of kJ per mole and most likely to take longer to increase in temperature. As you can see from the information above the longer molecules need more energy to break it’s bonds compared to a smaller molecule e.g. Methanol. So I can safely say that methanol will need less energy than Octanol simply because it has less bonds to beak. I can therefore generally summarizes that, the more bonds there are holding the carbon, oxygen and hydrogen atoms together, the more energy will be required to separate them.
Method
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Measure 50cm3 of water in to a glass beaker
- Place the beaker into the clamp stand.
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Weigh the spirit burner with the lid on
- Record the starting temperature of the water
- Put the chosen alcohol burner under the beaker allowing the flame to just touch the beaker.
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Leave to heat up until he temperature of the water is 10oC more than the original temperature.
- Weigh the spirit burner
The variables which must remain constant throughout the experiment are
-
Temperature rise of 10oC
-
Mass of the water (50cm3)
- Type of beaker- 100ml beaker
- The height of the beaker from the wick- 2cm
- Using the same scales- 3 d.p.
- Keeping the lid on the spirit burner
The variable which must be changed
Results
So that I was positive that my results were correct, I carried out the investigation twice. This was so that I not only had one set of results but two, so that I could take an average and therefore hopefully have more accurate results.
From the sets of results we cannot actually come to maybe conclusions with ease. Therefore I can look at the rmm which helps me to find out the number of kJ per mole in each alcohol. I am also going to find an average for the length of time and the average weight difference.
Now that I have got the average weight difference I can now find the Enthalpy change. I do this by the following formula,
Enthalpy change = mass (volume of water) x 4.2 (SHC) x 10 (temp. rise)
= 50 x 4.2 x 10
= 2100joules
I then want to find out the number of moles in each alcohol, I do this by,
No. of moles burnt = Weight difference
rmm
e.g. Methanol No. of moles burnt = Weight difference (average)
rmm
0.298 = 0.0093125moles
32
Now that I have found the number of moles and the Enthalpy change I can find the kJ per mole. I do this by
Enthalpy change = kJ moles
Number of moles burnt x 1000
For methanol again, this would be
2100
0.0093125 x 1000
2100
9.3125
= 225.6 kJ per mole
The table below shows all of my results,
As you can see from my obtaining evidence section I have recorded all the information from the results that I need, to draw some conclusions.
If you look at the results I have obtained I have proven my original hypothesis. It is clear that the number of carbon and hydrogen atom in the alcohol effects the enthalpy change and therefore kJ per mole per gram burnt. I proven that more energy per gram given off , the greater the number of carbon atom and hydrogen atoms the alcohol has. The reason why more fuel has been burnt is because of the large number of carbon atoms and large molecular length, hence the surface area is larger allowing more energy to be released. Due to there being more energy released than used in bond breaking we can say the reaction was exothermic. The bonds could have been formed in two ways, ionic or covalent. Our results were ionic, meaning that there was a transfer of electrons from one atom to another. There is a clear pattern with time as well. I can see that the greater the number of carbon and hydrogen atoms there are, the slower it takes the water to increase in temperature by 10o C. This may be due to the way the electrons, neutrons and protons are arrayed in the atom. There was one anomalous results, for which I don’t have a reason for. It may have something to do with the accuracy of our measuring or perhaps we did not set up to experiment correctly that time. However we were exact with our experimenting and I am surprised due to our double checking of apparatus and our accuracy that we made sure we maintained? As you can see my graph clearly increased as the number of carbon and hydrogen atoms increased. I do not believe any of our results were anomalous and I believe we completed our experiment with full precision. Due to the clear increase in my graph I can confirm that my results were reliable.
From my original Theoretical Enthalpy of Combustion chart I obtained some results. These were in fact different to the results I obtained however this is to be expected. However both in the end agreed with the final statement which was that the more carbon and hydrogen atoms the alcohol has the more energy it gives off per gram.
I believe my results were very inaccurate. Below is a list of why it was an inaccurate experiment.
- Heat conducted and convected away through the air.
- Radiation of heat was given off into the atmosphere.
- The metal clamp transfers heat.
- Heat may have been lost through gusts of wind
- Energy given off through sound and light
- Incomplete combustion
- The water not being tested accurately for temperature, only a basic thermometer.
- The flame size changed due to the type of alcohol, hence it was a different distance away from the beaker each time.
- The size of the wick
- Evaporation of water so there will be less water to heat, making the water hotter.
All of these points would either decrease of increase the results and therefore can been seen as inaccurate. The equipment I used in this experiment was accurate enough to give me good clear results however they were not totally accurate. To collect heat, it is almost impossible not to loss any part of it therefore these experiment can never be totally accurate. I feel that the most limiting factoring the experiment is the convection of air and the water. Also during the experiment water will have been lost and therefore the reading will have been slightly inaccurate.
I believe that if I wanted to improve the accuracy of this experiment I could have tested more alcohols. Despite already having clear results they would have been even more easy to confirm if I had other alcohols such and ethanol and butanol. Next time I repeat this experiment I will focus on reducing heat lost. Improving insulation techniques would be a valuable asset in 100% accurate results. Another error is that of incomplete combustion. Complete combustion occurs if there are lots of oxygen atoms available when the fuel burns, then you get carbon dioxide, carbon atoms bond with two oxygen atoms.
If in the case of a limited supply of oxygen then you get carbon monoxide, each carbon atom can only bond with one oxygen atom. This is known as incomplete combustion. This is because the carbon monoxide can react to make more carbon dioxide.
If I were to repeat the investigation I would improve the accuracy of my results, however I am satisfied with them. They have given clear evidence and I can draw conclusions from them.
