I think it will happen for the following scientific reasons: As the size of the carbon chain grows, more bonds are added to the structure of the alcohol. This means that each time, more energy from the surroundings must be extracted in order to break these starting bonds (endothermic stage). Yet, the more energy that is taken to break the bonds, the more energy is used to form the product’s bonds and this makes the energy of the products greater than that of the reactants.
The alcohols start off with methanol, which has 0 carbon – carbon bonds, 3 carbon – hydrogen bonds, 1 carbon – oxygen bond and 1 oxygen – hydrogen bond.
When the products are formed, excess energy is released which is mainly due to the amount of C = O bonds being made. The products of methanol have 2 C = O bonds and 4 O – H bonds. Since a C = O bond has a high energy value of 805, many of them will ensure that the energy of the products is greater than that of the reactants. Here are the theoretical values for the input, output and exothermic heat energy of methanol along with a diagram and the bond energy values: Bond type Energy Value (Kj) Bond type Energy Value.
Now let’s take ethanol as an example. The bonds are 1 C – C bond, 5 C – H bonds, 1 C – O bond and 1 O – H bond. The number of C – H bonds has risen by 2 and the carbon bond has gone up by one. For the products, there are 4 C = O bonds and 6 O – H bonds which is two more bonds than methanol. Here is the input and output calculation for ethanol: The theoretical input energy for the ethanol is greater than that of methanol as is the output energy and the exothermic heat energy difference. The same is shown with proponal, butanol, pentanol, heptanol and octanol.
The pattern that we see is that when the alcohols gain a carbon, they have to break an extra 2 C – H bonds (and C – C bonds if it is connected to another carbon) and this makes it need more energy from the surroundings which comes out as more energy when the products are formed. The theoretical values show that the more bonds in the reactants, the greater the bonds in the products and the more the theoretical energy difference. Also, since there is a regular change in the structure of the alcohols, then there is probably a regular change in the energies too.
There is a regular change in the exothermic energy given out. Here is a graph of my theoretical values in order to show the relationship between the energy given out and the number of carbons: The number and range of results I will need, to obtain reliable evidence are: I will need to have at least 20 results; twice for each different alcohol, perhaps thrice if possible. If each experiment is done at least once, then averages of mass differences can be obtained and will make the ‘energy given out by one mole of alcohol readings’ of each, much more accurate.
The range of these results will be from the alcohol ‘ethanol’ to the alcohol ‘octanol’ which is 7 alcohols in all as we are excluding methanol and heptanol. This will enable me to make bond energy charts for each alcohol to easily identify and illustrate the relative heat energies released. I will require the apparatus for my investigation: Spirit burners of each alcohol, 5 heatproof mats, metal stand and clamp, thermometer, can, ruler, measuring cylinder, splint and a cardboard can lid. The way I will use this apparatus to obtain reliable evidence is shown below:
Once the apparatus has been assembled as shown, fill the measuring cylinder with a specific volume of water and pour into the can. The starting temperature of the water should be recorded and the temperature it will go up to should be calculated by adding the original temperature to a temperature rise (e. g. 20i?? C + 7i?? C =27i?? C). Check that the distance between the alcohol and spirit burner and that the temperature rise is the same for every experiment. Weigh the alcohol before the experiment and then place in the shelter of the windbreakers, so it is directly underneath the can.
Light the alcohol using a lighted splint and close it in with another heatproof mat. Safety glasses must be worn as the alcohols are flammable and if some gets on your hands, they must be washed immediately in case any flame touches your hand. Also, it would be safer and less of an equipment hazard if one Bunsen burner were used. Put the cardboard lid on top of the can and the thermometer through it (punch hole in lid and slide thermometer through). After the water temperature goes up to the one decided, put out the alcohol and weigh its finishing mass.
This should be recorded. To prepare for a repeat – empty out the can and refill and change the alcohol with another of the same type. Repeat the procedure for other alcohols and take average. I have used the following to help me plan my investigation: I have used the following books to help me plan my investigation: ‘Physics for you’ by Keith Johnson. On page 37 to 39, I found out information concerning measuring heat energy, its values, specific heat capacity and a table displaying specific heat capacities of different substances.
‘Revision guide for GCSE Double Science – Physics’ where on page 67 to 68, there is information about heat transfer, evaporation, the conduction of heat and vibrating particles. ‘Chemistry for you’ by Lawrie Ryan (Revised National Curriculum Edition). On page 178 to 179, there is information regarding organic molecules such as alcohol; their structures, physical properties and homologous series. On page 182, there is information on the combustion of alcohols, a brief explanation of the combustion experiment and combustion formulas.
On page 186, which is about energy transfer, there is more detailed coverage of incomplete/complete combustion and fuels. On page 190 to 195, there are things about exothermic/endothermic reactions, what happens to the temperature during these reactions, energy level diagrams, making and breaking bonds and finally bond energy calculations. ‘Nuffield Book of Data’ sheet has exothermic heat energies of all the alcohols combustion reactions and all the varied bond energy values.
I have done the following experiments to help plan my investigation: periment on page 191 to see what substances have endothermic or exothermic reactions. Previous practise experiment of the alcohol investigation, which helped me to find corrections for certain procedures, various precautions, tips that would produce less inaccuracy, discover key factors and how to set up the apparatus. Here is the set up of my results table as an example of what I learnt from the practise experiment.