The aim of the investigation was to find a relationship between the molecular size of different alcohols and the associated energy change involved in the reaction. A preliminary experiment had been carried out already and errors were noted accordingly and were adjusted in this experiment where possible. This investigation looked into the energy changes when different alcohols were combusted, this is the reaction of the alcohol with oxygen in the surrounding air.
The alcohols are a homologous series of compounds that contain -OH as the functional group (a group of atoms in a structure that determines the characteristic reactions of a compound). For this experiment, only methanol, ethanol, propanol, butanol and pentanol will be used. Alcohol Molecular formula, CnH2n+1OH Boiling point (i?? C) Methanol CH3OH 65 Ethanol C2H5OH 78 Propan-1-ol C3H7OH 97 Butan-1-ol C4H9OH 117 Pentan-1-ol C5H11OH 137 The structures of alcohols Here’s where the alcohol structures go, sorry I couldn’t send them! In general, alcohol + oxygen –> carbon dioxide + water.
E. g. ethanol + oxygen –> carbon dioxide + water C2H5OH(l) + 3O2(g) –> 2CO2(g) +3H2O(l) Hypothesis The larger the molecular size, i. e. the more carbon atoms it has, the larger the amount of energy given out. Some chemical reactions are capable of releasing a lot of energy; usually it is through heat. The combustion of alcohols is a good example, but it is known that they do give off different amounts of heat. When an alcohol is combusted, it reacts with the oxygen in the air and the products are carbon dioxide and water: alcohol + oxygen –>carbon dioxide + water.
During this reaction, as with all others, bonds are first broken and then new bonds are made. In alcohol, hydrogen atoms are covalently bonded to hydrogen atoms. In oxygen gas, the atoms are held together in diatomic molecules. During this reaction, all these bonds must be broken. Chemical bonds are forces of attraction between atoms or ions, and to break these bonds or forces, energy is required; and so energy must be taken in to pull atoms apart. Breaking chemical bonds takes in energy from the surroundings, and is an endothermic process.
When the new bonds are made between carbon and oxygen to make carbon dioxide and between hydrogen and oxygen to form water, it gives out energy. Making chemical bonds gives out energy to the surroundings, and is an exothermic process. When the alcohols react with oxygen, the total energy given out is greater that that taken in to break the bonds, and so overall, this reaction gives out energy and is an exothermic reaction. This energy is released as heat. The energy change involved in the combustion of alcohols is called the heat of combustion, and is what we will be finding in this investigation.
The overall change in energy for this exothermic reaction can be shown in an energy level diagram: In this reaction, energy is given out because the bonds in the products (CO2 and H2O) are stronger than those in the reactants (CH4 and O2). This shows that the products are more stable than the reactants. Some bonds are stronger than others and so require more energy to break them, but they also give out more energy when they are formed. Because it takes more energy to break some bonds, it shows that these are more stable. For example, Type of bond.
Bond energy on formation or breaking (kJ/mol) C – C 347 C – H 435 O – H 464 C = O 803 O = O 497 C – O 358 The bond energies can be used to define the strength of the bond. Using the above information we can work out the predicted energy released per mole of the alcohol. Methanol: 3 (C – H) + C – O + O – H 3(435) + 358 + 464 = 2127 kJ/mol Ethanol:
Pentanol: 11 (C – H) + 4 (C – C) + C – O + O – H 11(435) + 4(347) + 358 + 464 = 6995 kJ/mol My hypothesis was that the more carbon atoms an alcohol had then the more energy per mole would be released. This is because it has more bonds and so has more energy, so when these bonds are made, then heat energy will be released due to the rule of exothermic reactions, and the more bonds that are made then the more energy will be released.
Because all the alcohols are part of a homologous series (i. e.each homologue is related to each other and have the same general formula, in this case it is CnH2n+1OH), the amount of energy they release should have a relationship with the molecular size. For ease of purpose, we can substitute the phrase “molecular size” with “the number of carbon atoms present”; this is because with each proceeding alcohol, the number of carbon atoms increases by one. GCSE Chemistry Coursework Investigation into energy changes in the combustion of alcohols Procedure In this investigation our aim was to determine the amount of energy released through the combustion of different alcohols.
Our method involved heating a can of water by a certain temperature to harness the energy given off by the burning alcohol underneath. The time is measured so that the rate of energy released can be measured. The mass of alcohol loss is also noted to see how much was used in the process. The experiment was kept as fair as possible with the use of draft excluders. Preliminary Result Through our preliminary experiment, the following theoretical errors were noted which caused results to be inaccurate: a) Heat loss to environment – convection currents from windows, other experiments etc.
b) Heat stored in actual aluminium/steel of can. c) Difference in flame sizes d) Incomplete combustion of fuel, i. e. alcohol + oxygen –> carbon monoxide + water. e) Uneven heating of the water in the can. Hence we adjusted the method according to the above and changes made are as follows: a) Use of a draft excluder around the apparatus. b) Mass and material of can noted along with its specific heat capacity and so allowing results to be adjusted accurately. c) Bottom of can and tip of flame measured and kept constant throughout. d) Distance between can and flame enlarged and allowing a good oxygen supply to flame.
e) Can constantly stirred throughout the experiment while it was heating. Variables In order to conduct a fair experiment, variables will be kept constant. We will use the same procedure throughout and only change the fuel, i. e. the alcohol each time.
The factors that affect the amount of energy released: Fuel used: it is known that different fuels release different amounts of energy, so we will change the fuel we are using each time – this includes methanol, ethanol, propanol, butanol and pentanol.
Period of time: to keep this constant we will only heat the water by 20i??C, according to the initial starting temperature and time how long it takes. Amount of fuel combusted: we will measure the amount of alcohol used to heat the water by 20i??
C. Amount of water heated: the more water heated, the more dispersed the energy is and so we will use a constant amount of water each time; we will use 100ml of water. Energy lost: energy is either lost to the environment or to the can as we found in our experiment: a) Energy lost to environment: this is minimised with the use of a draft excluder around the apparatus, which was a failure in the preliminary.
b) Energy lost to the can: as we found in our preliminary experiment, which was a problem, as we could not measure the energy lost to the can. In this experiment, we have weighed the mass of the can and with known specific heat capacities of the material of the can (steel or aluminium) we can calculate the energy lost to the can and adjust results accordingly to enable accurate results to be produced. Specific heat capacities: it is understood that different materials are able to store different amounts of energy in a period of time, hence we use the same can throughout the experiment.
Energy released: to allow suitable and accurate comparison between results of the five different alcohols, and to allow an accurate relationship to be produced, we will convert all taken results into a value of total energy released. Accuracy To produce results with the maximum accuracy suitable, we repeated the experiment twice in order that we could find an average between the results and hence allowing us to identify and eliminate the anomalous results. We used the most accurate measuring equipment available, including digital balance to 2 decimal places and a thermometer to the nearest degree.
We also learnt from the causes of errors in the preliminary and adjusted our method accordingly; i. e. the use of draft excludes and taking into accounts the energy stored in the can. We kept the variable as constant as possible by using the same equipment throughout, based on the concept that different materials have different specific heat capacities and so store different amounts of energy; even if energy is lost we can either account for it or know that it is a constant amount each time, hence still allowing us to make an accurate comparison between results.