How does the Relative Molecular Mass change in heat combustion of an alcohol?
Planning
Introduction
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As alcohol burns in air it gives out energy as heat and light. I am going to investigate how the energy output of an alcohol in combustion changes, with increased relative molecular mass, or RMM. RMM is the sum of the atomic masses of every atom in the molecule. Using the alcohols: Methanol, Ethanol, Propan-1-ol, Butan-1-ol and Pentan-1-ol,
I will plan, and complete an experiment that tests the prediction below. ======================================================================
Prediction And Theory
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In the combustion of alcohols in air, the alcohol reacts with oxygen molecules, to create carbon dioxide and water. Many bonds are broken in the process using up energy. At the same time, the atoms reforming into the new molecules of carbon dioxide and water give out energy. In the combustion of alcohols, the energy created, when forming bonds will always be more that what is lost, when breaking bonds, this gives us excess energy. This energy is given out primarily as heat, but also as light and sound. As energy is given out it is called an exothermic reaction. If the opposite were true, it would be an endothermic reaction. It is never possible to calculate exact energy change by experimentation due to inaccuracies and energy waste, so we use bond energy calculations give the exact theoretical energy change.
Bond energy calculations show that the higher the RMM the more energy will be produced for the same weight of fuel (RMM is the sum of the atomic masses of every atom in the molecule). This is because as the
RMM increases there are more atoms and therefore, more bonds to be broken and then made. As, when burning alcohols, this process gives out energy, the more bonds go through this process, ie as the RMM increases the more energy should be released. The calculations also suggest that for every carbon atom you add to the chain of an alcohol the energy out should increase by
618 Kj/mol. I predict then, that as the RMM goes up then the energy change will get increasingly more negative i.e. more energy is given off. The RMM will be proportional to the final energy created as both should increase by the same number each time, (RMM by 14 as one C and
2 H atoms are added, and the energy out by 618KJ/mol). This will therefore result in a straight-line on the graph. The bond energy calculations show how much energy should be released, accounting for experimental inaccuracies however, I expect the experimental output to be considerably less.
Proposed Method
I am going to test how the energy output per mole in the combustion of
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The change in enthalpy for the combustion of magnesium metal. Abstract = == ==
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In the lab the reaction that took place was a synthesis reaction. A synthesis reaction, is a type of chemical reaction in which two or more simple substances combine to form a new product. The reactants may be elements or compounds. In this case it is a gas and a metal that will react and produce a compound. The general form of a synthesis reaction is, A + B → AB. In order for this lab to be done successful you need knowledge on, percent composition, the empirical and molecular formula, the law of conservation of mass, moles and molar mass, qualitative and quantitative. To begin, the percent composition of a compound is the percent of the total mass that each element has in that compound. Every compound would have a certain percent composition. To calculate percent composition of a compound, you would have to determine the total molecular mass of the compound. For example, for H2O the total molar mass would be 18.00g/mol. You would then input the mass of one of the elements and the molar pass into the equation % by weight (mass) of element = (total mass of element present ÷ total mass of compound) x 100 to find out the percent composition. So for Oxygen it would be, % of O = (16.00g ÷ 18.00g/mol) x 100 which would equal 88.9%. Therefore the percent composition of O in this compound is roughly 88.9%. Furthermore, the molecular formula is the number and types of atoms that are existing in a single molecule of a substance. The empirical formula also known as the simplest formula is the ratio of elements present in the compound. The key difference between these two is that the empirical formula shows the simplest positive integer ratio of atoms of each element present in a compound whereas the molecular formula of a compound is a way ...
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