Alcohols Coursework

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GCSE Chemistry - Alcohols Coursework

Planning

This investigation involves burning alcohol in the air. Key science- Chemistry by Eileen Ramsden says that " an alcohol is a series of organic, homologous compounds, with the general formula Cn H2n+1OH". The alcohol reacts with the oxygen in the air to form the products water and carbon dioxide:

Cn H2n+1OH +(n+n/2)-1O2 ? nH2O + nCO2

The structure of the molecules in this reaction is:

H H | | H - C - C - O- H + 3[O=O] ? 1/2[O=C=O] + 3[H-O-H] | | H H

This reaction is exothermic, as heat is given out. This is because the amount reactant energy is more than the product energy the difference between this is ?H, therefore some energy has been given out in the form of heat.The energy is given out when forming the bonds between the new water and carbon dioxide molecules. This can be shown in an energy level diagram: Reaction co-ordinate ?H is the heat content, which is the enthalpy, which is negative in exothermic reactions as the diagram shows that energy is 'lost' as heat. Enthalpy is defined as the energy of reaction, or the heat energy associated with a chemical change. Chemical Principles By Master & Slowinski says that "For any reaction carried out directly at a constant pressure, the heat flow is exactly equal to the difference between enthalpy of products and that of the reactants", or: Qp = Hp - Hr = ?H Where Qp is the heat flow at constant pressure, Hp is heat energy of products, and Hr is the heat energy of the reactants.

To measure H given off, we must use this energy to heat something, this will be water. This is assuming that all the heat produced by combustion of fuel (?H) will equal the amount of heat absorbed by the water (q). So I will measure the amount of energy required to do so. This can be worked out by useing the formula: q = mass X specific heat capacity X temperature rise 1000 Where q is the quantity of heat. The specific heat capacity is the amount of energy required to heat the substance, and is calculated using the formula q = MC?, where q is the enthalpy, is the specific heat capacity and ? is the temperature rise. I chose to use water as it is safe, easily obtainable, and has a constant, reliable specific heat capacity of 4.2J/?C.

The bonds which are made in a exothermic reaction "are forces of attraction between the atoms or ions in a substance" according to Key science- Chemistry by Eileen Ramsden. These can be of two types: covalent, in which the atoms share electrons. Examples of this are water and carbon dioxide, which has a double covalent bond because it shares two pairs of electrons are shared. The other type of bonds are ionic, where a metal is involved. This is where electrons are transferred from one ion to another, so there is an electrostatic force between the ions.

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The variables that must be controlled are:

 

* Mass of water

* Amount of wick on burner

* Type of alcohol

* Height of can above flame

* Type of can

* Time of burning

 

The alcohols used in this experiment will be from methanol, to hexanol, their formulas and predicted enthalpy changes are:

  Substance Formula - Predicted enthalpy change (KJ/mole)

Alcohol CnH2n+1OH

Methanol CH3OH -730

Ethanol C2H5OH -1370

Propanol C3H7OH -2010

Butanol C4H9OH -2670

Pentanol C5H11OH -3320

Hexanol C6H13OH 3980 N.B. The ...

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Here's what a star student thought of this essay

Although grammar is overall good, at some points there are some errors such as incomplete sentences which could be avoided easily, by re-reading the coursework before submitting it. The use of a table for the presentation of data could make the results easier to see. The student characterizes the results as "highly inaccurate" in one paragraph and in the next paragraph they proceed to describing them as "small inaccuracies", which shows inconsistency in answering of the question. The coursework is well-structured, however repetion of points previously mentioned should be avoided, for example the same controlled variables might appear twice in the essay. The student uses 2 books for information. Further research could be carried out to develop a better understanding of the topic.

The student did not specify an aim, although a prediction (hypothesis) was developed and disproven in the evaluation of results. An excellent list of controlled variables was included. The student also used known enthalpy changes to compare with the results obtained, which helped in reaching valid conclusions. An excellent example of preliminary work is shown: the student carried out an investigation to determine the best volume of water to use in his apparatus. In the method section, the steps undertaken are clearly stated. The response shows some other positive features such as good reference to minimizing the sources of error, suggesting that no firm conclusion could be reached with clear explanation why, as well as suggestions for any improvements which could be made and further work which could be carried out. In the section titled "Analysis", the explanation given is unclear.

The student shows a fair understanding of the key-principles involved in the combustion of alcohols and provides a good explanation of what happens during combustion, however a better scientific explanation of what the student predicted to happen could be provided and the analysis of the results could be improved. The student answers the question accurately, although information not required to explain combustion appears in places, for example explanation of what ionic bonds are.