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Investigating the Enthalpy Changes of Combustion of Alcohols.

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Introduction

Investigating the Enthalpy Changes of Combustion of Alcohols AIM The aim of this investigation is to determine the enthalpies of combustion of some alcohols and, if possible, identify any patterns or trends. Alcohols are a series of homologous carbohydrates. As the description suggests, they are compounds that contain carbon, hydrogen and oxygen. They are a similar (homologous) group of compounds as they are made of a alkane chain with an -OH functional group instead of the normal -H that is found in alkanes. For example: Methane (an alkane) Methanol (an alcohol) As with alkanes, alcohols vary in the number of carbons that are present in the chain. The general chemical formula of alcohols is CnH2n+2O, where n is the variable. However, a better way to represent the chemical formula of alcohols is CH3(CH2)nOH as it helps to visualise the structure of the compounds. Ethanol Propanol Butanol Pentanol Hexanol Combustion is the combination of a material with oxygen (i.e. oxidation) where heat and or light are produced (both are forms of energy). Therefore combustion is an exothermic reaction as energy is released from the reaction (combustion=exothermic oxidation). The enthalpy of combustion (?H�c) is a measure of the amount of energy released. It is measured in joules per mole of a specific substance (J mol-1). However, because combustion reactions tend to release vast amounts of energy (especially with organic compounds), the enthalpy of combustion is measured in kilojoules per mole (kJmol-1). A NOTE ABOUT THE ENTHALPY SYMBOL (?H�C) ? - Greek letter 'delta' and symbolises 'difference of' or 'change in' H - Means enthalpy or energy � - Means standard conditions, that is room temperature - 298 K (25�C) and standard pressure - 1 atm. (101 kPa, kNm-2) C - Refers to the fact that is an enthalpy of combustion. (Note: This means complete combustion, i.e. all of the reactant is oxidised) [Completely] oxidising an alcohol (or any other carbohydrate for that matter) ...read more.

Middle

Using 200 cm3 requires a large calorimeter and this could increase the heat loss, therefore it is better to use the smaller calorimeter. Measuring the volume of the water using a cylinder is very inaccurate as the readings are very close to each other. Therefore, I've decided to weigh the water instead. As water has the density of 1 gcm-3, its mass in grams is equal to its volume in cm3. The mass scales available measure to 1/100 of a gram (i.e. 0.00 g, or 2 d.p.). The measuring cylinders are far less accurate. It is hard to distinguish between one mil and the next on a measuring cylinder, let alone 1/100. Another problem with inaccuracy arose with the thermometer. Just like the measuring cylinder, the readings on the thermometer were too close to one another. This problem was helped by my decision to use a data logger, which measures temperature to 1/10 of a degree (i.e. 0.0 �C, 1 d.p.). This will further improve the accuracy of my experiments. (NOTE: The above section can also be considered as part of THE EVALUATION) I have decided to investigate as many alcohols as are available to me (i.e. the ones the school can afford to buy) and I am aiming to test each alcohol four times, as it will be easier to identify anomalous results and will make better averages. CALIBRATING THE HEAT CAPACITY In order to calculate the heat capacity of the apparatus plus the surrounding air, I need to know the amount of energy that went towards heating them up and the temperature rise caused by that energy. Once I have these two values, I can work out the heat capacity of the surroundings. Here, the mass of the surroundings is irrelevant, as it will be kept constant throughout all of the experiments. I have chosen methanol as the base alcohol to calculate the heat capacity of the surroundings, as it is the first in the series. It is the most basic alcohol. ...read more.

Conclusion

1 O-H 464 kJmol-1 (as in H2O) (1) 1 O=O 498.3 kJmol-1 (as in O2) (1) Bonds Made: 1 C O 1076.5 kJmol-1 (as in CO)(5) 4 O-H 464 kJmol-1 (as in H2O) (1) Enthalpy = {(3x413) + (336) + (464) + (498.3)} - {(1076.5) + (4x464)} = 2537.3 - 2932.5 = -395.2 kJmol-1 This shows that if methanol was to undergo incomplete combustion, only 54% of the energy given out when compared with complete combustion (-726.0 kJmol-1). So clearly, incomplete combustion can cause a large error (46%). Incomplete Combustion Of Ethanol: Bonds Broken: 5 C-H 413 kJmol-1 (average) (1) 1 C-O 358 kJmol-1 (average) (1) 1 O-H 464 kJmol-1 (as in H2O) (1) 1 C-C 347 kJmol-1 (average) (1) 2 O=O 498.3 kJmol-1 (as in O2) (1) Bonds Made: 2 C O 1076.5 kJmol-1 (as in CO)(5) 6 O-H 464 kJmol-1 (as in H2O) (1) Enthalpy = {(5x413) + 358 + 464 + 347 + (2x498.3)} - {(2x1076.5) + (6x464)} = 4230.6 - 4937 = -706.4 kJmol-1 The problem of incomplete combustion is highlighted here again. If ethanol were completely combusted, it would release -1367.3 kJmol-1. Only 52% of this is released when the ethanol is incompletely combusted (as shown above), making a 48% error. REFERENCE (1) Salters Advanced Chemistry: Activities and Assignments Folder Heinemann Educational Publishers 1994 (First Publication) Data Sheets Table 18: Bond length and bond enthalpies Pg. 421 (1) Salters Advanced Chemistry: Activities and Assignments Folder Heinemann Educational Publishers 1994 (First Publication) Data Sheets Table 17: Organic compounds: physical and thermo-chemical data. Pg. 420 (3) http://hypertextbook.com/physics/thermal/heat-sensible NOTE: On the website, all of the heat capacities are stated in Jkg-1K-1 whereas the in my write up, they are stated in Jg-1K-1. So all of the heat capacities in my write-up are a thousandth of those on the website. (4) Salters Advanced Chemistry: Activities and Assignments Folder Heinemann Educational Publishers 1994 (First Publication) DF1.2 (Measuring the Enthalpy Change of Combustion of Different Fuels) Pg. 20-21 (5) http://www.webelements.com/webelements/elements/text/C/enth.html (6) Salters Advanced Chemistry: Chemical Ideas Heinemann Educational Publishers 1994 (First Publication) Unit 4.1 "Energy out, energy in" - Figure 3 Pg. 47 1 ...read more.

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