1 2 Introduction My inspiration for this investigation came from listening to a CBC podcast on sustainable energy sources during my car ride to school As society is becoming more aware of the consequences of global warming and new renewable energy sources such as crude oil and petroleum is becoming more scarce and costly scientists are more eager to discover alternative fuel sources to meet the increasing demand for sustainable energy In 2017 alcohols are becoming more recognized as promising renewable fuel sources due to their high octane rating and fuel efficiency For instance ethanol acts as a popular sustainable fuel substitute to gasoline for powering vehicles in Brazil the United States and Europe Ethanol has many advantages such as it s carbon neutral properties positive energy balance and lower levels of greenhouse gases upon combustion Ethanol blended fuels such as E85 composed of 85 ethanol and 15 gasoline can reduce the net emissions of greenhouse gases by 37 1 This made me wonder why is only ethanol used as a primary fuel source opposed to methanol propanol and butanol What is the relationship between the chemical composition of an alcohol and the amount of energy they release
The aim for my internal assessment is to investigate the effects of the number of carbon atoms in an alcohol chain in the same homologous series of primary alcohols on their standard enthalpy change of combustion through conducting my own investigation where I can apply the knowledge I have obtained in my chemistry class outside of the classroom to a situation that is relevant to modern day society 1 3 Background Information Alcohols are organic compounds with the general formula CnH2n 1OH in which the hydroxyl functional group OH is bonded to a saturated carbon atom For industrial purposes their highly flammable properties make alcohols attractive as an alternative fuel source and are commonly used as solvents in medicine and cosmetics Hydrogen bonding dipole dipole interactions and london dispersion forces all exist as intermolecular forces within the organic compound Primary alcohols only have one linkage between the carbon atom with the OH and the alkyl group In a homologous series each consecutive alcohol has an additional methylene group CH2 lengthening the hydrocarbon chain See Figure 1 Figure 1 Chemical Composition of First Three Primary Alcohols The general molecular equation for the combustion of alcohols can be expressed as CnH2n 1OH 1 5n O2 n 1 H2O nCO2 Energy Combustion is a exothermic redox reaction that occurs under high temperatures between a fuel alcohol and an oxidant atmospheric oxygen producing energy with water and either carbon dioxide complete or carbon monoxide carbon in the form of soot incomplete The standard enthalpy change of combustion ΔH c is the enthalpy change when one mole of a substance is completely combusted in oxygen under the standard conditions of 298 K and 100 kPa pressure
The ΔH c value is negative exothermic as energy will always be released when alcohols are burned The ΔH c can by dividing the negative heat of combustion value by the number of moles of the alcohol burnt shown in the following formula ΔH c mass m x specific heat capacity c x temperature change ΔT moles of the alcohol burnt According to the IB Chemistry Data booklet in the same homologous series of primary alcohols the literature value of the standard enthalpy change of combustion ΔH c becomes more negative as the number of methylene groups CH2 in an alcohol chain increase This is due to the structural and chemical composition of the alcohols As the number of carbon atoms increase more energy is required to break the intermolecular forces as an increase in mass of the compound strengthens the london dispersion forces between atoms and an two extra carbon hydrogen an extra carbon carbon need to be broken 2 Methodology 2 1 Hypothesis Theoretical Hypothesis The standard enthalpy change of combustion ΔH c will become more exothermic as the number of carbon atoms in an alcohol chain increases in the same homologous series of primary alcohols As the number of methylene groups CH2 increase in the chemical composition of an alcohol down a homologous series the hydrocarbon chain becomes longer and more energy will be required to break the intermolecular forces between atoms
The alcohol will also increase in molecular mass which strengthens the london dispersion forces between atoms resulting standard enthalpy change of combustion ΔH c to become more exothermic releasing more energy Null Hypothesis The number of carbon atoms in an alcohol chain in the same homologous series of primary alcohols has no effect on their respective standard enthalpy change of combustion ΔH c There will be no correlation in experimental results if there is inconsistencies in heat loss between trials evaporation of water temperature changes wind etc Thus a preliminary experiment must be conducted before I proceed to ensure that my experimental design is functional
