Renewable Ethanol Flashcards
describe and account for the many uses of ethanol as a solvent for polar and non-polar substances
Ethanol is used as a solvent in dissolving medicines and food flavourings and colourings that do not dissolve easily in water. Once the non-polar material is dissolved in the ethanol, water can be added to prepare a solution that is mostly water.
The ethanol molecule has a water loving (hydrophilic) -OH group that helps it dissolve polar molecules and ionic substances. This occurs through hydrogen bonding, dipole-dipole attraction or ion-dipole attraction. The short, water fearing (hydrophobic) hydrocarbon chain CH3CH2- can attract non-polar molecules. The non-polar component of an ethanol molecule bonds to non-polar molecules through dispersion forces. Thus ethanol can dissolve both polar and non-polar substances.
Industrially and in consumer products, ethanol is the second most important solvent after water. Ethanol is the least toxic of all the alcohols as it is poisonous in moderate amounts rather than small amounts. Consumer products listed as containing alcohol practically always contain ethanol as the alcohol.
outline the use of ethanol as a fuel and explain why it can be called a renewable resource
Ethanol combusts in air, releasing carbon dioxide, water and heat. Because the ethanol molecule contains an O atom, the combustion is practically always complete. There is hardly any formation of the polluting CO or C forms, which form from the incomplete combustion of many other hydrocarbons.
A familiar example .
When the oxygen supply in a Bunsen burner is adequate for complete combustion, you get a hotter, colourless, almost invisible flame, like the flame of burning ethanol.
In contrast, when you reduce the oxygen supply to a Bunsen flame, you get a yellow smoky flame, due to carbon.
The presence of an oxygen atom in ethanol minimises the formation of carbon in an ethanol flame.
Ethanol can be called a renewable resource because ethanol can be made from plant material and the products of its combustion, carbon dioxide and water, are the reactants needed by plants for photosynthesis.
describe conditions under which fermentation of sugars is promoted
The conditions that promote the fermentation of sugar are:
a suitable micro-organism such as yeast
water
a suitable temperature for the fermenting yeast
low oxygen concentrations favouring the fermenting yeast
a small amount of yeast nutrients such as phosphate salt.
Once the ethanol concentration reaches 14-15% by volume, the yeast cannot survive, and the fermentation process stops.
summarise the chemistry of the fermentation process
Cane sugar waste, such as molasses, is rich in sucrose (C12H22O11), however, it is uneconomic to separate.
If water and yeast is added, the sucrose reacts with water producing glucose and fructose, both of which have the molecular formula C6H12O6.
sucrose + water -> glucose + fructose
Fermentation can then occur:
glucose or fructose -> + carbon dioxide
Define the molar heat of combustion of a compound and calculate the value for ethanol from first-hand data
The molar heat of combustion is the heat change when one mole of the substance is combusted to form products in their standard states (that is, solid, liquid or gas) at 105 Pa (100 kPa) and 25oC (298K).
To calculate the value for ethanol from first hand data:
Write a balanced equation for the complete combustion of ethanol including states of matter required for the molar heat of combustion definition.
Use your measurement of heat released per gram for ethanol as your first-hand data to calculate the molar heat of combustion for ethanol.
Compare your calculated value with a published value from a text or data book. Explain any difference in measured value and published value by considering how you carried out the measurement.
An example . . .
A student measures the temperature rise of a measured mass of water heated by a burner containing ethanol. Weighing of the burner before and after use, gives a difference equal to the mass of ethanol that burned.
The student does a calculation (deltaH = –mCdeltaT) to find the amount of heat released. From these two measurements, the heat released per gram and then per mole is calculated.
assess the potential of ethanol as an alternative fuel and discuss the advantages and disadvantages of its us
An assessment is a judgement of value, quality, outcomes, results or size. Make sure you include advantages and disadvantages in your assessment of the potential of ethanol as an alternative fuel.
Ethanol can be used in internal combustion engines if it can be economically produced from renewable resources or subsidised as a fuel to reduce air pollution.
The advantages of using ethanol include its complete combustion with minimal pollution. It can also be made in a number of ways. See the diagram below.
Ethanol cycle
The disadvantages of using ethanol include the need to modify fuel lines and even the engine if the ethanol is more than 10-15% when mixed with petrol. Another disadvantage is the low price of still readily available petroleum. Large tracts of land would need to be allocated to growing plants to use in the production of ethanol rather than food.
describe the dehydration of ethanol to ethylene and identify the need for a catalyst in this process and the catalyst used
describe the addition of water to ethylene resulting in the production of ethanol and identify the need for a catalyst in this process and the catalyst used
The following information addresses the above two syllabus points together.
ethylene and ethanol are easily interchanged by addition of water (hydration) and removal of water (dehydration). Catalysts such as sulfuric acid, phosphoric acid or heated ceramic solids can be used to catalyse these dehydration and hydration reactions.
Addition of water to ethene and dehydration of ethanol
Additional background information
Countries rich in petroleum or natural gas, e.g. around the Persian Gulf, or petroleum refining and cracking facilities, e.g. Singapore, can make ethanol by hydration of ethylene.
Countries rich in land and climate suitable for growing crops that could be used to produce ethanol, e.g. Brazil, can make ethylene by dehydration of ethanol.
