ThermochemistryThermochemistry

Heat combustion of alcoholsHeat combustion of alcohols

Today’s focusToday’s focus

Determine experimentally which of Determine experimentally which of the three alcohols methanol, ethanol, the three alcohols methanol, ethanol, and propan-1-ol would be the best and propan-1-ol would be the best alternative to petrol by obtaining alternative to petrol by obtaining their heats of combustion. their heats of combustion.

Complete combustion of alcoholsComplete combustion of alcohols

Alcohols burn readily in the presence Alcohols burn readily in the presence of oxygen with a non-smoky flame to of oxygen with a non-smoky flame to form CO2 and H2Oform CO2 and H2O

Example (Ethanol)Example (Ethanol) C2H5OH + 3 O2 → 2 CO2 + 3 H2O(l)C2H5OH + 3 O2 → 2 CO2 + 3 H2O(l) (ΔHc = −1364 kJ/mol) (ΔHc = −1364 kJ/mol) Heat energy is released so Heat energy is released so

exothermicexothermic

ExperimentExperiment Heating water to get Heating water to get ΔT (Remember the specific ΔT (Remember the specific

heat capacity of water is 4.184 J/g/degrees heat capacity of water is 4.184 J/g/degrees Celsius)Celsius)

From this we can work out From this we can work out ΔE (ΔE = m x ΔT x ΔE (ΔE = m x ΔT x 4.184)4.184)

Calculate the mass of alcohol burnt, and Calculate the mass of alcohol burnt, and therefore the energy released per gram. therefore the energy released per gram.

Finally calculate the heat of combustion in kJ/mol Finally calculate the heat of combustion in kJ/mol Practical- page 147 of prac bookPractical- page 147 of prac book Work in pairsWork in pairs Safety glasses to be worn!!!!Safety glasses to be worn!!!!

FermentationFermentation Ethanol fermentationEthanol fermentation, also referred to as , also referred to as

alcoholic fermentationalcoholic fermentation, is a biological process , is a biological process in which sugars such as in which sugars such as glucoseglucose, , fructosefructose, and , and sucrosesucrose are converted into cellular energy and are converted into cellular energy and thereby produce thereby produce ethanolethanol and and carbon dioxidecarbon dioxide as as metabolic waste products. Because metabolic waste products. Because yeastsyeasts perform this process in the absence of perform this process in the absence of oxygenoxygen, , ethanol fermentation is classified as anaerobic. ethanol fermentation is classified as anaerobic. Ethanol fermentation occurs in the production of Ethanol fermentation occurs in the production of alcoholic beverages and ethanol fuel, and in the alcoholic beverages and ethanol fuel, and in the rising of bread dough. rising of bread dough.

C6H12O6 → 2C2H5OH + 2CO2 C6H12O6 → 2C2H5OH + 2CO2

So where to for alcohols as fuels?So where to for alcohols as fuels?

The proposed alcohol to replace current fossil fuels will be The proposed alcohol to replace current fossil fuels will be produce through fermentation (biofuel). produce through fermentation (biofuel).

1st generation biofuel is produced from feedstock, such as 1st generation biofuel is produced from feedstock, such as sugarcane and maize. Implications? sugarcane and maize. Implications?

22ndnd generation biofuel is produced from non-food crops. generation biofuel is produced from non-food crops. Implications? Implications?

33rdrd generation biofuel is produced from algae (sewage generation biofuel is produced from algae (sewage plants). Called algaeculture. Suggested that the area plants). Called algaeculture. Suggested that the area required to produce enough ethanol form ‘algae ponds’ required to produce enough ethanol form ‘algae ponds’ would be 7 times less than if it was produced from 1would be 7 times less than if it was produced from 1stst generation techniques. Two ways in which ethanol can be generation techniques. Two ways in which ethanol can be produced from algae? produced from algae?