2Reactions with sodium dichromate. Sodium dichromate is an oxidising agent, ie it oxidises other chemicals, being reduced in the process.Orange chromium (vi) is reduced togreen chromium (iii) when heated with acid.Cr2O H+ + 6e- → 2Cr3+ + 7H20
3Oxidation of primary alcohols. Primary alcohols are oxidised as orange chromium (vi) is reduced togreen chromium (iii) when heated with acid.Two organic products are possible; Aldehydes or Carboxylic Acids.
4Aldehydes vs Carboxylic Acids Aldehydes have a carbonyl group C=O, at one end.Eg PropanalCarboxylic acids have the functional group;-CO2HEg; Ethanoic acid
5The alcohol is oxidised; Dichromate is reduced; CH3CH2OH → CH3HC=O + 2H+ + 2e-Ethanol EthanalDichromate is reduced;Cr2O H+ + 6e- → 2Cr3+ + 7H20The aldehyde is then distilled as fast as it is formed so that it is unable to react further to form the carboxylic acid.
6Oxidising alcohols to aldehydes using distillation. Aldehydes evaporate
7Oxidation of aldehydes. Aldehydes are oxidised as orange chromium (vi) is reduced togreen chromium (iii) when heated with acid.Only one organic product is possible; a Carboxylic Acids.
8Reflux produces the carboxylic acid. Aldehyde condensesReflux produces the carboxylic acid.So the aldehyde can be oxidised to a carboxylic acid.Aldehyde evaporates.
9The carboxylic acid is then obtained by distillation. Carboxylic acid evaporates
10The aldehyde is oxidised; CH3CHO + H2O→ CH3CO2H + 2H++ 2e- Ethanal Ethanoic acidDichromate is reduced.Cr2O H+ + 6e- → 2Cr3+ + 7H20NB Instead of H+ [H] or ‘H’ can be used in writing half equations.
11Distinguishing between aldehydes and carboxylic acids. Aldehydes are neutral.Aldehydes turn blue Cu(ii) orange on heating.Carboxylic acids are acidic.Carboxylic acids do not react.
12Carboxylic acids do not. Aldehydes give a silver mirror when heated with ammonical silver nitrate.Carboxylic acids do not.
13Effects of ethanol on the body. Ethanol depresses the activity of the central nervous system.Reducing stress, tension and anxiety and inducing a feeling of relaxation.Unfortunately it also reduces reaction times and impairs judgement.Blood Alcohol Concentration (BAC) is measure in mg/100ml of blood.The legal limit is 80mg/100ml.
14The UK legal limit for drivers is 80 mg of alcohol per 100 ml of blood Detecting alcoholAlcohol is not digested on absorption nor chemically changed in the blood.The UK legal limit for drivers is 80 mg of alcohol per 100 ml of bloodBut it takes time to analyse blood…
15Breathalysers It is much easier to analyse exhaled air. The oxidation reaction of ethanol is the basis of the breathalyser.
16As the blood flows through the lungs some of the alcohol moves across the alveoli into the air, because it is volatile.The concentration of alcohol in the alveolar is directly proportional to the concentration of alcohol in the blood.As the alveolar air is exhaled the alcohol can be detected by a breathalyzer, giving an instant result.
17Biological recognition layer How Breathalyzers WorkBreathsamplewith alcoholAmplifierBiological recognition layerPhotosystem cellThe reacted mixture is compared to a vial of unreacted mixture in a photosystem cell that produces an electric current, displayed in a meter.Enzyme catalyses the reaction of the alcohol with potassium dichromate to chromium sulphate, potassium sulphate, acetic acid and water. The reddish-brown dichromate ion changes to a green chromium ion.
18½O2 + 2H++ 2e- → H2O Modern breathalysers use a fuel cell. At one electrode ethanol is oxidised;CH3CH2OH+ H2O → CH3CO2H+ 2H++ 2e-At the other oxygen is reduced;½O2 + 2H++ 2e- → H2OThe voltage produced at varying concentrations of ethanol can then be measured to calibrate the machine.A green light will then indicate no ethanol, amber a little and red on or near the limit.
19Oxidation of secondary alcohols. Secondary alcohols are also oxidised as orange chromium (vi) is reduced togreen chromium (iii) when heated with acid.Only one organic product ispossible; a Ketone.
21Aldehydes vs Ketones Aldehydes have a carbonyl group C=O, at one end. Ketones also have a carbonyl group, but in the interior of the molecule.
22Ketones cannot be oxidised by acidified potassium dichromate. Oxidation of KetonesKetones cannot be oxidised by acidified potassium dichromate.To oxidise them a much stronger oxidising agent is needed that can break C/C bonds.
23Oxidation of tertiary alcohols Tertiary alcohols cannot be oxidised by acidified potassium dichromate.To oxidise them a much stronger oxidising agent is needed that can break C/C bonds.
24Ethanol as a renewable fuel Ethene is derived from crude oil, a non-renewable resource.But sugars are renewable.Ethanol derived from fermentation can be burnt as a renewable fuel.In some countries ethanol, from sugar cane, is mixed with petrol to form gasohol.
25Carbon footprints of biodiesel and ethanol vs diesel. The carbon in diesel was taken out of circulation hundreds of millions of years ago.When it is burnt it therefore results in a net increase of carbon dioxide in the atmosphere.But the carbon in biodiesel or ethanol from fermentation was only recently taken up, so its release does not result in an increase in atmospheric carbon dioxide, ie it is carbon neutral.
26Carbon neutrality 6CO2 + 6H2O→C6H12O6 + 6O2 C6H12O6 → 2C2H5OH + 2CO2 Carbon dioxide is taken up during photosynthesis.6CO2 + 6H2O→C6H12O6 + 6O2But released upon fermentation.C6H12O6 → 2C2H5OH + 2CO2And when ethanol is burnt;2[C2H5OH → 2CO2 + 3H2O]
27Carbon budget Carbon uptake = 6CO2 Carbon production = 2CO2 +2[ 2CO2] = 6CO2So there is no net change, but there are hidden costs in ethanol production and distribution which mean that ethanol is in fact not completely carbon neutral.