Unit 2 Nature s Chemistry Revision Notes

Transcription

1 Unit 2 Revision Notes Homologous series: a group of compounds with the same general and similar chemical properties that show a gradual change in physical properties. Examples include the alkanes, alkenes, cycloalkanes, alkanols and alkanoic acids. Alkanes General : C n H 2n+2 methane CH 4 CH 4 ethane CH 3 CH 3 C 2 H 6 propane CH 3 CH 3 C 3 H 8 butane CH 3 CH 3 C 4 H 10 pentane CH 3 CH 3 C 5 H 12 hexane CH 3 CH 3 C 6 H 14 heptane CH 3 CH 3 C 7 H 16 octane CH 3 CH 3 C 8 H 18 Combustion: hydrocarbon + oxygen carbon dioxide + water, e.g. propane + oxygen carbon dioxide + water C 3 H 8 + O 2 CO 2 + H 2 O C 3 H 8 + 5O 2 3CO 2 + 4H 2 O 1

2 Alkenes General : C n H 2n ethene = C 2 H 4 propene CH 3 CH= C 3 H 8 butene CH 3 CH= C 4 H 8 pentene CH 3 CH= C 5 H 10 hexene CH 3 CH= C 6 H 12 heptene CH 3 CH= C 7 H 14 octene CH 3 CH= C 8 H 16 Alkenes are unsaturated hydrocarbons and can undergo addition reactions. In this reaction bromine decolourises rapidly. This reaction is used as a test for unsaturation. Hydrogenation - the addition of hydrogen, can convert alkenes into alkanes. 2

3 Cycloalkanes General : C n H 2n cyclopropane C 3 H 8 cyclobutane C 4 H 8 cyclopentane CH2 C 5 H 10 cyclohexane C 6 H 12 cycloheptane H C 7 H 14 2 C The fact that the cycloalkanes and the alkenes have the same general, C n H 2n, allows us to conclude that cycloalkanes are isomers of the corresponding alkene with the same number of carbon atoms. For example; is an isomer of Isomers: same molecular different e. Isomers have different properties, e.g. propene decolourises bromine solution, cyclopropane does not. 3

4 Systematic names e can be drawn and molecular e written from systematic names and vice versa. Rules 1. Identify and name the longest chain of carbon atoms. 2. Identify the branch and name it according to the number of carbon atoms in the branch. 3. Number the branch so that it has the lowest possible number. 3-ethylhexane CH 3 CH( CH 3 ) CH 3 3-methylheptane CH 3 CH(CH 3 ) CH 3 4. Alkenes are named by numbering the carbon atoms from the end that gives the carbon of the double bond the lowest number. 5. Where there are branches, the double bond takes priority over the branch. but-1-ene but-2-ene CH=CH CH 3 CH 3 CH=CHCH 3 4-methylpent-2-ene CH 3 CH(CH 3 )CH=CHCH 3 4

5 Alcohols Functional group: hydroxyl group (-OH) Alkanols: homologous series of alcohols General : C n H 2n+1 OH methanol CH 3 OH CH 3 OH ethanol CH 3 OH C 2 H 5 OH propanol CH 3 OH C 3 H 7 OH butanol CH 3 OH C 4 H 9 OH pentanol CH 3 OH C 5 H 11 OH hexanol CH 3 OH C 6 H 13 OH heptanol CH 3 OH C 7 H 15 OH Isomers e.g. butanol butan-1-ol butan-2-ol CH 3 OH CH 3 CH(OH)CH 3 Alcohols are effective solvents, highly flammable, and burn with very clean flames resulting in their use as fuels. 5

6 Carboxylic acids Functional group: carboxyl group (-COOH) Alkanoic acids: homologous series of carboxylic acids General : C n H 2n+1 COOH methanoic acid HCOOH HCOOH ethanoic acid CH 3 COOH CH 3 COOH propanoic acid CH 3 COOH C 2 H 5 COOH butanoic acid CH 3 COOH C 3 H 7 COOH pentanoic acid CH 3 COOH C 4 H 9 COOH hexanoic acid CH 3 COOH C 5 H 11 COOH heptanoic acid CH 3 COOH C 6 H 13 COOH Vinegar is a solution of ethanoic acid. Vinegar is used in household cleaning products designed to remove limescale (a build up of insoluble carbonates on plumbing fixtures) and as a preservative in the food industry. Esters An ester can be made by reacting a carboxylic acid with an alcohol. Esters are used in food flavouring, industrial solvents, fragrances and materials. 6

7 Energy from fuels Alkanes and alcohols can be used as fuels. Combustion reactions are exothermic reactions. Exothermic reactions release heat energy to the surroundings. Endothermic reactions take in heat energy from the surroundings. Calculations based on equations When a substance is combusted the reaction can be represented using a balanced e equation. The quantities of reactants and products in these reactions can be calculated. E.g. Calculate the mass of oxygen required to burn 50g of butan-1-ol. C 4 H 9 OH(l) + 6O 2 (g) 4CO 2 (g) + 5H 2 O(l) 1mol reacts with 6mol 74g 192g 50g 50 x = 130g Energy calculations Different fuels provide different quantities of energy and this can be measured experimentally and calculated using E h = cmδt. thermometer Specific heat capacity of water (kjkg -1o C -1 ) = 4.18 Energy (kj) copper can water E h = cmδt Mass of water heated (kg) 1cm 3 = 0.001kg Change in temperature ( o C) fuel Energy per gram (kjg -1 ) = E h Mass of fuel burned (g) 7