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Properties of Hydrocarbons

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Presentation on theme: "Properties of Hydrocarbons"— Presentation transcript:

1 Properties of Hydrocarbons

2 Physical Properties of Alkanes
Non polar compounds Insoluble in water Less dense than water and so will float on top of the water Dissolve in organic solvents (eg dry cleaning fluid) and in each other (eg Petrol is a mixture of alkanes) Low melting and boiling points

3 Chemical Properties of Alkanes
Very unreactive Don’t react with concentrated sulfuric acid or concentrated bases Don’t react with strong oxidants (dichromate or permanganate ions) Will react with bromine but only if heated or in strong sunlight

4 Substitution Reactions
Where an atom or group of atoms is displaced by an atom or group of atoms CH4 + Br2 → CH3Br + HBr Non specific reaction. Can not control which hydrogen is substituted or how many are substituted CH3Br + Br2 → CH2Br2 + HBr

5 Substitution reactions
Very slow with heat or sunlight Product is a mixture of bromoalkanes Products would need to be separated by fractional distillation giving only a very small yield of each



8 Combustion of Alkanes Very useful group as fuels
Fuels burn completely when there is sufficient oxygen, forming CO2 and H2O If oxygen is limited then incomplete combustion occurs and carbon (soot) and carbon monoxide (CO) are formed as well as CO2 & H2O

9 Combustion of Alkanes Incomplete combustion is less efficient and so produces less energy per mole that complete combustion

10 Balancing Combustion Equations
Write correct formula Alkane + O2 → CO2 + H2O Balance carbon Balance hydrogen Balance oxygen (Fractional coefficients are acceptable in combustion equations)

11 Physical Properties of Alkenes
Non polar molecules so they are insoluble in water Less dense than H2O, so floats on water Low melting and boiling points 11

12 Chemical Properties of Alkenes
Combustion is incomplete in air but will burn completely in pure O2 Burn with a smoky flame in air due to unburnt carbon. Higher carbon to hydrogen ratio means some carbon remains unburnt More reactive than alkanes because the can undergo addition and oxidation reactions 12

13 Addition Reactions The colour of bromine rapidly disappears when shaken with an alkene C2H4 + Br2 → C2H4Br2 The reaction site is the alkene double bond. One bromine adds to the carbon on one end of the double bond and the other bromine to the other end. The double bond becomes a single bond 13

14 Addition Reactions 1,2 dibromo ethane 14

15 Addition Reactions An alkene can be converted to the corresponding alkane by the addition of hydrogen gas The reaction conditions for this to occur are heat, high pressure and a nickel catalyst This reaction is called catalytic hydrogenation 15

16 Addition Reactions This reaction is used to turn a pourable oil into a spreadable fat (margarine) 16

17 Other Addition Reactions
Alcohols can also be made from alkenes by the addition of water in the presence of an acid catalyst (conc. Sulfuric acid) 17

18 Other Addition Reactions
Alkenes can add to each other to form polymers (very large molecules) polyethylene ethene 18

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