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Comparing the Chemical Properties of Alkanes and Alkenes

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Introduction

Comparing the Chemical Properties of Alkanes and Alkenes Aim To investigate and compare the chemical properties (notably the reactivity) of the alkane Cyclohexane and the alkene Cyclohexene. Hypothesis Based on previous knowledge of alkanes and alkenes I suggest the chemical properties of the two will be similar as both are fuels, however, as alkenes (but not alkanes) contain a carbon double bond (C=C) some of the chemical properties will differ. Method Apparatus: * Hard glass watch glass * Wood splints * 10 dry test tubes (two of which covered in aluminium foil) * 10 tight fitting bungs (to seal the test tubes) * Test tube rack * Bromine dissolved in an inert solvent (Bromine Water) * Acidified Potassium Manganate (VII) solution * Concentrated Sulphuric acid (H2SO4) Procedure A: Combustion 1. Place 3-4 drops of cyclohexane onto the watch glass 2. Light a splint and use this to light the cyclohexane 3. Record observations (e.g. Colour of the flame, amount of soot produced) Procedure B: Reaction with Bromine 1. Place 3-4 drops of cyclohexane into a test tube and the same again into one of the aluminium wrapped tubes. ...read more.

Middle

When kept in the dark, however, there is no apparent change to the bromine or cyclohexane. There has been no obvious reaction between the two substances and we can therefore deduce that the previous reaction is photochemical in nature. C. Acidified Potassium Manganate(VII) KMn: There was no apparent reaction, change in temperature or colour. The cyclohexane would not mix at all with the KMn and in fact seemed to settle on top of the KMn. D. Concentrated Sulphuric Acid (H2SO4): The two substances mixed but did not seem to react in any way. Having mixed they then separated to form two distinct layers. Alkene (Cyclohexene) Observations: A. Combustion: Much like the cyclohexane however the flame did seem a little smokier. B. Bromine: Whether or not in the presence of light when mixed with cyclohexene the Bromine almost instantly decolourised. C. Acidified Potassium Manganate (VII) KMn: An almost instant decolourisation followed by separation into two distinct clear layers. D. Concentrated Sulphuric Acid (H2SO4): There was an extreme exothermic reaction together with an almost instant colour change from clear to cloudy brown. ...read more.

Conclusion

Below is a diagrammatic representation of the structure of this molecule, which is consistent for all alkanes: Compare this to Ethene which (although having the same number of carbon atoms as ethane, cannot contain the same number of hydrogen atoms due to the presence of a carbon-carbon double bond (C=C). It is this double bond that characterises an alkene and due to the double bond the formula for alkenes is as follows: CnH2n. Where with the alkanes there was an extra two hydrogen atoms, the positions available for these hydrogen atoms have been lost as a result of the carbon atoms forming bonds with themselves. Because of this alkenes are referred to as being unsaturated. It is referred to as unsaturated because the presence of a double bond offers a site of reactivity so that other atoms can add to the molecule. The results from our tests show that the alkenes were more 'unstable' and willing to react. With the only difference between the alkane and the alkene being the presence of the double bond I can quite safely state that the reactions observed (but not necessarily combustion test) where as a result of the alkene having a double bond which offered a site of reactivity and the chemical changes to take place. ...read more.

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