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AIM OF EXPERIMENT: TO DETERMINE THE RATE OF REACTION OF HALOGENOALKANES

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Introduction

AIM OF EXPERIMENT: TO DETERMINE THE RATE OF REACTION OF HALOGENOALKANES EQUIPMENTS REQUIRED * Eye protection * Marking pens or labels * A test tube rack and 6 test tubes * Beaker ( 250cm3) * Bunsen burner, tripod stand * Measuring cylinder (10cm3) * Thermometer ( 0oC-100oC) * Stop clock * Source of laser light. * 1-bromobutane* * 1-chlorobutane # * 1-iodobutane* * Silver nitrate solution (0.1mol dm-3) * Ethanol# * Dropping pipettes : 1 for each bottle * Source of hot water * Harmful. Eye protection must be worn # Highly inflammable. Keep tubes and bottles away from naked flame. INTRODUCTION A halogenoalkane is a compound which has a carbon (C) - halogen(X) bond in its carbon chain. The C?+? X?-bond is polarised due to the difference in electro negativity between the carbon atom and the halogen atom. This induces the slightly positive charge on the carbon atom and the slightly negative charge on the halogen atom. The slightly positive charge on the carbon atom makes it open to nucleophilic attack. This results in the displacement of the halide ion. ...read more.

Middle

12) Start a stop clock immediately 13) Direct the laser light through the mixture while shaking the contents of the test tube to ensure that the solutions mix. 14) When the laser light fails to pass through the solution, stop the stop clock and record the time obtained 15) Repeat steps 11-14 by adding the solution in tube B to tube 2 and also adding the solution in tube C to tube 3. 16) Perform 3 repeats of the whole experiment. 17) The final results should be the average of the 3 set of results obtained. PREDICTION Element Electronegativity+ Bond *Bond energy(kJ mol-1 Carbon 2.5 Carbon-Fluorine 467 Iodine 2.5 Carbon-Iodine 228 Bromine 2.8 Carbon-bromine 290 Chlorine 3.0 Carbon-Chlorine 346 + Pauling scale * http://www.chemguide.co.uk/mechanisms FACTORS WHICH DETERMINE REACTIVTY The bond energy of carbon-halogen bond decreases from chlorine to iodine The difference between the carbon-halogen decreases from chlorine to iodine. The difference in electronegativity causes the shared pair of electrons to exist closer the to the halogen atom. ...read more.

Conclusion

+ H2O(l) ? R?OH(l) + AgX(s) + HNO3(aq) However, halogenoalkanes which are covalently bonded are not soluble in water which is polar so an organic solvent like Ethanol which is miscible in water can be added to the reactants. Therefore, ethanol acts as a mutual solvent which mixes the halogenoalkane and the silver ions. MECHANISM OF REACTION The mechanism of the reaction is nucleophilic substitution. When a nucleophile approaches a halogenoalkane, it is attracted to the slightly positive carbon atom. The lone pair of electrons on the nucleophile repels the shared pair of electrons between the carbon and halogen which shifts the shared pair of electrons closer to the halogen. At a middle stage of the reaction, the halogen and the nucleophile are both bonded to the same carbon atom partially. This is a transition state which doesn't last very long. As the shared pair of electrons shifts closer to the halogen atom, the halogen atom breaks off heterolytically with the pair electrons forming a halide ion. The nucleophile forms a new coordinate covalent (dative) bond with the carbon atom. OR The halide ion combines with the silver ion to form a precipitate of silver halide. http://www.psigate.ac.uk/newsite/reference/plambeck/chem1/p01244a.htm ?? ?? ?? ?? ...read more.

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Response to the question

The author has produced a clear set of instructions to discover the rate of reaction of the halogenoalkanes in water, and discussed the theory behind it. To improve the instructions, I would have added a labelled diagram of the set ...

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Response to the question

The author has produced a clear set of instructions to discover the rate of reaction of the halogenoalkanes in water, and discussed the theory behind it. To improve the instructions, I would have added a labelled diagram of the set up of the equipment. They clearly understand this challenging topic. However, the piece begins as if it is the write-up of an experiment, but the experiment clearly hasn’t been done as there are no results. I would have expected the coursework to end with a description of a completed experiment and analysis of results – for example whether the predicted results were seen. It is unclear whether the task was to describe an experiment one could do in the future, or the report an incomplete write-up of an experiment. I would have explained this more clearly at the beginning of the piece.

Level of analysis

The writer clearly has a very good understanding of the topic, and they have explained polar bonds and hydrolysis very well. They have given values for the electronegativity and bond enthalpy and used these to consider the outcome of the experiment, although I would also have described the potential for the decreased difference in bond polarity down the group to have an effect on the reaction speed due to the decreased attractions with the nucleophile, and discussed whether this is a more important factor than the bond enthalpy. However, this is a very challenging topic, so this level of thought may not be required at GCSE.

Quality of writing

The author’s quality of written communication is excellent. The report is well structured and easy to follow. Their spelling, punctuation, and grammar are very good. They have listed sources throughout the piece, which makes it clear where all the information has come from, although this could also have been added as a bibliography at the end of the piece to avoid them interrupting the analysis. They have used a variety of appropriate technical terms such as ‘electronegativity’ and ‘nucleophile’, and clearly fully understand the meanings of these words. Although they have briefly mentioned the safety hazards of the chemicals, I would have improved the report by including safety hazards and precautions in a separate section as these are very important to consider when carrying out an experiment.


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