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.
This experiment will compare the rate of hydrolysis of 1-chlorobutane, 1-bromobutane and 1-iodobutane where H2O acts as a nucleophile.
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Peer Reviews
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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.
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.
Response to 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.
