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Rates of reactions of halogenoalkanes

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Introduction

AS: ASSESSED PRACTICAL CHEMISTRY (Skill P) Rates of reactions of halogenoalkanes The relative rates of the reaction are observed by adding silver nitrate to the reaction mixture and timing the first appearance of the silver halide precipitate. This is the equation of hydrolysis of halogenoalkanes by aqueous silver nitrate. The X represents the halogen, e.g.; I, Br, Cl or halide I- , Br-, and Cl- The reaction below will be the same for all halogens. You just need to replace the halogen or halide, with any of the above. CH3 CH2 X + OH- CH3 CH2 OH + X- Ag+(aq) + X- (aq) AgX Nucleophilic Substitution of a halogenoalkane: *the bromine halogen can be replaced with any other halogen Prediction: I predict that the reactivity of the halogenoalkanes with AgNO3 is; *R = represents an alkyl group Most Reactive Least Reactive R-I > R-Br > R-Cl So I predict that alkyl iodide will have the fastest rate of reaction than alkyl bromide than alkyl chloride being the slowest. ...read more.

Middle

The reason for the decrease in bond enthalpy down the group of halogens is a result of increase in atomic size. Iodine's atomic size is the greatest and therefore the forces needed to be broken are the smallest. The large atomic size causes the bond enthalpy to be reduced and there is an increase in reactivity due to greater shielding of the outer electrons, and greater distance from the nucleus. Apparatus: * 3 test tubes * 45 �C water bath * Burette * 3 beakers * Ethanol ( 3cm3 per experiment) x3 * Silver Nitrate ( 3cm3 per experiment) x3 * Iodobutane 3.0 cm3 * Bromobutane 2.0 cm3 * Chlorobutane 0.9 cm3 * Fine pipette * Thermometer * Stop clock/ timer * Piece of paper with an X on it Method: The best test for halogenoalkanes is to warm with water and then test the halide using aqueous silver nitrate solution. The experiment is successful because the reactions between the ions can be seen by the formation of a visible silver halide precipitate. ...read more.

Conclusion

5. Now using a stop watch, place a piece of paper with an X on it behind the test tube and see how long it takes for the X to disappear. This will give the time needed for the reaction to take place, consequently differing the rates of reaction 6. Record results in an appropriate table like this one, and then repeat the experiment. Halogenoalkane First test, time taken in seconds Second test, time taken in seconds Iodobutane Bromobutane Chlorobutane Hazards and Precautions Silver nitrate is a toxic compound, and so are halogenoalkanes. So when using the above chemicals, the experiments should be done in a well ventilated area or in a fume cupboard where possible, as this removes the toxic gases. As ethanol and halogenoalkanes are highly flammable, care should be taken that they are not near naked flames. Eye protection should be worn at all times, as well as protective clothing to prevent injury caused by splash back or entry of any of the reagents or the hot water. ...read more.

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