Comparing the Rates of Hydrolysis Reactions

Question 1

why can silver nitrate solution be used in a test for halide ions

Answer 1

• because the silver ions in silver nitrate react with the halide ions formed in the hydrolysis (with water in the solution)
• to give a precipitate

Question 2

what is the general formula for this occurring

Answer 2

Ag+ + X- = AgX

Question 3

how could we use this to determine how quickly the hydrolysis reaction occurs

Answer 3

be observing how quickly the precipitate of AgX forms

Question 4

why couldnt you tell the rate of the reactions without the percipitate

Answer 4

because both reagents and the products are colourless

Question 5

what are the three things the comparison of the rates of reaction involves

Answer 5

• using ethanol as the solvent for the mixture
• controlling variables like temperature and the concentration and quantity of the halogenoalkanes
• timing the appearance of the precipitate

Question 6

why does ethanol need to be used as a solvent for the mixture

Answer 6

• because halogenoalkanes and aqueous silver nitrate dont mix
• but from separate layers
• ethanol is needed to prevent this

Question 7

why is difficult to time the appearance of the precipitate

Answer 7

because a precipitate may appear faint but become thicker over time

Question 8

what is the general trend you should observe about how much precipitate is formed as you go down group 7

Answer 8

• more and thicker precipitates are formed as you go down the group
• due to the halogenoalkanes being less reactive as you go down group 7

Question 9

why would you expect the halogens at the top of the group to form the precipitates the quickest regarding their bonds

Answer 9

• the electronegativity of the elements in group 7 increase as you go up the group
• meaning the o+ charge on the carbon atom would also increase
• so the attacking nucleophile should be attracted more strongly

Question 10

despite that prediction, why do the elements down the group experience the fastest results when reacted with silver nitrate in the same conditions

Answer 10

• the bond energies between the carbon atom and the halogen decrease as you do down the group
• weaker bonds break more easily than stronger bonds, so the bonds between the carbon atom the halogen as you go down the group require less energy to break
• this means that for the same conditions, the C-X bond breaks most easily as you go down the group, so the precipitate forms more quickly

Question 11

what is the trend in how quickly precipitates are formed regarding what the classification of halogenoalkane is

Answer 11

• tertiary halogenoalkanes react the quickest with the silver nitrate to form the precipitate the quickest
• primary halogenoalkanes are the slowest
• with secondary ones being in between