The method and the results are on the halogenoalkanes lab. Do the analysis yourself first, then check the answers below.
Quick answer: Alkanes react with halogens in UV light by free radical substitution, a chain reaction of initiation, propagation and termination. These answers go with the Chemistry 11-12 lab on halogenoalkanes: the graph, the calculations, the uncertainties and the conclusion explained, for all 7 analysis questions.
Here are the halogenoalkanes lab answers for chemistry grades 11-12: graph, calculations, uncertainties and conclusion. First do the lab yourself, then compare, and go back to the lesson if needed.
These model answers use the sample times in the table. Your times may differ, but iodo must be faster than bromo, chloro slowest, and tertiary fastest of the bromo compounds.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: IB R3.4
Halogenoalkanes lab answers: the analysis explained
Question 1 Identify the precipitates
Chloro gives no precipitate in the time (white AgCl would form slowly), bromo gives cream AgBr and iodo gives yellow AgI. Ionic equation: Ag⁺(aq) + I⁻(aq) → AgI(s).
Question 2 Effect of the halogen
The rate rises in the order Cl < Br < I. The C–I bond is the weakest (228 kJ mol⁻¹) and so breaks fastest, while C–Cl (346 kJ mol⁻¹) is strongest.
Rate increases primary < secondary < tertiary. The tertiary compound reacts by SN1: the halide leaves to give a stable tertiary carbocation, which water then attacks.
Question 5 Controlled variables
Control temperature, volume of silver nitrate and number of drops (and concentration). Pre-warming ensures the reaction starts at 50 °C, so times are comparable.
Question 6 Source of error
Judging when the solution first turns cloudy is subjective. Use a black cross or lamp behind the tube and the same observer, and repeat to average.
Question 7 Hydrolysis equation
(CH₃)₃CBr + H₂O → (CH₃)₃COH + HBr, with the Br⁻ then precipitating with Ag⁺.
