Halogen Displacement Reactions

For the last 2 weeks, i have given several sets of O Level Chemistry Questions on “Tough Question Series” to students in our Sec 4 O-Level & IP Chemistry Tuition Classes. These are not really very tough questions but are more like tricky application questions that test their understanding and concepts.

Let’s check out one of the question on the topic of “Periodic Table & Group Trends”.

Question 1:
You are provided with bromine water, aqueous sodium chloride and aqueous sodium iodide, together with a supply of test tubes. Using these solutions only, describe two experiments to show the relative reactivity of chlorine, bromine and iodine.

For each experiment, describe what you would do and what observation you would make. State how the results of the experiments confirm the relative reactivity and write an equation for any reaction that occurs. [8]

Experiment I:

Experiment II:

Conclusion on relative reactivity:

It came to my awareness that many students are not apt in answering questions on “Describing an experiment….” and make some common errors in their answers.

Below are some of the answers that i have received and the common errors that students tend to make year after year, batch after batch (this is how Up Your Chemistry Grades Now! came about).

Answer by Student A:

Answer by Student B:

Answer by Student C:

Answers by all three students are similar in one way

• it shows that all three of them understand the reactivity of Halogens (Gp VII elements) as it goes down the group

However, some are:

• not sure about the colour of solutions and elements
• not sure about writing of chemical equations (balanced)
• not sure how to present a “Describe an Experiment…” type of questions

Please see my suggested answer below:

Experiment I:

Add bromine water to sodium iodide solution.
The colourless sodium iodide solution will turns brownish.
Bromine has displaced iodine from the sodium iodide solution.
This shows that bromine is more reactive than iodine.

Br₂ (aq) + 2NaI(aq) → 2NaBr(aq) + I₂(aq)

Experiment II:

Add bromine water to sodium chloride solution.
No visible observations.
This shows that bromine is less reactive than chlorine.

Br₂(aq) + NaCl(aq) → No Reaction

Conclusion on relative reactivity:

Reactivity in decreasing order: Chlorine > Bromine > Iodine

Hope the above discussion is useful to your learning for O-Level and IP Pure Chemistry.

Before we go off, how about trying out the following question:

Question 2:
Excess bromine is shaken with a mixture of sodium chloride and sodium iodide solutions.
What will the final solution contain?

A. Sodium chloride and sodium bromide
B. Sodium chloride and sodium iodide
C. Sodium iodide and sodium bromide
D. Sodium chloride, sodium bromide and sodium iodide

PS: Let me know your answers and the reasons for choosing it by typing it in the comment section below.

PPS: If you would like to see more discussion on Periodic Table & Group Trends, Click HERE for more O-Level and IP Pure Chemistry exam-based questions and answers.