Question

(a) Use the following reactions to prepare an activity series for the halogens: $$ \begin{aligned} \mathrm{Br}_{2}(a q)+2 \mathrm{NaI}(a q) & \longrightarrow 2 \mathrm{NaBr}(a q)+\mathrm{I}_{2}(a q) \\ \mathrm{Cl}_{2}(a q)+2 \mathrm{NaBr}(a q) & \longrightarrow 2 \mathrm{NaCl}(a q)+\mathrm{Br}_{2}(a q) \end{aligned} $$ (b) Relate the positions of the halogens in the periodic table with their locations in this activity series. (c) Predict whether a reaction occurs when the following reagents are mixed: \(\mathrm{Cl}_{2}(a q)\) and \(\mathrm{KI}(a q) ; \mathrm{Br}_{2}(a q)\) and \(\mathrm{LiCl}(a q)\).

Short answer

The activity series for the given halogens is Cl₂ > Br₂ > I₂, showing that the reactivity decreases down group 17 in the periodic table. A reaction will occur between Cl₂(aq) and KI(aq) as Cl₂ is more reactive than I₂, forming KCl(aq) and I₂(aq). However, no reaction will occur between Br₂(aq) and LiCl(aq) as Br₂ is less reactive than Cl₂ and cannot displace it from LiCl.

Step-by-step solution

1. Understand the given reactions

The given reactions are displacement reactions between halogens and their salts. They show the ability of one halogen to displace another from its salt, which helps us in creating an activity series. In both reactions, one halogen displaces the other from its salt.

2. Create an activity series

Using the given reactions, we see that Br₂ is able to displace I₂ from NaI (forming NaBr) and Cl₂ is able to displace Br₂ from NaBr (forming NaCl). This means that Cl₂ is more reactive than Br₂, and Br₂ is more reactive than I₂. So, the activity series for the given halogens is Cl₂ > Br₂ > I₂.

3. Relate the positions in the periodic table and activity series

In the periodic table, the halogens belong to group 17, with Fluorine (F₂) at the top, followed by Chlorine (Cl₂), Bromine (Br₂), Iodine (I₂), and Astatine (At₂). The reactivity of halogens generally decreases down the group, so Cl₂ is more reactive than Br₂, and Br₂ is more reactive than I₂. This is the same order as the activity series that we derived in step 2. The relationship between the positions of halogens in the periodic table and the activity series is that the order of reactivity decreases from top to bottom in group 17.

4. Predict the occurrence of reactions

(a) Cl₂(aq) and KI(aq): Since Cl₂ is more reactive than I₂ (based on our activity series), it will displace I₂ from the potassium iodide (KI) salt, forming the products KCl(aq) and I₂(aq). So, a reaction will occur in this case. \( Cl_{2}(aq) + 2KI(aq) \longrightarrow 2KCl(aq) + I_{2}(aq) \) (b) Br₂(aq) and LiCl(aq): According to the activity series, Br₂ is less reactive than Cl₂. This means Bromine cannot displace Chlorine from Lithium Chloride (LiCl); hence, no reaction will occur in this case.

Key concepts

Displacement Reactions

In the world of chemistry, displacement reactions form one of the foundations of predictive reactions. They involve replacing one element in a compound with another, more reactive, element. Consider it like a game of musical chairs played by chemical elements; the more reactive one finds a seat, while the less reactive one is left 'standing', so to speak.

For example, when we look at the reaction of bromine (\textnormal{Br}\(_2\)(aq)) with sodium iodide (\textnormal{NaI}(aq)), the bromine 'takes the seat' of iodine in the compound, resulting in sodium bromide (\textnormal{NaBr}(aq)) and free iodine (\textnormal{I}\(_2\)(aq)). This happens because bromine is more reactive than iodine. The general form of such displacement reactions is \textnormal{A} + \textnormal{BC} \rightarrow \textnormal{AC} + \textnormal{B}, where \textnormal{A} is more reactive than \textnormal{B}. Understanding this concept helps us predict the outcome of reactions between halogens and halide salts.

Halogens Reactivity

Diving deeper into halogen chemistry, we notice a trend in their reactivity. This trend corresponds to their position in the activity series, a ranking based on the reactivity of elements, particularly halogens in this context.

In our exercise, we established that chlorine (\textnormal{Cl}\(_2\)) is more reactive than bromine (\textnormal{Br}\(_2\)), which in turn is more reactive than iodine (\textnormal{I}\(_2\)). A vital takeaway here is that a more reactive halogen can displace a less reactive one from its compound. Since Fluorine (\textnormal{F}\(_2\)) is the most reactive of the halogens, but was not part of the given reactions, we can infer it would sit atop the activity series, making our series read \textnormal{F}\(_2\) > \textnormal{Cl}\(_2\) > \textnormal{Br}\(_2\) > \textnormal{I}\(_2\).

Periodic Table Trends

The periodic table is an incredible tool for predicting the behavior of elements, including the reactivity of halogens. As part of group 17, halogens showcase a trend of decreasing reactivity as we move down the group. This aligns with what our activity series suggested.

Starting from fluorine at the top, which isn't directly observed in our reactions but known to be the most reactive, down to iodine at a lower position, reactivity decreases. It is because as we go down the group, the atoms get larger, and the outer electrons are further away from the nucleus. This makes it more difficult for these elements to attract additional electrons, which is a key aspect of halogen reactivity. By understanding these trends, students can not only memorize the order of reactivity but also grasp the 'why' behind the sequence.