Problem 58 - Chemistry: The Central Science

Question

The following reactions (note that the arrows are pointing only one direction) can be used to prepare an activity series for the halogens: $$ \begin{gathered} \mathrm{Br}_{2}(a q)+2 \mathrm{NaI}(a q) \longrightarrow 2 \mathrm{NaBr}(a q)+\mathrm{I}_{2}(a q) \\ \mathrm{Cl}_{2}(a q)+2 \mathrm{NaBr}(a q) \longrightarrow 2 \mathrm{NaCl}(a q)+\mathrm{Br}_{2}(a q) \end{gathered} $$ (a) Which elemental halogen would you predict is the most stable, upon mixing with other halides? (b) Predict whether a reaction will occur when elemental chlorine and potassium iodide are mixed. (c) Predict whether a reaction will occur when elemental bromine and lithium chloride are mixed.

Step-by-Step Solution

Answer

(a) The most stable halogen upon mixing with other halides is iodine (I\(_2\)), as it is the least reactive in the activity series for these halogens. (b) A reaction will occur when elemental chlorine (Cl\(_2\)) and potassium iodide (KI) are mixed, producing 2 KCl(aq) and I\(_2\)(aq). (c) No reaction will occur when elemental bromine (Br\(_2\)) and lithium chloride (LiCl) are mixed, since bromine is less reactive than chlorine.

Step 1: Understand the given reactions

From the given reactions with unidirectional arrows: 1. Br\(_2\)(aq) + 2 NaI(aq) → 2 NaBr(aq) + I\(_2\)(aq) 2. Cl\(_2\)(aq) + 2 NaBr(aq) → 2 NaCl(aq) + Br\(_2\)(aq) We can deduce the following from these equations: - Elemental bromine (Br\(_2\)) can displace iodine (I\(_2\)) from its salt (NaI) - Elemental chlorine (Cl\(_2\)) can displace bromine (Br\(_2\)) from its salt (NaBr)

Step 2: Prepare the activity series for halogens

With the information from the given equations, we can say that chlorine is more reactive than bromine, and bromine is more reactive than iodine. Hence, our activity series for these three halogens is: Cl\(_2\) > Br\(_2\) > I\(_2\)

Step 3: Answer part (a) - Predict the most stable halogen

The least reactive halogen will be the most stable halogen when mixed with other halides. According to our activity series, the least reactive halogen is iodine (I\(_2\)). So, the elemental iodine would be the most stable halogen when mixed with other halides.

Step 4: Answer part (b) - Predict the reaction between chlorine and potassium iodide

We are asked to predict whether a reaction will occur when elemental chlorine (Cl\(_2\)) and potassium iodide (KI) are mixed. Based on our activity series, we know that chlorine is more reactive than iodine. Therefore, chlorine will displace iodine from potassium iodide and a reaction will occur. The reaction is: Cl\(_2\)(aq) + 2 KI(aq) → 2 KCl(aq) + I\(_2\)(aq)

Step 5: Answer part (c) - Predict the reaction between bromine and lithium chloride

We need to predict whether a reaction will occur when elemental bromine (Br\(_2\)) and lithium chloride (LiCl) are mixed. According to our activity series, bromine is less reactive than chlorine. Therefore, bromine will not displace chlorine from lithium chloride and no reaction will occur in this case.

Key Concepts

Chemical Reactivity and the Halogens

Chemical reactivity refers to the ability of a substance to undergo a chemical change. This property varies across the periodic table of elements, particularly within groups such as the halogens. The halogens include fluorine, chlorine, bromine, iodine, and astatine. They are known for their high reactivity, especially with metals, forming a wide range of salts. For example, when a halogen reacts with a metal, a halide (salt) is typically formed.

In the context of reactivity, an 'activity series' is a helpful tool. It ranks elements like halogens based on their ability to displace other halogens from their compounds in chemical reactions. This series is crucial for predicting the outcomes of such reactions. In our textbook exercise, we examined reactions involving bromine and chlorine to establish an activity series for these halogens.

Displacement Reactions Among Halogens

Displacement reactions occur when a more reactive element displaces a less reactive one from a compound. In the case of halogens, a displacement reaction typically involves halogens and halide salts. The halogen displacing another must be higher in the activity series for the reaction to occur.

For instance, we saw that chlorine (Cl₂) reacts with sodium bromide (NaBr) to displace bromine (Br₂) and form sodium chloride (NaCl). This reaction only occurs because chlorine is more reactive than bromine. When teaching these concepts, it's important to clarify that displacement reactions demonstrate the differences in reactivity among elements in the same group. In this case, the ability of one halogen to displace another from its salt is directly related to its position in the activity series.

Reactivity Trend of Halogens

The reactivity trend of halogens is crucial for understanding why certain halogens can displace others. Within the group of halogens, reactivity decreases as you move down the periodic table. Fluorine is the most reactive, followed by chlorine, bromine, and iodine, with astatine being the least reactive due to its instability and rarity.

This trend is due to the atomic structure of halogens; as you move down the group, each element has more electron shells, which shield the attractive force of the nucleus from the valence electrons. This makes it more difficult for the element to attract additional electrons, hence the decrease in reactivity. In the exercise solutions, this trend is illustrated through the successful displacement of iodine by bromine and bromine by chlorine, confirming that Cl₂ is more reactive than Br₂, and Br₂ more than I₂.