Problem 58 - Chemistry: The Central Science in SI Units, Global Edition

Question

The following reactions (note that the arrows are pointing only one direction) can be used to prepare an activity series for the halogens: $$ \begin{array}{c} \mathrm{Br}_{2}(a q)+2 \mathrm{NaI}(a q) \longrightarrow 2 \mathrm{NaBr}(a q)+\mathrm{I}_{2}(a q) \\ \mathrm{Cl}_{2}(a q)+2 \mathrm{NaBr}(a q) \longrightarrow 2 \mathrm{NaCl}(a q)+\mathrm{Br}_{2}(a q) \end{array} $$ (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) Iodine is the most stable elemental halogen when mixed with other halides. (b) Yes, a reaction will occur when elemental chlorine and potassium iodide are mixed: \[ Cl_2(aq) + 2 KI(aq) \longrightarrow 2 KCl(aq) + I_2(aq) \] (c) No, a reaction will not occur when elemental bromine and lithium chloride are mixed because bromine cannot displace chlorine from the compound.

Step 1: Reaction 1: Bromine and sodium iodide

The first reaction is: \[ Br_2(aq) + 2 NaI(aq) \longrightarrow 2 NaBr(aq) + I_2(aq) \] In this reaction, bromine (Br2) displaces iodine (I2) from its sodium iodide (NaI) compound, forming sodium bromide (NaBr).

Step 2: Reaction 2: Chlorine and sodium bromide

The second reaction is: \[ Cl_2(aq) + 2 NaBr(aq) \longrightarrow 2 NaCl(aq) + Br_2(aq) \] In this reaction, chlorine (Cl2) displaces bromine (Br2) from its sodium bromide (NaBr) compound, forming sodium chloride (NaCl).

Step 3: Activity series of halogens

From the given reactions, we can prepare an activity series for the halogens. In reaction 1, bromine is able to displace iodine from the compound, and in reaction 2, chlorine can displace bromine from the compound. This indicates the following activity series for the halogens: 1. Chlorine 2. Bromine 3. Iodine

Step 4: (a) Stability of elemental halogens

According to the activity series, iodine is at the bottom, which means it is the least reactive or most stable halogen when mixed with other halides.

Step 5: (b) Reaction between elemental chlorine and potassium iodide

From the activity series, chlorine is more reactive than iodine. So, when elemental chlorine is mixed with potassium iodide (KI), a reaction will occur, and chlorine will displace iodine from the compound. This can be represented as: \[ Cl_2(aq) + 2 KI(aq) \longrightarrow 2 KCl(aq) + I_2(aq) \]

Step 6: (c) Reaction between elemental bromine and lithium chloride

From the activity series, bromine is less reactive than chlorine. So, when elemental bromine is mixed with lithium chloride (LiCl), no reaction will occur, as bromine cannot displace chlorine from the compound. Therefore, there is no reaction in this case, and the substances remain unchanged.

Key Concepts

Redox Reactions

In chemistry, redox reactions (short for reduction-oxidation reactions) are vital for understanding how elements behave in compounds. During a redox reaction, there is a transfer of electrons between substances, which leads to changes in oxidation states. The substance that loses electrons undergoes oxidation, while the one that gains electrons undergoes reduction.
For example, in the given reactions, when bromine (\(Br_2\) ) reacts with sodium iodide (\(NaI\) ), bromine displaces iodine. Here, iodine is oxidized as it forms iodine molecules (\(I_2\) ), and bromine is reduced to bromide ions (\(Br^-\) ). Understanding redox reactions helps us interpret why certain substances displace others in reaction series.

• Oxidation: increase in oxidation state by loss of electrons.
• Reduction: decrease in oxidation state by gain of electrons.
• These reactions are essential for many industrial and biological processes.

Halogen Displacement

Halogen displacement reactions showcase the competition between halogens to form compounds by displacing another. When a more reactive halogen comes into contact with a halide, it can "kick out" the less reactive halogen from its compound. The displaced halogen is released as a free element.
This concept is evident in the given example of chlorine and sodium bromide, where chlorine (\(Cl_2\) ) displaces bromine (\(Br_2\) ) from sodium bromide (\(NaBr\) ). Chlorine, being more reactive, ends up forming sodium chloride (\(NaCl\) ), while bromine is liberated.

• Reactive halogen displaces a less reactive halogen from its salt.
• Displacement follows the order of the activity series of halogens.
• Demonstrates chemical reactivity and bonding preference.

Reactivity of Halogens

The reactivity of halogens is an important concept in understanding how different halogens will act in chemical reactions. Halogens are ranked based on their ability to displace each other from compounds, which can be visualized in an "activity series." Generally, the activity decreases as you move down the group in the periodic table.
In our case, chlorine, bromine, and iodine demonstrate this pattern. Chlorine is the most reactive of the three and is capable of displacing both bromine and iodine. Bromine can displace iodine, but not chlorine. Iodine is the least reactive, making it the most stable when mixed with other halides.

• Activity series: Chlorine > Bromine > Iodine.
• Reactivity determines halogen displacement ability.
• Higher reactivity means a higher tendency to form compounds and displace other halogens.