Which of the following reaction is possible: (a) \(\mathrm{Cl}_{2}+2 \mathrm{NaBr} \rightarrow \mathrm{Br}_{2}+2 \mathrm{NaCl}\) (b) \(\mathrm{I}_{2}+2 \mathrm{NaCl} \rightarrow \mathrm{Cl}_{2}+2 \mathrm{NaCl}\) (c) \(\mathrm{I}_{2}+2 \mathrm{NaBr} \rightarrow \mathrm{Br}_{2}+2 \mathrm{NaI}\) (d) \(\mathrm{Br}_{2}+2 \mathrm{NaCl} \rightarrow \mathrm{Cl}_{2}+2 \mathrm{NaBr}\)

The reactivity series of halogens is an important concept in chemistry that ranks the halogens — fluorine, chlorine, bromine, iodine, and astatine — according to their reactivity. This series is essential for predicting the outcomes of halogen displacement reactions, which are a type of redox reaction.

In this series, the reactivity decreases as we move down the group in the periodic table. The order from most to least reactive is fluorine (F₂), chlorine (Cl₂), bromine (Br₂), iodine (I₂), and astatine (At₂), although astatine is rarely discussed due to its radioactivity and rarity.

Understanding the Activity Trend

The trend in reactivity can be attributed to the atomic structure of the halogens. As you move down the group, atomic size increases, and the outer electrons are further from the nucleus. This makes it more difficult for the halogen to attract additional electrons, thus reducing its reactivity.

This knowledge allows us to predict which halogen displacement reactions are possible. A key point from the exercise is that a more reactive halogen will displace a less reactive halogen from its salts. For example, chlorine can displace bromine because it is higher in the reactivity series.

Chemical reactivity in halogens is governed by their ability to gain electrons and form negative ions called halides. This electronegativity is the driving force behind their reactivity and is essential when considering displacement reactions. Halogens with higher electronegativities tend to be more reactive.

Factors Affecting Halogen Reactivity

Not only does atomic size play a role in halogen reactivity, but other factors such as electron shielding and nuclear charge also influence it. With increased electron shielding in larger atoms, the effective nuclear charge felt by the valence electrons decreases, making the atom less reactive.

In the context of the provided exercise, we note that when halogens act as oxidizing agents in displacement reactions, the more reactive halogen can oxidize the ions of the less reactive halogen. For instance, chlorine (Cl₂) can oxidize bromide ions (Br⁻) to bromine (Br₂), but iodine (I₂) cannot oxidize chloride ions (Cl⁻) to chlorine (Cl₂).

Predicting chemical reactions involves understanding the reactivity of the elements involved and using this information to determine if a reaction will proceed. This is where the reactivity series becomes instrumental, particularly for halogen displacement reactions.

Applying the Reactivity Series

We can predict the feasibility of a reaction by comparing the position of each halogen in the reactivity series. Only reactions where a more reactive halogen displaces a less reactive halogen from its compound are possible. Applying this to the provided exercise, we can deduce that chlorine, being more reactive than bromine, can displace bromine from sodium bromide (NaBr), making reaction (a) possible.

By contrast, reaction (b) is not feasible because iodine is less reactive than chlorine and cannot displace chlorine ions from sodium chloride (NaCl). This logical approach simplifies the prediction of such chemical reactions and is fundamental in studying and understanding chemistry.