Question

The following reactions (note that the arrows are pointing only one direction) can be used to prepare an activity series for the halogens: $$\begin{array}{c}{\mathrm{Br}_{2}(a q)+2 \mathrm{Nal}(a q) \longrightarrow 2 \mathrm{NaBr}(a q)+\mathrm{I}_{2}(a q)} \\ {\mathrm{Cl}_{2}(a q)+2 \mathrm{NaBr}(a q) \longrightarrow 2 \mathrm{NaCl}(a q)+\mathrm{Br}_{2}(a q)}\end{array}$$ (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.

Short answer

(a) The most stable halogen upon mixing with other halides is iodine (\(I_2\)), as stability increases going down the group of halogens. (b) A reaction will occur when elemental chlorine and potassium iodide are mixed, producing potassium chloride (\(KCl\)) and elemental iodine (\(I_2\)). (c) No reaction will occur when elemental bromine and lithium chloride are mixed, as bromine is less reactive than chlorine.

Step-by-step solution

1. Identify the Trends in Halogens

Recall that the halogens form a group in the periodic table (Group 17). Their reactivity decreases going down the group, making the higher halogens more stable than the lower ones. Using this information, we can list the halogens in decreasing order of reactivity: \(\\ Cl_2 > Br_2 > I_2\\ \).

2. Compare the Reactivity to Predict Stability

(a) Since reactivity decreases as we go down the halogens, stability increases. Therefore, the most stable halogen would be the one at the bottom of the list. In our case, it is iodine (\(I_2\)).

3. Predict the Outcome of Mixing Elemental Chlorine and Potassium Iodide

(b) The given reaction is: \(\\ Cl_2 (aq) + 2KI (aq)\\ \). Following the trend in reactivity, if the elemental chlorine is more reactive than iodine, the reaction will occur. Looking at our reactivity trend, we see that: \(\\ Cl_2 > I_2\\ \), so elemental chlorine is more reactive than iodine. Hence, the reaction will occur, producing potassium chloride (\(KCl\)) and elemental iodine (\(I_2\)): \[Cl_2(aq) + 2KI(aq) \longrightarrow 2KCl(aq) + I_2(aq)\]

4. Predict the Outcome of Mixing Elemental Bromine and Lithium Chloride

(c) The given reaction is: \(\\ Br_2 (aq) + 2LiCl (aq)\\ \). Similar to the previous step, if the elemental bromine is more reactive than the chloride ion (Cl), the reaction will occur. Comparing the reactivity, we see that bromine is less reactive than chlorine: \(\\ Cl_2 > Br_2\\ \), so elemental bromine is less reactive than the chloride ion. Therefore, no reaction will occur when elemental bromine and lithium chloride are mixed.

Key concepts

Periodic Table

The periodic table is an essential tool in chemistry that arranges elements based on their atomic number, electron configuration, and recurring chemical properties. Elements are organized into rows called periods and columns known as groups. Each group contains elements with similar properties and behaviors. For instance, Group 17 consists of the halogens, which include fluorine, chlorine, bromine, iodine, and astatine.

Understanding the periodic table allows chemists to predict how different elements will react with one another. It helps to identify trends in reactivity, atomic size, and electronegativity across different groups and periods. In particular, as you move from top to bottom in a group, you'll notice that metallic character increases, while non-metals like the halogens become less reactive.

Chemical Reactions

Chemical reactions occur when substances combine to form new products, involving the breaking and forming of bonds. In the case of halogen reactions, these reactions often involve the transfer of electrons, resulting in ionic compounds.

When elemental halogens react with halide compounds, one element can replace another based on their reactivity. This type of reaction is known as a single displacement reaction. For instance, when chlorine (\(Cl_2\)) reacts with potassium iodide (\(KI\)), it displaces iodine due to being more reactive. This reaction highlights the concept of reactivity in chemical reactions, showcasing how some elements are capable of displacing others based on their position in the activity series.

Halogens

The halogens are a group of highly reactive non-metal elements in Group 17 of the periodic table. They include fluorine, chlorine, bromine, iodine, and astatine, each with unique properties. Halogens are known for their high electronegativities and their ability to gain an electron to form negative ions called halides.

The reactivity of halogens decreases as you move down the group from fluorine to astatine. This trend is because the atomic size increases, making it harder for the nucleus to attract additional electrons. Despite this decrease in reactivity, halogens remain very active compared to other element groups and can form compounds with most elements.

Stability and Reactivity

The concepts of stability and reactivity are interconnected. Reactivity refers to the tendency of an element to undergo chemical reactions, while stability indicates how resistant an element is to changing its state. Generally, as reactivity decreases, stability increases.

For the halogens, the topmost elements in the group are more reactive than those lower down. Hence, iodine, being at the bottom, is the most stable when mixed with other halides. This increased stability occurs because iodine's larger atomic size results in weaker attraction of additional electrons, making it less likely to react compared to higher halogens like chlorine.

Activity Series

The activity series is a ranking of elements based on their reactivity. This series is particularly useful in predicting the outcomes of single displacement reactions among metals and halogens. For halogens, the activity series is: chlorine (\(Cl_2\)) > bromine (\(Br_2\)) > iodine (\(I_2\)).

In a halogen displacement reaction, a more reactive halogen can replace a less reactive halide in a compound. The activity series helps in determining whether a reaction will occur. For example, since chlorine is above iodine in the activity series, it can replace iodine in compounds. Conversely, bromine cannot replace chlorine due to its lower reactivity, which helps predict the outcome of halogen reactions.