Which of the following substitution/redox reactions with halogens (F, Cl, Br, I) can take place?

Question

For the following reactions:

1. NaI(aq) + Cl2(g)

2. NaF(aq) + Cl2(g)

3. NaCl(aq) + Br2(aq)

4. NaF(aq)+Br2(aq)

Which reaction are allowed?

Answer 1

The reactions you're looking at are between halogens (F, Cl, Br, I), which exist primarily in their diatomic forms (F2, Cl2, Br2, and I2) and halide ions (F-, Cl-, Br-, and I-).

To determine which halogen reacts with which halide ion, you need to look at the periodic table. Electronegativity increases to F. Therefore:

F > Cl> Br> I electronegativity

The higher the electronegativity, the stronger the oxidizing power of oxidizing agents. Therefore:

F2> Cl2> Br2> I2 oxidizing agent strength

The oxidizing agent gains electrons (is reduced) in the reaction to produce its reduced form in the products. The reduced form is a reducing agent which can lose electrons (be oxidized). The strength of the corresponding reducing agents is opposite to oxidizing strength. Therefore:

I- > Br- > Cl- > F- reducing agent strength

For a reaction to proceed, the oxidizing agent and reducing agent in the reactants (left side of equation) must be stronger than the oxidizing and reducing agents they form in the products (right side of equation).

Let's look at the reactions provided to compare oxidizing and reducing agents.

1. For NaI (aq) + Cl2 (g)

If the reaction is possible, the balanced equation would be:

2NaI (aq) + Cl2 (g) -> I2 (aq) + 2NaCl (aq)

I- is the reducing agent in reactants which forms I2, the oxidizing agent in the products.

Cl2 is the oxidizing agent in the reactants which forms Cl-, the reducing agent in the products.

I- > Cl- reducing agent strength

Cl2 > I2 oxidizing agent strength

The reactant agents are stronger. This reaction WILL proceed as follows:

2NaI (aq) + Cl2 (g) -> I2 (aq) + 2NaCl (aq)

2. For NaF(aq)+Cl2(g)

If the reaction is possible, the balanced equation would be:

2NaF (aq) + Cl2 (g) -> F2 (aq) + 2NaCl (aq)

F- is the reducing agent in reactants which forms F2, the oxidizing agent in the products.

Cl2 is the oxidizing agent in the reactants which forms Cl-, the reducing agent in the products.

F- < Cl- reducing agent strength

Cl2< F2 oxidizing agent strength

The agents are stronger in the products, so this reaction WILL NOT proceed.

3. NaCl(aq)+Br2(aq)

If the reaction is possible, the balanced equation would be:

2NaCl (aq) + Br2 (aq) -> Cl2 (g) + 2NaBr (aq)

Cl- is the reducing agent in reactants which forms Cl2, the oxidizing agent in the products.

Br2 is the oxidizing agent in the reactants which forms Br-, the reducing agent in the products.

Cl- < Br- reducing agent strength

Br2 < Cl2 oxidizing agent strength

The agents are stronger in the products, so this reaction WILL NOT proceed.

4. For NaF(aq)+Br2(aq)

If the reaction is possible, the balanced equation would be:

2NaF (aq) + Br2 (aq) -> F2 (g) + 2NaBr (aq)

F- is the reducing agent in reactants which forms F2, the oxidizing agent in the products.

Br2 is the oxidizing agent in the reactants which forms Br-, the reducing agent in the products.

F- < Br- reducing agent strength

Br2< F2 oxidizing agent strength

The agents are stronger in the products, so this reaction WILL NOT proceed.

These reactions are known more as redox reactions than as substitution reactions. But, as one atom displaces another you can interpret it as substitution.