Question

Is this reaction a redox reaction? Explain your answer. $$ 2 \mathrm{~K}(\mathrm{~s})+\mathrm{Br}_{2}(\ell) \rightarrow 2 \mathrm{KBr}(\mathrm{s}) $$

Short answer

Yes, it is a redox reaction due to the transfer of electrons (oxidation and reduction).

Step-by-step solution

Identify the Oxidation States

Determine the oxidation states of potassium (K) and bromine (Br) in both the reactants and products. Initially, in its elemental form, potassium (K) has an oxidation state of 0, and bromine (Br) also has an oxidation state of 0 in its diatomic form (Br₂). In the product, potassium bromide (KBr), K has an oxidation state of +1 and Br has an oxidation state of -1.

Identify Changes in Oxidation States

Compare the oxidation states of each element before and after the reaction. Potassium changes from 0 to +1, indicating it has lost electrons (oxidation). Bromine changes from 0 to -1, indicating it has gained electrons (reduction).

Determine Redox Nature of Reaction

Since there is a simultaneous oxidation (K from 0 to +1) and reduction (Br from 0 to -1), this reaction is classified as a redox reaction. Redox reactions are characterized by the transfer of electrons between elements.

Key concepts

Oxidation States

Oxidation states, also known as oxidation numbers, help us understand the electron distribution within molecules. They indicate the number of electrons an atom effectively gains or loses when forming a chemical bond in a compound. For elemental potassium (K) and bromine (Br), the oxidation state is 0. This is because they exist in their natural, uncombined form. This changes when they form compounds, like potassium bromide (KBr). Here, potassium has an oxidation state of +1, and bromine has -1.

• Potassium (K) starts with an oxidation state of 0 and becomes +1 in KBr.
• Bromine (Br) starts with an oxidation state of 0 and becomes -1 in KBr.

Recognizing these changes is crucial in identifying whether a reaction involves the shift of electrons, which is a key feature of redox reactions.

Electron Transfer

Electron transfer is at the heart of redox reactions. It refers to the movement of electrons from one species (usually an atom or molecule) to another. In the chemical equation provided, the transfer is clear. Potassium (K) loses electrons, signaling oxidation, while bromine (Br) gains electrons, indicating reduction.
Key points of electron transfer:

• Oxidation involves the loss of electrons. Potassium (K) goes from 0 to +1, losing an electron.
• Reduction involves the gain of electrons. Bromine (Br) goes from 0 to -1, gaining an electron.

This complementary process creates a balanced transaction, where the number of electrons lost equals the number gained, ensuring conservation of charge.

Oxidation and Reduction

Oxidation and reduction are intertwined processes in redox reactions. Oxidation involves a substance losing electrons, while reduction is the gaining of electrons by another. In the example reaction:

• Potassium (K) is oxidized since it loses electrons, changing from an oxidation state of 0 to +1.
• Bromine (Br) is reduced as it gains electrons, changing from an oxidation state of 0 to -1.

These processes occur simultaneously; thus, whenever oxidation occurs, reduction happens too. This dual process is why the overall reaction is considered a redox reaction. Recognizing these paired changes helps determine the nature of reactions and can explain many chemical processes observed in lab experiments and daily life.