Question

For the reaction \(2 \mathrm{Al}(s)+3 \mathrm{Br}_{2}(l) \rightarrow 2 \mathrm{AlBr}_{3}(s),\) show how electrons are gained and lost by the atoms.

Short answer

During the reaction \(2\mathrm{Al}(s) + 3\mathrm{Br}_{2}(l) \rightarrow 2\mathrm{AlBr}_{3}(s)\), aluminum (Al) loses 6 electrons as its oxidation number changes from 0 to +3, hence it is oxidized. Bromine (Br) gains these 6 electrons as its oxidation number changes from 0 to -1, indicating it is reduced. The balanced equation shows the total number of electrons lost by Al is equal to the total number of electrons gained by Br, resulting in the formation of AlBr3.

Step-by-step solution

Assign Oxidation Numbers

Before the reaction, Al is in its elemental form with an oxidation number of 0. Similarly, Br2 is in its elemental form, and each Br atom also has an oxidation number of 0. In AlBr3, Al has an oxidation number of +3, and each Br atom has an oxidation number of -1. We now have enough information to understand the gain and loss of electrons.

Determine Oxidation and Reduction

During the reaction, the oxidation number of Al changes from 0 to +3. Since the oxidation number increases, Al loses electrons and is oxidized in the process. On the other hand, the oxidation number of Br changes from 0 to -1, meaning that Br gains electrons and is reduced.

Balance the Reaction

Now that we have established which elements are oxidized and reduced, we can illustrate this in a balanced chemical equation: 2Al(s) + 3Br2(l) → 2AlBr3(s) From the balanced equation, we can see that for every 2 moles of Al, 3 moles of Br2 are required to form 2 moles of AlBr3. This also ensures that the total number of electrons lost by Al is equal to the total number of electrons gained by Br.

Determine Electron Transfer

In order to calculate the amount of electrons gained and lost, we need to consider the change in the oxidation numbers of both species and the stoichiometry of the reaction. Since Al loses 3 electrons (oxidation number change from 0 to +3), and there are 2 moles of Al in the balanced equation, a total of 6 electrons are lost by Al: 2 moles Al × 3 electrons/Al = 6 electrons lost Two moles of AlBr3 are formed, and within each AlBr3, there are 3 Br atoms, meaning 6 Br atoms are involved in the reaction in total. The oxidation number of Br changes from 0 to -1, indicating that each Br atom gains 1 electron. Therefore, the total number of electrons gained by Br is: 6 moles Br × 1 electron/Br = 6 electrons gained In conclusion, 6 electrons are lost by Al and gained by Br during the reaction, resulting in the formation of AlBr3.

Key concepts

Oxidation Numbers

Understanding oxidation numbers is crucial to grasping the concepts of oxidation and reduction. They're like bookkeeping tools in chemistry that help track the electron transfer during a chemical reaction.
Oxidation numbers assign a value to each atom in a compound based on its charge, assuming the compounds are fully ionic.

• In their elemental forms, like Al and Br₂, oxidation numbers are zero.
• When bonded in compounds, they reflect the loss or gain of electrons during bonding.

For instance, in the reaction provided, Al transforms from an element with an oxidation number of 0 to a compound where it has an oxidation number of +3. Similarly, Br goes from 0 to -1 in AlBr₃. Assigning these oxidation states helps us identify changes in electron ownership during the reaction.

Electron Transfer

Electron transfer is at the heart of redox (reduction-oxidation) reactions. This involves the movement of electrons from one element to another.
In the given reaction, Al loses electrons, while Br gains them.

• Each atom of aluminum loses 3 electrons, increasing its oxidation number as it becomes Al³⁺.
• Bromine gains those electrons, each Br atom receiving one electron.

The total transfer involves 6 electrons, as 2 moles of Al lose electrons to precisely balance with the 6 Br atoms that gain them. This balance is key and aligns with the conservation of charge, an essential principle in chemistry.

Balanced Chemical Equation

A balanced chemical equation is a powerful tool in understanding chemical reactions.
It ensures that the number of atoms and the charge are conserved throughout the reaction.
For the reaction 2Al(s) + 3Br₂(l) → 2AlBr₃(s), balancing helps in several ways:

• It shows that two moles of aluminum solids react with three moles of diatomic bromine.
• This forms two moles of aluminum bromide.

The equation not only signifies the quantitative aspect but also the qualitative, indicating how substances transform and provide clues about the electron transfer. Correctly balancing ensures that electrons lost in oxidation are gained in reduction, achieving consistent stoichiometry between reactants and products.