Question

In balancing oxidation-reduction equations, why is it permissible to add water to either side of the equation?

Short answer

In balancing oxidation-reduction equations, it is permissible to add water to either side of the equation because it helps balance the oxygen and hydrogen atoms within half-reactions without affecting the overall charge balance. Water is chemically consistent and serves as a convenient source of oxygen and hydrogen atoms needed for achieving balanced half-reactions, ultimately leading to a balanced overall redox equation.

Step-by-step solution

Understand Half-Reactions of a Redox Equation

A redox equation can be split into two half-reactions: one that represents the oxidation process and the other that represents the reduction process. In each half-reaction, the number of electrons lost or gained must be equal so that the overall reaction remains balanced in terms of charge.

Balancing Atoms in Half-Reactions

For each half-reaction, atoms other than hydrogen and oxygen should be balanced first. Then, we can balance oxygen atoms in half-reactions that involve molecular oxygen (O2) or water (H2O) by adding the necessary number of oxygen atoms in the form of water molecules. Similarly, we can balance hydrogen atoms in half-reactions by adding the required number of hydrogen atoms in the form of water molecules or hydrogen ions (H+).

Balance Charge in Half-Reactions

In order to balance the charge in each half-reaction, the number of electrons gained or lost should be equal. So, if the charges on both sides of the half-reaction are unequal, we can add electrons to the side where the charge is more positive to balance the charge in the half-reaction.

Combine Half-Reactions

After balancing atoms and charge in the two half-reactions, we can combine the half-reactions to obtain the complete balanced redox reaction. The electrons that are gained by one species should be equal to the electrons that are lost by the other species. If necessary, the coefficients of the half-reactions can be multiplied by suitable integers so that the number of electrons gained and lost is equal.

Balance Overall Equation

At this point, check the overall balanced redox equation to make sure that it meets the following requirements: (1) The total number of atoms for each element is the same on both sides of the equation, (2) The net charge on both sides of the equation is the same, and (3) The number of electrons lost and gained is equal. If these criteria are met, then the redox equation is correctly balanced. In conclusion, adding water to a redox equation is permissible because it allows for the balancing of oxygen and hydrogen atoms within half-reactions, ultimately helping to balance the overall equation. It does not affect the overall charge balance and is chemically consistent as long as the added water molecules are factored into the balancing process.

Key concepts

Half-Reactions

Understanding half-reactions is a crucial step in mastering redox chemistry. A redox (oxidation-reduction) reaction can be deconstructed into two simpler parts, known as half-reactions. Each half-reaction represents either the loss of electrons (oxidation) or the gain of electrons (reduction).

In the context of redox equations, the process starts by identifying and writing the individual half-reactions. Atoms and electrons are accounted for separately in these half-reactions. This separation allows us to focus on balancing each process one at a time, which simplifies the overall task of balancing the redox equation.

Oxidation and Reduction

To fully grasp redox reactions, it's important to understand oxidation and reduction. Oxidation refers to the loss of electrons by a molecule, atom, or ion. On the flip side, reduction is the gain of electrons. These processes are inseparable; whenever one species undergoes oxidation, another undergoes reduction.

This electron transfer is what chemistry is all about. For example, when iron rusts, it is oxidized to form iron oxide, and oxygen is reduced. Learning to identify which species is oxidized and which is reduced helps in writing accurate half-reactions, setting the foundation for eventually balancing the entire reaction.

Balancing Charge

Half-reactions often start out unbalanced in the charge department. The electron loss in the oxidation half-reaction and electron gain in the reduction half-reaction must eventually cancel each other out. To achieve this, we add electrons as reactants or products within the half-reactions to balance the overall charge.

Electrons function as the currency of redox reactions, and just like in economics, the books must balance. Ensuring that the charge is consistent on both sides of each half-reaction paves the way for the electrons to cancel out when the half-reactions are recombined to form the full equation—demonstrating charge conservation.

Stoichiometry

Stoichiometry is like the mathematical backbone of chemistry, ensuring that reactants and products are accounted for in a balanced equation. In the realm of redox reactions, once half-reactions are balanced for mass and charge, stoichiometry helps establish the final, balanced equation.

It may involve multiplying the coefficients of each half-reaction to ensure that the electrons lost in oxidation are equal to those gained in reduction. This careful calculation guarantees that mass and charge are conserved in the complete reaction, following the law of conservation of mass and the principle of charge neutrality.