Question

When balancing reactions in Chapter 3, we did not mention that reactions must be charge balanced as well as mass balanced. What do charge balance and mass balance mean? What happens in a redox reaction that requires charge to be balanced?

Short answer

Mass balance refers to the conservation of mass in a chemical reaction, ensuring the number and type of atoms are equal on both sides of the equation. Charge balance refers to the conservation of charge in a reaction, ensuring the total charge of reactants is equal to the total charge of products. In redox reactions, where electrons are transferred between atoms, maintaining charge balance is crucial as every electron lost in oxidation must be gained in reduction to keep the system charge-neutral.

Step-by-step solution

Definition of Mass Balance

Mass balance refers to the principle that the total mass of reactants in a chemical reaction must be equal to the total mass of products, indicating that matter cannot be created or destroyed during the reaction. This principle is based on the conservation of mass. In other words, the number and type of atoms on both sides of a chemical reaction must be identical.

Definition of Charge Balance

Charge balance refers to the principle that the total charge of reactants in a chemical reaction must be equal to the total charge of products. This is because charge is also conserved during processes such as chemical reactions. In other words, the sum of charges on both sides of a chemical equation must be equal to maintain charge neutrality.

Redox Reactions

Redox reactions are chemical reactions in which the oxidation states of some atoms change due to transfer of electrons. They comprise two half-reactions: reduction and oxidation. During reduction, an atom gains electrons and its oxidation state decreases. In the oxidation half-reaction, an atom loses electrons, which causes its oxidation state to increase.

Charge Balance in Redox Reactions

In redox reactions, maintaining charge balance is crucial as electrons are transferred between species. This means that, for every electron lost by one species (oxidation half-reaction) during the reaction, there must be an equal number of electrons gained by another species (reduction half-reaction). This ensures that the overall charge of the reaction remains balanced and the system remains charge-neutral. To summarize, charge balance and mass balance are two essential principles that must be followed when balancing chemical reaction equations. In redox reactions, the charge balance plays a critical role in ensuring the conservation of both mass and charge.

Key concepts

Mass Balance

When you hear about mass balance in chemical equations, think about a seesaw. Both sides should have equal weight, or in this case, equal mass. Mass balance is all about ensuring that the total mass of the reactants (those are the substances you start with) equals the total mass of the products (the new substances formed by the reaction).
This might sound straightforward, but it’s crucial. It follows the principle of conservation of mass, meaning mass cannot magically appear or disappear in a reaction. For instance, if you have 2 oxygen atoms in your reactants, you must have 2 oxygen atoms in your products.
The key takeaway here is:

• Matter must not be created or destroyed.
• The number and type of each atom should match on both sides of the equation.

Charge Balance

Charge balance is somewhat similar to mass balance, but it's all about keeping the electrical charge even. In any chemical reaction, the total charge of reactants and products must be the same. This is important because, just like mass, charge is conserved during chemical reactions.
Imagine you're at a theme park, and you need a specific number of tickets for entry. If you give away five tickets (reactants) but only one ticket is taken (product), something is off balance. For equations, this means that the sum of all positive and negative charges from reactants should match in the products.

• The total charge on each side of the equation must be equal.
• This ensures no unbalanced electrical charges are floating around.

Redox Reactions

Redox reactions might sound complex, but they're just reactions where some atoms lose electrons (oxidation) and some gain electrons (reduction). Imagine a dance where partners switch. One leads (by losing electrons), and the other follows (by gaining those electrons).
These reactions involve two halves: one where oxidation happens and one where reduction takes place. The charge balance is vital here because the electrons lost must equal the electrons gained to keep everything neat and neutral.
Here's what you should remember:

• Redox stands for 'Reduction-Oxidation' reactions.
• They include electron transfer between species.
• Charge balance maintains neutrality in these reactions.

Understanding these concepts not only helps in balancing equations but in grasping how many chemical reactions occur in real life.