Introduction

Revisiting Chemistry: Reactions and Equations

Every study of chemistry begins with chemical reactions and equations. They are the backbone of chemistry: essential to understand how atoms rearrange to form new substances.

🧪 Chemical Reactions: Processes that transform one or more substances (reactants) into other substances (products), resulting in fundamental chemical changes.

🍃 Chemical Equations: Ways to represent chemical reactions using symbols and chemical formulas, indicating the relative quantity of reactants and products.

Mastering the balancing of chemical equations, especially redox equations, is a fundamental skill that unlocks the understanding of more advanced topics and opens the doors to comprehending complex chemical processes, both in the laboratory and in nature.

Contextualization

In the vast and fascinating universe of chemistry, Redox Equations are among the most versatile and powerful tools. Their applications range from the field of electronics to medicine and, of course, in the analysis of chemical reactions in general.

This study of Reactions: Balancing Redox Equations is a bridge to future explorations in the curriculum, allowing us to understand more complex processes, such as oxidation, reduction, and electron transfer.

Here, you will learn to decipher the 'language' of redox equations, working with concepts of oxidation numbers, acidic and basic mediums, and the famous 'Zn hint', which will be explored in detail throughout this note.

But before we delve into this rich area of chemistry, let's review some basic concepts.

Theoretical Development

Components of Redox Chemical Equations

• Redox Reactions: The type of chemical reaction that involves electron transfer between chemical species is known as a redox reaction or oxidation-reduction reaction. One reactant loses electrons (oxidation) and another gains electrons (reduction). Ox and Red, Ma O Reduzem.

• Oxidation Numbers (Nox): Tools we use to track the flow of electrons in a redox reaction are oxidation numbers, or Nox. These numbers are assigned to each atom in a formula and, together, must sum up to the total charge of the species.

  • Oxidation: Increase in the oxidation number (loss of electrons).
  • Reduction: Decrease in the oxidation number (gain of electrons).

• Acidic or Basic Medium (pH): The environment in which the reaction occurs has a significant effect on how hydrogen ions (H+) and hydroxide ions (OH-) are treated. If the reaction occurs in an acidic medium, H+ ions are added to neutralize negatively charged substances. In a basic medium, OH- ions are added.

Key Terms

• Redox Equation: It is a representation of a redox reaction using chemical formulas and symbols to indicate the electron transfer.
• Balancing: The process of equalizing the number of each type of atom on both sides of the chemical equation.
• Anode and Cathode: In the context of redox equations, the anode is where oxidation occurs (loss of electrons) and the cathode is where reduction occurs (gain of electrons).

Examples and Cases

• Oxidation Number Method: Technique used to balance redox equations, involving changing the coefficients of reactants and products to equalize the oxidation numbers of all atoms.

  • Example: Consider the unbalanced redox equation: Fe + HCl -> FeCl3 + H2. Here, iron changes from Nox 0 to Nox +3 and hydrogen from Nox +1 to Nox 0. To balance the equation, we need to insert appropriate coefficients to make the oxidation numbers equal on both sides. The balanced equation will be: 6Fe + 6HCl -> 2FeCl3 + 3H2.

• Ion-electron Method (or Half-Reaction): An alternative method for balancing redox equations, which separates the reaction into two half-reactions, one for oxidation and one for reduction.

  • Example: Consider the unbalanced redox equation: MnO4- + I- -> I2 + Mn2+. We separate the equation into two half-reactions: one for oxidation (MnO4- -> Mn2+) and one for reduction (I- -> I2). By balancing the half-reactions and then adding them, we get: 5MnO4- + 8H+ + I- -> 5Mn2+ + 4H2O + I2.

• Example of Redox Equation in Basic Medium: In the case of a reaction occurring in a basic medium, it is necessary to add OH- to neutralize the H+ ions.

  • Example: Redox equation in basic medium: MnO4- + I- -> I2 + MnO2 + H2O. First, balance the equation in acidic medium: MnO4- + 5e- + 8H+ + I- -> Mn2+ + 4H2O + I2. Then, add OH- to neutralize the H+: MnO4- + 5e- + 8H+ + I- -> Mn2+ + 4H2O + I2. Now, add H2O to balance the O (on the right side, we already have 4). Now, we have the balanced redox equation in basic medium: MnO4- + 5I- + 8H+ -> Mn2+ + 4H2O + 5e- + I2 + OH-.

Detailed Summary

Key Points

• Redox Chemical Reactions: Fundamental processes involving the oxidation and reduction of species. Learning to identify them transforms our view of reactions, as all of them can be classified as redox or non-redox.

• Oxidation Numbers (Nox): Vital tools to establish the movement of electrons in a chemical reaction. Learning to determine Nox is an essential skill.

• Balancing Redox Equations in Acidic Medium: This is the most crucial step in solving redox chemical equations. Zinc, being a solid, is an elegant option to track electrons.

• Balancing Redox Equations in Basic Medium: Unlike in an acidic medium, where H+ ions are used, balancing in a basic medium is done with OH- ions.

Conclusions

• Intrinsic Relationships: Balancing redox equations reveals the deep relationships between reactants and products, and is a tangible indication of the conservation of mass and charge in chemical reactions.

• Skill Development: Working with redox equations enhances our logical and critical thinking skills, as well as improves our proficiency in manipulating chemical formulas.

• Applicability: The skills of balancing redox equations are widely used, not only in the classroom or laboratory but also in technology, medicine, and engineering fields.

Exercises

Exercise 1: Balance the following redox equations:

• a) Al + O2 -> Al2O3 (acidic medium)
• b) Cr + HCl -> CrCl3 + H2 (acidic medium)
• c) KNO3 + Al -> KAlO4 + NH3 (basic medium)

Exercise 2: Identify the oxidation and reduction reactions in the following redox equations:

• a) Na2Cr2O7 + 14HCl -> 2NaCl + 2CrCl3 + 7H2O + 3Cl2
• b) Cl2 + 2KI -> 2KCl + I2
• c) MnO4- + Fe2+ + H+ -> Mn2+ + Fe3+ + H2O.

Exercise 3: Determine the oxidation number of the highlighted element in the following substances:

• a) NaCl (Cl)
• b) SO2 (O)
• c) Fe2O3 (Fe)