Part 2. Topic 5. Algorithm for Writing Redox Reaction Equations

Part 2.
TOPIC 5. Algorithm for Writing Redox Reaction Equations

When writing redox reaction equations, it is important to follow a specific sequence:

1. Identify the oxidizer and the reducer among the reactants.

To do this, it is first necessary to assign oxidation states to the elements and compare their redox properties.
2. Write the products of the reaction.

A correct determination of the reaction products will depend on your knowledge of the atomic structure of the elements, their properties, and the properties of their compounds. It is important to remember that in interatomic and intermolecular redox reactions, free oxygen is generally not released. Oxygen is released when peroxides react with strong oxidizers, as well as in some reactions involving ozone and fluorine. For example,

2KMnO4 + 5Na2O2 + 8H2SO4 = 2MnSO4 + 5O2↑ + 5Na2SO4 + K2SO4 + 8H2O
SiO2 + 2F2 = SiF4 + O2↑
PbS + 4O3 = PbSO4 + 4O2↑

In other cases, oxygen atoms from the oxidizer (or reducer) molecules combine to form water molecules with the involvement of H+ ions, for example:

HNO2 + HI = NO + I2 + H2O

Nitric Acid.
When nitric acid reacts with metals, depending on its concentration and the metal's activity, nitrates, water, and various products of acid reduction are formed, as shown in the table below.

Sulfuric Acid.
Diluted sulfuric acid is not an oxidizer and reacts with metals like any other acid. If the metal is located before hydrogen in the electrochemical series, hydrogen is released upon interaction with the acid. If the metal is located after hydrogen in the series, no reaction occurs.

Concentrated sulfuric acid, when reacting with active metals (up to Zn), is reduced to S and H2S, while with less active metals (located after hydrogen or near it in the electrochemical series), it is reduced to SO2. Noble metals—gold (Au) and platinum (Pt)—are not oxidized even by concentrated sulfuric acid. Active metals such as Al, Fe, and Cr are passivated under normal conditions and do not react with concentrated H2SO4, but reactions are possible when heated.

Test.

1. A redox reaction is:
   A) Interaction of potassium with water;
   B) Interaction of potassium oxide with water;
   C) Interaction of carbon dioxide with calcium oxide;
   D) Decomposition of aluminum hydroxide.

2. Oxidizers in chemical reactions:
   A) Do not change oxidation state;
   B) Increase oxidation state;
   C) Decrease oxidation state;
   D) Do not participate in the chemical reaction.

3. The oxidizing ability of halogens decreases from top to bottom in the group:
   A) Increases
   B) Decreases
   C) Remains unchanged
   D) Halogens are not oxidizers

4. Sulfur acts as an oxidizer in a reaction with:
   A) Oxygen
   B) Metals
   C) Chlorine and fluorine
   D) Nitric acid

5. In the reaction with the equation: 3P + 5HNO3 + 2H2O = 3H3PO4 + 5NO, phosphorus:
   A) Is oxidized
   B) Is reduced
   C) Accepts electrons
   D) Does not change oxidation state

6. A substance that acts as both an oxidizer and reducer is:
   A) Fluorine
   B) Sodium permanganate
   C) Sodium chromate
   D) Sodium sulfite

7. Oxidizers can be substances with:
   A) The highest oxidation state
   B) The lowest oxidation state
   C) An intermediate oxidation state
   D) In their free state

8. The photosynthesis process that occurs in nature is a:
   A) Combination reaction
   B) Decomposition reaction
   C) Redox reaction
   D) Substitution reaction

9. The oxidation state of chromium in potassium chromate is:
   A) +3 B) +6 C) 0 D) +2

10. Bromine is a reducing agent in a chemical reaction if its oxidation state:
    A) Increases
    B) Decreases
    C) Remains unchanged
    D) Becomes 0

11. 5H2O2⁻ + 2KMn⁺7O4 + 3H2SO4 → 2Mn²⁺SO4 + 5O₂↑ + K₂SO4 + 8H2O
    The key substances are listed.
    Reducer: O₂⁻ – 2 e = O₂
    Oxidizer: Mn⁺7 + 5 e = Mn²⁺

12. 3P⁻3 + 8HMn⁺7O4 → 8Mn⁺4O2 + 3H₃PO₄ + 4H₂O
    Key substance missing on the right: H₃PO₄ (oxidation product of PH₃)
    Reducer: P⁻3 – 8 e = P⁺5
    Oxidizer: Mn⁺7 + 3 e = Mn⁺4

13. Cr₂(SO₄)₃ + 6KMn⁺7O4 + 16KOH → 2K₂CrO₄ + 6K₂MnO₄ + 3K₂SO₄ + 8H₂O
    Key substance missing: K₂MnO₄ (reduction product of KMnO₄)
    Reducer: Cr⁺3 – 6 e = Cr⁺6
    Oxidizer: Mn⁺7 + 1 e = Mn⁺6

14. 2N⁺2 + 3KCl + 2KOH → 2KN⁺5O₃ + 3KCl⁻ + H₂O
    Key substances are listed.
    Reducer: N⁺2 – 3 e = N⁺5
    Oxidizer: Cl⁺1 + 2 e = Cl⁻1

15. P⁻3 + 8Ag⁺1NO₃ + 4H₂O → 8Ag⁰ + H₃PO₄ + 8HNO₃
    Key substance missing: H₃PO₄ (oxidation product of PH₃)
    Reducer: P⁻3 – 8 e = P⁺5
    Oxidizer: Ag⁺1 + 1 e = Ag⁰