How many electrons are transferred in each overall reaction?

(a) 4Al + 12HCl --> 4AlCl3 + 6H2


(b)  4 CrO42- + 10 H2O --> 4Cr3+ + 3O2 + 20 OH-

Let's analyze the two reactions step by step to determine how many electrons are transferred in each case. To do this, we need to break down the oxidation states of the elements involved and examine the changes.

### (a) \( 4 \text{Al} + 12 \text{HCl} \rightarrow 4 \text{AlCl}_3 + 6 \text{H}_2 \)

- **Aluminum (Al)**: 
  - In elemental form, aluminum has an oxidation state of 0.
  - In \( \text{AlCl}_3 \), aluminum has an oxidation state of \( +3 \).
  - The change in oxidation state for each aluminum atom is \( 0 \rightarrow +3 \), meaning each aluminum atom loses 3 electrons.

- **Total electrons lost**: 
  Since there are 4 aluminum atoms, the total electrons lost is:
  \[
  4 \times 3 = 12 \text{ electrons lost}.
  \]

- **Hydrogen (H)**:
  - In \( \text{HCl} \), hydrogen has an oxidation state of \( +1 \).
  - In \( \text{H}_2 \), hydrogen has an oxidation state of 0.
  - The change in oxidation state for each hydrogen atom is \( +1 \rightarrow 0 \), meaning each hydrogen atom gains 1 electron.

- **Total electrons gained**: 
  Since there are 12 HCl molecules and each HCl molecule provides 1 hydrogen atom (and two atoms form each \( \text{H}_2 \) molecule), the total number of hydrogen atoms gaining electrons is 12. Each hydrogen gains 1 electron, so the total electrons gained is:
  \[
  12 \times 1 = 12 \text{ electrons gained}.
  \]

Since the number of electrons lost and gained must be equal in a redox reaction, **12 electrons are transferred** in this reaction.

### (b) \( 4 \text{CrO}_4^{2-} + 10 \text{H}_2\text{O} \rightarrow 4 \text{Cr}^{3+} + 3 \text{O}_2 + 20 \text{OH}^- \)

- **Chromium (Cr)**: 
  - In \( \text{CrO}_4^{2-} \), chromium has an oxidation state of \( +6 \).
  - In \( \text{Cr}^{3+} \), chromium has an oxidation state of \( +3 \).
  - The change in oxidation state for each chromium atom is \( +6 \rightarrow +3 \), meaning each chromium atom gains 3 electrons.

- **Total electrons gained**: 
  Since there are 4 \( \text{CrO}_4^{2-} \) ions, the total number of electrons gained is:
  \[
  4 \times 3 = 12 \text{ electrons gained}.
  \]

- **Oxygen (O)**:
  - In \( \text{CrO}_4^{2-} \), oxygen has an oxidation state of \( -2 \).
  - In \( \text{O}_2 \), oxygen has an oxidation state of 0.
  - The change in oxidation state for each oxygen atom is \( -2 \rightarrow 0 \), meaning each oxygen atom loses 2 electrons.

- **Total electrons lost**: 
  Since 3 molecules of \( \text{O}_2 \) are produced, there are \( 3 \times 2 = 6 \) oxygen atoms involved. Each oxygen atom loses 2 electrons, so the total electrons lost is:
  \[
  6 \times 2 = 12 \text{ electrons lost}.
  \]

Thus, **12 electrons are transferred** in this reaction as well.

### Summary:
- (a) 12 electrons are transferred.
- (b) 12 electrons are transferred.

Would you like further details on the oxidation states or any other clarification?

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Here are 5 questions to expand your understanding:
1. How do you determine the oxidation state of an element in a compound?
2. What are the general rules for assigning oxidation states?
3. Can you explain the difference between oxidation and reduction in a redox reaction?
4. How can you identify which element is oxidized and which is reduced in a reaction?
5. What role do electron transfers play in balancing redox reactions?

**Tip:** When balancing redox reactions, use the half-reaction method to separate oxidation and reduction processes for clarity.

How many electrons are transferred in each overall reaction? (a) 4Al + 12HCl --> 4AlCl3 + 6H2 (b) 4 CrO42- + 10 H2O --> 4Cr3+ + 3O2 + 20 OH-

Learn how to calculate the number of electrons transferred in redox reactions such as 4Al + 12HCl and 4 CrO42- + 10 H2O. This guide covers key concepts like oxidation states and electron balance, with a detailed analysis suitable for Grades 10-12.

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