2.1.2 Oxidation-Reduction in Terms of Change in Oxidation Number

In the previous sections, we learned that redox reactions involve the transfer of electrons. We also learned how to assign oxidation numbers to atoms in a compound. Now, we will combine these two ideas to look at oxidation and reduction in terms of changes in oxidation numbers. This is a very practical way to identify what is happening in a chemical reaction.

Main Idea

• Oxidation is an increase in oxidation number (loss of electrons).
• Reduction is a decrease in oxidation number (gain of electrons).

By comparing the oxidation number of an element before and after a reaction, you can immediately tell whether it has been oxidized or reduced.

Detailed Explanation

Let's look at the examples from the textbook to see how this works in practice.

Example 1: Formation of Sodium Chloride

Consider the reaction between sodium metal and chlorine gas: 2Na_{(s)} + Cl_{2(g)} ⟶ 2NaCl_{(s)}

Step 1: Assign oxidation numbers. - Reactants: Both Na and Cl are in their elemental forms. According to Rule 1, their oxidation numbers are 0. 2Na^0_{(s)} + Cl^0_{2(g)} ⟶ … - Products: In sodium chloride (NaCl), sodium is a Group 1 metal (Rule 6), so its oxidation number is +1. Chlorine is a Group 17 nonmetal, and in a binary compound with a metal, it typically has an oxidation number of -1 (from the rule that the sum must be zero). … ⟶ 2Na^{+1}Cl^{-1}_{(s)}

• Sodium (Na): The oxidation number changes from 0 to +1. This is an increase. Therefore, sodium is oxidized.
• Chlorine (Cl): The oxidation number changes from 0 to -1. This is a decrease. Therefore, chlorine is reduced.

In this reaction, sodium loses electrons (oxidation) and chlorine gains those electrons (reduction).

Example 2: Combustion of Natural Gas

The textbook also provides an example of the combustion of methane (natural gas) to identify which elements are oxidized and reduced: CH_{4(g)} + 2O_{2(g)} ⟶ CO_{2(g)} + 2H_2O_{(g)}

• Reactants:
• In CH_4, hydrogen is +1. To make the sum zero, carbon must be -4.
• In O_2, oxygen is in its elemental form, so its oxidation number is 0.
• Products:
• In CO_2, oxygen is -2. To make the sum zero, carbon must be +4.
• In H_2O, hydrogen is +1 and oxygen is -2.

• Carbon (C): The oxidation number changes from -4 in CH_4 to +4 in CO_2. This is an increase, so carbon is oxidized.
• Oxygen (O): The oxidation number changes from 0 in O_2 to -2 in both H_2O and CO_2. This is a decrease, so oxygen is reduced.

Why is this useful?

• The element that is oxidized (its oxidation number increases) is the reducing agent.
• The element that is reduced (its oxidation number decreases) is the oxidizing agent.

In the first example, Na is oxidized and acts as the reducing agent. Cl is reduced and acts as the oxidizing agent. In the second example, Carbon is oxidized and acts as the reducing agent. Oxygen is reduced and acts as the oxidizing agent.

Intermediate Level

Defining redox in terms of oxidation number changes is very practical. It works for all reactions, even those involving covalent compounds where electrons are shared, not fully transferred. The key is to follow the rules to assign the oxidation states correctly first. Once you have the starting and ending numbers, the terms oxidation and reduction simply become "increase" and "decrease."

Important Points

• - Oxidation: An increase in oxidation number (loss of electrons).
• - Reduction: A decrease in oxidation number (gain of electrons).
• - The element that is oxidized is the reducing agent.
• - The element that is reduced is the oxidizing agent.
• - This method allows you to track electrons in any reaction, whether ionic or covalent.

Remember

• - Oxidation = Increase in oxidation number.
• - Reduction = Decrease in oxidation number.
• - Always assign oxidation numbers first before analyzing the change.
• - Increase = Oxidized = Reducing Agent.
• - Decrease = Reduced = Oxidizing Agent.

Common Mistakes

• - Forgetting to assign oxidation numbers to all elements. You need to know the starting and ending points for every element involved to make a complete analysis.
• - Confusing the change with the absolute value. An oxidation number going from -4 to +4 is an increase of 8, not a decrease.
• - Mixing up the agents. Remember: the substance that is oxidized is the reducing agent (because it reduces the other one). The substance that is reduced is the oxidizing agent (because it oxidizes the other one).
• - Assuming elements with the same oxidation number on both sides aren't involved. Some elements may not change their oxidation number. Only the ones that change are part of the redox process.

Quick Revision

You can identify oxidation and reduction by tracking the oxidation numbers of elements in a reaction. An increase in oxidation number is oxidation, and a decrease is reduction. The substance that is oxidized acts as the reducing agent, and the substance that is reduced acts as the oxidizing agent. This method is a reliable way to analyze any redox reaction, even those involving covalent compounds.

60-Second Revision

In terms of oxidation number, oxidation is an increase and reduction is a decrease. By assigning oxidation numbers to all atoms before and after a reaction, you can see which elements are oxidized (their number increases) and which are reduced (their number decreases). The element that is oxidized is the reducing agent; the one that is reduced is the oxidizing agent.

Frequently Asked Questions (FAQs)

Q1. Define oxidation and reduction in terms of change in oxidation number. Answer: Oxidation is an increase in the oxidation number of an element, which corresponds to a loss of electrons. Reduction is a decrease in the oxidation number of an element, which corresponds to a gain of electrons.

Q2. How does the change in oxidation number relate to the gain or loss of electrons? Answer: An increase in oxidation number means the element has lost electrons (oxidation). For example, a change from 0 to +2 means the element lost 2 electrons. A decrease in oxidation number means the element has gained electrons (reduction). For example, a change from 0 to -1 means the element gained 1 electron.

Q3. Identify the elements undergoing oxidation or reduction in the reaction CH_4 + 2O_2 → CO_2 + 2H_2O. Answer: Carbon is oxidized because its oxidation number increases from -4 in CH_4 to +4 in CO_2. Oxygen is reduced because its oxidation number decreases from 0 in O_2 to -2 in both CO_2 and H_2O.

Q4. In the reaction Zn + Cu^{2+} → Zn^{2+} + Cu, what are the changes in oxidation numbers, and which element is oxidized? Answer: The oxidation number of zinc changes from 0 to +2, an increase, so zinc is oxidized. The oxidation number of copper changes from +2 to 0, a decrease, so copper is reduced.

Q5. Why is defining redox in terms of oxidation number changes useful for covalent compounds? Answer: In covalent compounds, electrons are shared, not completely transferred. The oxidation number is an apparent charge that treats bonds as if they were ionic. This allows us to track the movement of electron density, so we can determine which atom is effectively losing electron density (oxidation) and which is gaining it (reduction), even without a full electron transfer.

Q6. If the oxidation number of an element increases by 3, how many electrons are lost? Answer: If the oxidation number increases by 3, the element has lost 3 electrons. This is because each electron carries a -1 charge, so losing electrons increases the positive character (or decreases the negative character) of the atom.

Q7. Can a reaction be redox if only one element changes its oxidation number? Answer: No, a redox reaction requires both oxidation and reduction to occur simultaneously. If one element's oxidation number increases (oxidation), another element's oxidation number must decrease (reduction) to conserve electrons. In some special cases like disproportionation, the same element can undergo both changes.

Q8. What is the relationship between the oxidizing agent and the change in oxidation number? Answer: The oxidizing agent is the substance that gets reduced. Its oxidation number decreases because it gains electrons from the reducing agent. It causes the other substance to be oxidized.

Multiple Choice Questions (MCQs)

• Q1. In the reaction 2Na + Cl_2 → 2NaCl, how does the oxidation number of sodium change?
• A. Decreases from 0 to -1
• B. Increases from 0 to +1
• C. Remains 0
• D. Increases from +1 to +2
• Correct Answer: B
• Explanation: Sodium goes from an elemental state (oxidation number 0) to a +1 ion in NaCl, so its oxidation number increases.

• Q2. In the reaction CH_4 + 2O_2 → CO_2 + 2H_2O, which element is reduced?
• A. Carbon
• B. Hydrogen
• C. Oxygen
• D. None
• Correct Answer: C
• Explanation: The oxidation number of oxygen changes from 0 in O_2 to -2 in the products, which is a decrease. Therefore, oxygen is reduced.

• Q3. An element's oxidation number changes from +5 to +2. What has occurred?
• A. Oxidation
• B. Reduction
• C. Neither
• D. Both
• Correct Answer: B
• Explanation: The oxidation number decreased, which means the element has gained electrons and has been reduced.

Q4. In the reaction Zn + Cu^{2+} → Zn^{2+} + Cu, which element acts as the reducing agent based on oxidation number changes? A. Zn B. Cu^{2+} C. Cu D. Zn^{2+} Correct Answer: A Explanation: Zn's oxidation number increases from 0 to +2 (it is oxidized), so it is the reducing agent.

• Q5. What is defined as an increase in oxidation number?
• A. Reduction
• B. Gain of electrons
• C. Oxidation
• D. Neutralization
• Correct Answer: C
• Explanation: Oxidation is defined as an increase in oxidation number, corresponding to a loss of electrons.

• Q6. If the oxidation number of an element changes from -3 to 0, the element has:
• A. Been reduced
• B. Gained 3 electrons
• C. Been oxidized
• D. Lost 3 protons
• Correct Answer: C
• Explanation: The oxidation number increased from -3 to 0, meaning the element has lost electrons and has been oxidized.

Q7. In a redox reaction, the element that is reduced is the: A. Reducing agent B. Oxidizing agent C. The one that loses electrons D. The one whose oxidation number increases Correct Answer: B Explanation: The element that is reduced (gains electrons, oxidation number decreases) is the oxidizing agent because it causes the other substance to be oxidized.

• Q8. In the reaction Fe + S → FeS, what is the change in oxidation number for iron?
• A. 0 to +2
• B. 0 to -2
• C. +2 to 0
• D. 0 to +3
• Correct Answer: A
• Explanation: Iron (Fe) starts as a free element with an oxidation number of 0. In FeS, it is in the +2 state, so its oxidation number increases from 0 to +2.

• Q9. Which statement is true about the element that is oxidized?
• A. It gains electrons and its oxidation number decreases.
• B. It loses electrons and its oxidation number increases.
• C. It is the oxidizing agent.
• D. Its oxidation number remains unchanged.
• Correct Answer: B
• Explanation: Oxidation is the loss of electrons, which corresponds to an increase in the oxidation number.

Q10. In the reaction 2H_2O_2 → 2H_2O + O_2, the oxygen in H_2O_2 (oxidation number -1) is: A. Only oxidized B. Only reduced C. Both oxidized and reduced D. Neither oxidized nor reduced Correct Answer: C Explanation: The oxygen in H_2O_2 (-1) is reduced to -2 in H_2O and oxidized to 0 in O_2. This is a disproportionation reaction.