Chemistry: Oxidation Numbers and Redox Reactions

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Questions and Answers

What is the oxidation number of elements in their elemental form?

Define oxidation and reduction in terms of electron transfer.

Oxidation is the loss of electrons, while reduction is the gain of electrons.

In the reaction 2Mg + O2 → 2MgO, which reactant is oxidized and which is reduced?

Mg is oxidized, and O2 is reduced.

What is the oxidation number of oxygen in peroxides?

-1 Signup and view all the answers

Identify the oxidizing agent in the reaction MnO4– + H2C2O4 → Mn2+ + CO2.

MnO4– is the oxidizing agent. Signup and view all the answers

What must be done to balance oxygen atoms in half-reactions?

Add H2O to balance the oxygen atoms. Signup and view all the answers

How should you adjust half-reactions to have the same number of electrons for summation?

You may need to multiply one or both half-reactions by a factor. Signup and view all the answers

What is the oxidation number of hydrogen when bonded to metals?

-1 Signup and view all the answers

In a redox reaction, what is the primary role of the reducing agent?

The reducing agent supplies electrons. Signup and view all the answers

What is the systematic rule for determining oxidation numbers in Group II elements?

+2 Signup and view all the answers

What are the products formed when zinc reacts with nitrate ions in basic solution?

The products are zinc tetrahydroxide and ammonia. Signup and view all the answers

After adding OH– ions to both sides of a half-reaction, what should be done with H+ ions?

The H+ ions should combine with OH– ions to form H2O. Signup and view all the answers

In the overall balanced equation 2MnO4– + 6H+ + 5H2C2O4 → 2Mn2+ + 10CO2 + 8H2O, what is the main charge balance required?

The total charge must balance on both sides of the equation. Signup and view all the answers

Which element in the activity series can reduce Cu2+ ions?

Zinc (Zn) can reduce Cu2+ ions. Signup and view all the answers

What should be done if H2O is present on both sides of a half-reaction during redox balancing?

H2O should be brought to one side of each half-reaction. Signup and view all the answers

In the equation 4Zn + 7OH– + NO3– + 6H2O → 4Zn(OH)42– + NH3, how is oxygen balanced?

Oxygen is balanced with 16 atoms on both sides. Signup and view all the answers

Describe the significance of combining half-reactions in the balancing process.

Combining half-reactions ensures the overall charge and mass are balanced in the redox reaction. Signup and view all the answers

What is the role of KMnO4 in redox reactions under acidic conditions?

KMnO4 acts as an oxidizing agent, facilitating the oxidation of other species. Signup and view all the answers

What happens to the manganese ion during the reaction of 2MnO4– in acidic solution?

Manganese is reduced from +7 oxidation state in MnO4– to +2 in Mn2+. Signup and view all the answers

In the reaction Br– → BrO3–, what type of reaction is occurring?

This is an oxidation reaction as the bromide ion is oxidized to bromate. Signup and view all the answers

Study Notes

Oxidation Numbers

The oxidation number of an atom is its effective charge.
An elemental substance has an oxidation number of zero.
The oxidation number of a monoatomic ion equals its charge.
The sum of oxidation numbers of all atoms in a species equals its total charge.
Group 1 elements have an oxidation number of +1.
Group 2 elements have an oxidation number of +2.
Group 3 elements (except boron) have an oxidation number of +3 for M³⁺ and +1 for M⁺.
Group 4 elements (except carbon and silicon) have an oxidation number of +4 for M⁴⁺ and +2 for M²⁺.
Hydrogen has an oxidation number of +1 when combined with non-metals, and -1 when combined with metals.
Fluorine always has an oxidation number of -1 in its compounds.
Oxygen has an oxidation number of -2 unless combined with fluorine, in peroxides (O₂²⁻) it's -1, in superoxides (O₂⁻) it's -½, and in ozonides (O₃⁻) it's -⅓.

Redox Reactions

Oxidation refers to electron loss.
Reduction refers to electron gain.
A redox reaction is a reaction where both oxidation and reduction occur.
An oxidizing agent removes electrons and is reduced in a reaction. The element in the oxidizing agent undergoes a decrease in oxidation number.
A reducing agent supplies electrons and is oxidized in a reaction. The element in the reducing agent undergoes an increase in oxidation number.

Redox Reactions in Acidic Solution

This section outlines steps to balance redox reactions in acidic solutions using the example of potassium permanganate (KMnO₄) and oxalic acid (H₂C₂O₄) reacting to form Mn²⁺ and CO₂.
Steps:
- Identify the oxidized and reduced species by looking at the changes in oxidation numbers.
- Write two skeletal equations for the half-reactions.
- Balance all elements (except O, H, and charge) by inspection.
- Balance oxygen atoms by adding H₂O to each half-reaction.
- Balance hydrogen atoms by adding H⁺ to each half-reaction.
- Balance the electric charges in each half-reaction by adding electrons.
- Multiply each half-reaction by a factor to equalize the number of electrons.
- Combine the two balanced half-reactions by adding them together to form a single balanced equation.
- Check the balanced equation by verifying the number of atoms and the net charge on each side.

Redox Reactions in Basic Solution

Balancing redox reactions in basic solution is similar to acidic solution, with additional steps:
Steps:
- Follow steps 1 to 5 from acidic solution balancing.
- Add the same number of OH⁻ ions to both sides of the equation as there are H⁺ ions in each half-reaction.
- Combine H⁺ and OH⁻ on each side of the equation to form H₂O.
- Move H₂O molecules to one side of the equation if present on both sides.
- Continue with Step 6 from acidic solution balancing.

Element Activity Series

This is a list of elements ordered by their reactivity.
A metal can reduce the cations formed by any metal below it in the list.
For example, Zinc (Zn) can reduce Cu²⁺, but copper (Cu) cannot reduce Zn²⁺.

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Description

Test your knowledge on oxidation numbers and redox reactions. This quiz covers essential concepts, including the effective charge of atoms and the rules governing oxidation states. Perfect for students studying general chemistry.