High School Chemistry : Help with Oxidation-Reduction Reactions

Study concepts, example questions & explanations for High School Chemistry

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Example Questions

Example Question #2 : Types Of Reactions

What process could be used to describe the conversion of  to ?

Possible Answers:

Neutralization

Oxidation

Combustion

Reduction

Radioactive decay

Correct answer:

Reduction

Explanation:

The process illustrated is the reduction of the chromium atom. Reduction occurs when electrons are gained, causing the oxidation number (charge) of the atom to decrease. We can examine this process by looking at the oxidations numbers of each atom involved.

Oxygen always has an oxidation number of , and the ion has a total charge of . This allows us to find the oxidation number for chromium. Note that there are two chromium atoms and seven oxygen atoms in the ion.

The initial oxidation state of chromium is .

Following the reaction, chromium has a charge of , meaning that it also has an oxidation number of .

 

During the process, the chromium atom went from an oxidation state of  to an oxidation state of . This means that it must have gained electrons (reducing the oxidation state). This is a reduction process.

Example Question #242 : High School Chemistry

Determine which compound or ion is the oxidizing agent in the given reaction. 

 

Possible Answers:

Correct answer:

Explanation:

The oxidizing agent is the element that causes the other element to be oxidized, and is itself being reduced in the equation. Reduction is defined as the gain of electrons, meaning the element would get "more negative."

In this reaction, aluminum transitions from an oxidation state of 3+ to an oxidation state of 0. This indicates a reduction, as the atom has gained more electrons and reduced charge.  is thus the oxidizing agent in this reaction.

Example Question #243 : High School Chemistry

What is the oxidation number of sulfur in the given compound?

Possible Answers:

Correct answer:

Explanation:

Use the rules for assigning oxidation numbers. We know that the sum of oxidation numbers of all atoms present in the compound must equal zero because the compound is neutral.

The oxidation number of hydrogen is  except in compounds with less electronegative elements (which is not the case here). Because there are two hydrogens, the overall oxidation is .

The oxidation number of oxygen is always . Multiply that by the total number of oxygens to get .

From here, simple math tells us that the oxidation number of sulfur must be  to make the total oxidation state equal to zero.

Example Question #244 : High School Chemistry

Which of the following describes a high energy, exothermic redox reaction that produces carbon dioxide and water?

Possible Answers:

Decomposition

Acid-base

Combustion

Nuclear

Boiling

Correct answer:

Combustion

Explanation:

A combustion reaction is characterized as an oxidation-reduction reaction because the oxidation number of the reactants changes, and oxygen is used to burn a fuel molecule. Carbon dioxide is often produced as a product. The following is an example of a combustion reaction:

 

Example Question #245 : High School Chemistry

Which of the following describes an oxidation-reduction reaction?

Possible Answers:

The transfer of electrons between species, and the subsequent changes in oxidation states 

A type of chemical reaction that always involves acids and bases

None of these

A type of reaction in which one reactant is broken down into two products

A reaction in which one species molecule is oxidized then reduced

Correct answer:

The transfer of electrons between species, and the subsequent changes in oxidation states 

Explanation:

In an oxidation reaction, one species is oxidized (loses electrons). During a reduction reaction, one species is reduced (gains electrons). These transfers of electrons results in the change of oxidation states. One way to help you remember which reaction is which, is by using the mnemonic: OIL RIG - Oxidation Is Loss of electrons; Reduction Is Gain of electrons. 

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