GRE Subject Test: Chemistry : Oxidation-Reduction Reactions

Study concepts, example questions & explanations for GRE Subject Test: Chemistry

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Example Questions

Example Question #1 : Oxidation Reduction Reactions

Based on the chemical equation given, which of the following compounds underwent reduction?

Possible Answers:

Correct answer:

Explanation:

The type of chemical reaction given is called an oxidation-reduction reaction. In this type of reaction, there is a transfer of electrons from one element to another. The species gaining an electron is said to be reduced and the species losing an electron is oxidized. Manganese ion goes from a  to a  oxidation state. Therefore, manganese is reduced in the chemical reaction because it gains electrons from the compound being oxidized (iron).

Example Question #1 : Principles Of Oxidation Reduction Reactions

Methane combusts in the presence of oxygen according to the following reaction:

Which of the following statements is true concerning the reaction?

Possible Answers:

Carbon has an initial oxidation state of

Carbon is oxidized in the reaction

Oxygen is oxidized in the reaction

will be the limiting reagent

Oxygen has a charge of  throughout the entire reaction

Correct answer:

Carbon is oxidized in the reaction

Explanation:

By comparing the oxidation number of an atom as a reactant and its oxidation number as a product, we can determine if the atom has been oxidized or reduced. In increase in oxidation number indicates a loss of electrons, or oxidation. A decrease in oxidation number signals a gain of electrons, or reduction.

For electrochemistry, you should familiarize yourself with the traditional oxidation states of hydrogen , halogens , oxygen , and elemental atoms

Carbon is initially in the form of methane, meaning that it is attached to four hydrogen atoms. The molecule is neutral, and each hydrogen has an oxidation number of . Carbon must have an initial oxidation state of in order to balance the molecular charge.

In the a product, carbon is attached to two oxygens, each with a charge of . Again, the molecule is neutral, so carbon must balance these charges. This means that carbon's final oxidation state is .

Since carbon went from an oxidation state of  to , we can conclude that carbon has been oxidized in the reaction.

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