High School Chemistry : Help with Oxidation-Reduction Reactions
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Example Question #1 : Types Of Reactions
What process could be used to describe the conversion of 
Oxidation
Reduction
Radioactive decay
Neutralization
Combustion
Reduction
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 
The initial oxidation state of chromium is 
Following the reaction, chromium has a charge of 
During the process, the chromium atom went from an oxidation state of 
Example Question #2 : Types Of Reactions
Determine which compound or ion is the oxidizing agent in the given reaction.
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. 
Example Question #1 : Help With Oxidation Reduction Reactions
What is the oxidation number of sulfur in the given compound?
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 
The oxidation number of oxygen is always 
From here, simple math tells us that the oxidation number of sulfur must be 
Example Question #1 : Help With Oxidation Reduction Reactions
Which of the following describes a high energy, exothermic redox reaction that produces carbon dioxide and water?
Boiling
Combustion
Decomposition
Nuclear
Acid-base
Combustion
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 #2 : Help With Oxidation Reduction Reactions
Which of the following describes an oxidation-reduction reaction?
The transfer of electrons between species, and the subsequent changes in oxidation states
A reaction in which one species molecule is oxidized then reduced
None of these
A type of reaction in which one reactant is broken down into two products
A type of chemical reaction that always involves acids and bases
The transfer of electrons between species, and the subsequent changes in oxidation states
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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