Nonane, C9H20, is a hydrocarbon found in JP-8, a type of jet fuel. Why is nonane a hydrocarbon? (A) It can be used as a fuel. (B) It contains hydrogen and carbon. (C) It is made from crude oil. (D) It contains only hydrogen and carbon. (E) It contains hydrogen and carbon in the correct ratio.
In this question, we are given a description of a chemical compound. The name of the compound is nonane, and its molecular formula is C9H20. We are also told that nonane is classified as a hydrocarbon. Some of the answer choices provided may be true or false statements about nonane. But our job is to find a true statement that best justifies why nonane is classified as a hydrocarbon.
We are told that nonane is used as a jet fuel. In fact, nonane is a fuel that is made from crude oil, which is also commonly called petroleum. But there are substances which are fuels which would not be classified as hydrocarbons. For example, hydrazine is a nonhydrocarbon fuel used in rocket fuel and some fuel cells. Another example of a nonhydrocarbon fuel is hydrogen, used by the rockets of the space shuttle and also in fuel cells. Most of the hydrogen fuel made today comes from the refining of crude oil.
While the statements in answer choices (A) and (C) are true statements about nonane, we’ve learned that there are substances which can be classified as fuels or made from crude oil which are not hydrocarbons. Therefore, we can eliminate these two answer choices. We learned that hydrazine and hydrogen are not hydrocarbons. These two molecules are classified as inorganic molecules. On the other hand, nonane, which we know is a hydrocarbon, is classified as an organic molecule.
Organic chemistry is a branch of chemistry that studies the structure, properties, and other aspects of carbon-based compounds. Hydrocarbons are the simplest carbon-based compounds. Some examples of more complex organic molecules that are not classified as hydrocarbons are alcohols, carboxylic acids, and esters. An example of an alcohol is ethanol, which has a molecular formula of C2H6O. Ethanol contains both carbon and hydrogen, but it also contains a third element in its composition, oxygen. Because compounds like alcohols, which contain carbon, hydrogen, and at least one other element, are not classified as hydrocarbons, we can eliminate answer choice (B).
There are a very large number of different hydrocarbons that exist due to the ability of a carbon atom to form stable bonds with four other atoms and to form long chains, which can be branched or ring shaped. In addition, carbon can also form carbon-to-carbon single bonds, carbon-to-carbon double bonds, and carbon-to-carbon triple bonds. All three molecules shown are classified as hydrocarbons, but the ratio of carbon to hydrogen in each molecule is different. In ethane, the ratio of carbon atoms to hydrogen atoms is two to six. And in ethene, the ratio of carbon to hydrogen atoms is two to four. Finally, in a molecule of ethyne, the ratio of carbon atoms to hydrogen atoms is two to two.
As we said, all three of these molecules are hydrocarbons. So the ratio of carbon to hydrogen in hydrocarbons can be different. Therefore, one specific ratio of carbon to hydrogen is not used to classify a compound as a hydrocarbon. And we can eliminate answer choice (E). As we’ve learned through our discussion, a hydrocarbon is best defined as a molecule composed of only carbon and hydrogen atoms. Therefore, nonane is a hydrocarbon because it contains only hydrogen and carbon. And the correct answer is answer choice (D).