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Discover the structure, physical properties, sources, and uses of the saturated hydrocarbon heptane (C7H16). Learn about heptane’s vital role in establishing the octane rating scale for gasoline.

There Wouldn’t Be an Octane Rating Without Heptane!

If you own a car, you might be familiar with octane ratings. Every time you go to the pump, you have to choose between gasolines rated with numbers such as 87, 89 or 91 — these are octane ratings.

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But wait, this lesson is supposed to be about heptane, right? Well, the most interesting fact about heptane is that it is used to define the zero point on the octane rating scale.By definition, when the octane rating scale was created, 100% pure heptane was given a rating of 0. Before being made available for sale, all gasolines are measured against pure heptane to determine their octane rating. So what exactly is heptane and where does it come from?

Structure and Properties of Heptane

Heptane is the name for the straight-chained saturated hydrocarbon with 7 carbon atoms and 16 hydrogen atoms. The 7 carbon atoms are connected in one long chain with no branches. It is said to be fully saturated because it contains no double bonds between carbons.

Hydrogens are attached such that each carbon makes 4 bonds to other atoms.Heptane’s molecular formula is C7H16, and its chemical structure can be drawn in many ways that are all equivalent to one another. Sometimes the name will be written ‘n-heptane’, where the n stands for normal and conveys that the seven carbon atoms are arranged in a straight line.At room temperature, heptane is a colorless liquid with a gasoline-like odor. It readily gives off fumes, and both the liquid and the fumes are highly flammable.

How Heptane is Used

Pure heptane has two main uses. The first use is as the standard zero point for the octane rating scale for gasoline.

As a driver, you may have noticed that higher octane fuel is more expensive because it is better for your engine. You can correctly conclude that a fuel rated zero octane would be terrible for your car. Indeed, pure heptane with its zero octane rating is a terrible fuel choice. The reason is that heptane burns very explosively, which is the source of engine knocking. Although heptane is not desirable in automotive fuel, actual gasoline is a complex mixture that does contain some heptane.The second use of pure heptane is as a laboratory solvent.

A good solvent should be able to dissolve the reagents (ingredients) of the chemical reaction, but not interfere with the reaction. Heptane does this well because it has almost zero reactivity with other molecules, and many organic molecules that won’t dissolve in water will dissolve in heptane.In other applications, heptane is present as part of a mixture. It is a frequent component in adhesive solvents, or solutions that can break up and dissolve sticky substances.

If you ever accidentally get a strong adhesive such as rubber cement in the wrong place and need to remove it, you’d need a specialized solvent to dissolve it away. For example, 3M’s Scotch Weld is a mixture of heptane and toluene and is suggested for cleaning up excess adhesives from parts and equipment.

Sources of Heptane

Heptane comes primarily from the refining of petroleum, where it is part of a mixture of refinery products. An estimated 100 to 500 million pounds of it are produce annually.

If pure heptane is needed (such as for the octane rating test), it is generally obtained from tree sources, specifically the Jeffrey pine in western North America. The heptane produced from crude oil has too many contaminants mixed in (and which are difficult to remove) to be used in the measurement of octane ratings.

Safety and Handling of Heptane

Heptane is highly flammable and should be stored away from heat in a well-ventilated area. The Material Safety Data Sheet for heptane lists it as slightly hazardous in case of skin contact, eye contact, ingestion or inhalation.

Gloves, goggles, and a vapor respirator are the recommended safety equipment for handling heptane.

Lesson Summary

Let’s review. Heptane is a straight-chained hydrocarbon used primarily as a standard of measure for gasoline octane ratings, as a solvent, and as a component in gasoline. It is a highly flammable, colorless liquid with a gasoline-like smell. Heptane can be obtained in large quantities from petroleum and in smaller, purer quantities from Jeffrey pine trees.

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