What is Cyanide? - Definition, Formula & Effects

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  • 0:00 What Is Cyanide?
  • 0:45 Chemical Formula Of Cyanide
  • 2:55 The Effects Of Cyanide
  • 5:50 Lesson Summary
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Lesson Transcript
Instructor: Nicola McDougal

Nicky has taught a variety of chemistry courses at college level. Nicky has a PhD in Physical Chemistry.

Cyanide is a poison which has been used since the ancient Egyptians, but how does it work? To help us answer this question, we will learn about cyanide's chemical structure, where it's found, and how it interacts with key biological molecules in the human body.

What Is Cyanide?

Cyanide is a fast-acting, potentially deadly chemical that can exist as both a gas and as a crystalline salt. Both forms can be lethal in high enough concentrations. Some people can detect the distinctive 'bitter almond' smell of cyanide, though this is not a reliable indicator as it doesn't always give it off, and not everyone can detect it.

Cyanide is found abundantly in nature as well as in man-made materials. Examples include the seeds of apples and peaches, as well as cigarette smoke and extermination materials for pests. It's also present in the air when a building is burning down, usually because plastic things - which contain cyanide - are burning. Cyanide kills by depriving the body of oxygen.

Chemical Formula of Cyanide

Cyanide is a very simple molecule consisting of one carbon and one nitrogen atom connected together by a triple bond. It's an ion and carries a negative charge of 1-. This diagram represents the chemical structure of the cyanide ion. The ion is shown in square brackets with the charge outside.

To help us understand how cyanide acts as a poison, we look at the Lewis Diagram, or the diagram that shows the bond between a molecule's atoms and electrons, to see where the electrons are. In this diagram, you will see a triple bond between the atoms and an unshared pair of electrons on each atom.

In the diagram, you can see there are ten electrons, six in the triple bond between the carbon and nitrogen atoms and an unshared pair of electrons on both the nitrogen and the carbon atom. The ten electrons represent the sum of all the valence electrons in the molecule plus or minus any charge if it's an ion. In cyanide, carbon brings four valence electrons and nitrogen brings five; plus there is the extra one from the charge on the ion. So, 4 + 5 + 1 = 10 electrons overall. We write the formula for the cyanide ion as CN-.

While the nitrogen atom is quite happy with three bonds and an unshared pair of electrons, the carbon atom is very unhappy indeed! To become stable, carbon tends to form four bonds. Here it has formed just three bonds, and it also has an unshared pair of electrons. This makes it very unstable. It's this instability that means the cyanide ion is highly reactive.

The cyanide ion doesn't exist alone and common forms of cyanide are hydrogen cyanide (HCN), which is normally breathed in as a gas, and the crystalline salts of potassium cyanide (KCN) and sodium cyanide (NaCN), which can be consumed in, or with, food and drink.

The Effects of Cyanide

The toxic effects of cyanide depend largely on how much you're exposed to, for how long you're exposed, and whether the cyanide is inhaled or ingested. It's possible to be exposed to cyanide by breathing air, drinking water, eating food, or touching soil that contains cyanide. Cyanide enters water, soil, or air as a result of both natural processes and industrial activities. When present in air, it's usually in the form of gaseous hydrogen cyanide, HCN.

All cyanide compounds are highly toxic and a relatively small amount can kill a person. A 160-pound adult has a 90% chance of death after consuming only about 0.4g of potassium cyanide. A teaspoon contains about 60 times this amount!

Crystalline potassium cyanide looks just like regular table salt or sugar and easily dissolves in water, making it a perfect candidate to poison someone and a favorite method portrayed in Hollywood blockbusters. However, unlike the movies where death is almost instantaneous after consuming the cyanide-laced champagne, death from ingestion normally takes between 15 to 45 minutes. Hydrogen cyanide gas is more toxic; due to the large surface area of the lungs allowing the cyanide to act very quickly, and in very high concentrations, death can be instant.

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