Chemistry Involved in Car Airbags and Fire Extinguishers

                                                                                                                      - Shiv K Sharma

Advancements in chemistry have also led to a better life for human beings. The chemistry advancements have led to better sanitation and hygiene technologies. Moreover, chemistry provides a range of important benefits in home and personal care products, clothing and sports equipment, automobiles, and electronics. As an example, chlorination disinfects drinking water. This chemical process kills waterborne bacteria and viruses that cause up to 3-4 million deaths per year. Thanks to innovations in chemistry, automobiles can be manufactured to be lightweight and more fuel efficient, building materials are more durable and resilient, and electronics like cell phones, computers, and televisions have enhanced functionality to meet the technology needs of today’s consumers.

Among different discoveries that save millions of lives every year, I am going to talk about the chemistry of two important discoveries that have equal contributions to saving human lives. Firstly, the chemistry of car airbags and secondly, the fire extinguisher.

Chemistry of Car Airbags

The airbag specifically for automobile use is credited independently to the American engineer John W. Hetrick, who filed for an airbag patent on 5 August 1952, which was granted #2,649,311 by the United States Patent Office on 18 August 1953. It is said that the concept of airbags came into the mind of John Hetrick when his family had a car accident. He was out on a Sunday drive with his wife and young daughter in 1952. Hetrick swung his car into a ditch in order to avoid a deer, and he and his wife flung their hands up to protect their daughter. While driving back home, he had an idea of an object coming out of the dashboard to soften the collision. 

                         

After these many years, there have been significant improvements in car airbags. However, the main chemical Sodium azide (NaN₃) has not been changed. Sodium azide is a rapidly acting, potentially deadly chemical that exists as an odorless white solid. An electrical charge triggered by automobile impact causes sodium azide to explode and convert to nitrogen gas inside the airbag. The reaction is as follows:

                                                                                2NaN₃ (s) -->  2Na (s) + 3 N₂ (g)

The problem, however, is that the reaction also forms sodium metal which reacts with moisture to generate sodium hydroxide, a highly corrosive substance. A burst airbag could wreak havoc. Chemical ingenuity, however, came to the fore. If potassium nitrate and silicon dioxide were also included with the sodium azide, the only products that would form in addition to nitrogen would be potassium silicate and sodium silicate. Both of these are inert, harmless substances.

 Chemistry of Fire Extinguishers

A fire extinguisher is a handheld active fire protection device usually filled with a dry or wet chemical used to extinguish or control small fires, often in emergencies. When talking about fire extinguishers, most people think of the device produces carbon dioxide gas that helps in extinguishing the fire. However, there are many types of fire extinguishers. The main types are water, foam, CO₂, powder, water mist, and wet chemicals. Each of the different types of fire extinguisher is suitable for different fire classes. It is important that we purchase the right fire extinguisher for our specific needs.

 There is also different chemistry involved in these different fire extinguishers. Today, I will talk about the chemistry of a foam fire extinguisher.

The foam fire extinguishers use the foam to cover the whole fire and smother it, taking away the air so the fire cannot keep on burning. It is made of diethylene glycol monobutyl ether, 1-propanol, and hexylene glycol. The aqueous film-forming foam (AFFF) is used for solid and regular fuels and also liquid fires like gasoline faster and without flashback. When we pull the pin, it discharges the fire extinguisher and then we pull the trigger which depresses the button releasing the extinguishing agent. The foam is in a liquid form, that is clear with a light straw color, with a mild sweet odor, and has a pH of 7.0-8.5. It is completely soluble in water, not flammable, non-explosive, and not an oxidizer. It is less vapor dense than air and has about the same density as water.

When you pull the lever of the fire extinguisher to fire the foam onto a fire, there releases carbon dioxide which then replaces all of the foam within the bottle. This is an example of a single replacement reaction. Then the foam is next where chemistry is used. It is aqueous film-forming foam and it needs a chemical reaction between water, fluoro tensides, and other various chemicals to create a firefighting foam. A carbon-based fire has an equation of:

                                                                              CH₄  + 2O₂ --> CO₂ + 2H₂O

which then the foam puts out the fire by smothering it or basically taking away the oxygen from the equation.

References

1. https://www.consumernotice.org/environmental/afff/

2. https://www.chemistryislife.com/the-chemistry-of-a-fire-extinguisher

3. https://www.mcgill.ca/oss/article/technology/fascinating-chemistry-airbags