The Chemistry of Fire - Part 2
Chemistry isn’t just a high school subject to skate by. It does have very real-world applications with fire and firefighting being one of the leading relationships. For this article, please use this link to the Periodic Table of Elements to facilitate following along.
https://drive.google.com/file/d/1QuiaiNbPizi5KJHoZnuyfo31e62HMb0r/view?usp=drive_link
To set the stage, at the top of the Table above each column is a number, starting on the left with one and going right to eighteen. Each of these columns are called a “Family”. For example, the first column contains seven elements some of which are Hydrogen (H), Lithium (Li), Sodium (Na) and Potassium (K) among others. All elements in each family will in a chemical reaction behave similarly relative to the number of valence electrons it will take or give away. Recall a valence electron are those in the outermost ring or shell of every element.
Within the box for each element in the upper right-hand corner is another number – sometimes more than one. For Family I this number is a +1. This means that all of these elements are likely to give away an electron resulting in an overall positive charge. This is because that element, now called an ion because it has a charge has one more positively charged proton than the number of negatively charged electrons.
As you might guess the elements in Family II have a +2 in the upper right-hand corner meaning these elements are more likely to give away two electrons resulting in an ion sporting two more protons. To contrast look at Family Seventeen – specifically Florine (F) and Chlorine (Cl). Both are more likely to add an electron thus creating an ion of -1. Further, all the elements in Family Eighteen have a zero because these elements (called the Noble gases) do not react with ANY other elements.
Why is all this important for firefighters? Have you ever been in a flashover simulator (or witnessed an actual flashover)? What about a hazmat situation? How about fighting a grease fire on a stove? As an officer have you had to make a decision for your crew to be on air? All of these situations and more involve chemistry and more specifically how elements combine.
Take the familiar element of oxygen (family 16 and element number 8). It is important to note that oxygen is NOT FLAMMABLE in and of itself but does support combustion because it’s an oxidizer meaning that it readily accepts electrons (typically 2) from other elements as fuel breaks down in a chemical reaction. The result is the release of heat and light i.e. fire. This is why any ventilation operation must be coordinated with interior firefighters or why a door must be controlled during forcible entry. The introduction of an oxidizer could change the flow path of a fire, cause a backdraft or flashover resulting in a potentially dangerous/deadly situation for firefighters.
That said, firematics is not the only aspect of firefighting where chemistry is applicable. In fact, chemistry is arguably most relevant to a hazmat situation. Large scale hazmat calls can be extremely dangerous and may require multiple agencies and tactics such as the use of drones to identify chemicals and constant water flow to cool and prevent an exothermic reaction i.e. an explosion. It doesn’t even have to be an industrial situation either. Take for example a call involving the combination of household cleaners such as ammonia and bleach. The resulting reaction releases pure chlorine gas which is deadly and even in small amounts causes irritation to the eyes, nose and throat. The combination of vinegar (or any acid) and bleach is similar. Here is the chemical reaction:
2HOCL + 2CH3COOH ? CL2+2H2O + 2CH3COO
Bleach Vinegar Chlorine gas Water Acetate
The protocol will likely be to ventilate the area but should firefighters be on air? In spite of the fact that vinegar and bleach are two common household items the answer is a resounding YES because pure chlorine gas can kill – even in small amounts.
OK so we have covered some potentially volatile chemical reactions but what about a situation where there is no chemical reaction. Take your Noble gases in Family eight. These gases have a full valence shell so their electrons are not shared and they don’t react with other elements. For this reason, these gases are often used to protect computer systems and important documents to prevent any degradation (as an aside the Declaration of Independence is encased in Argon for this reason). With all the new data centers popping up firefighters may run into calls with large computer filled rooms with no air/oxygen and will have to be on their own air in order to function. Officers need this information in advance so their firefighters are prepared, can remedy the situation and most importantly come home safely.
The last concept has mostly to do with hydrocarbons such as gasoline, oils and grease. The element carbon always has four bonds. Depending on the compound those bonds may be single, double or even triple – see diagram.
Single-bonded hydrocarbons tend to be solids but can turn to liquids when heat is added. Bacon grease is a great example and may be the cause of many kitchen fires. What happens when bacon grease is left to cool however? Because it is single bonded it reverts back to being a solid. When a hydrocarbon has a double or triple bond by nature those bonds are not evenly distributed and therefore these compounds tend to remain a liquid.
This may have an impact on how firefighters approach a hydrocarbon fire. Those that remain liquids will simply be pushed around by water flow potentially causing a larger fire. Throwing water on a candle fire with melted wax and it will only cause a severe spread. This video shows how this works: https://www.youtube.com/watch?v=ZI30ZnePn60. These fires are put out most effectively by cutting off the oxygen supply.
Chemistry is an amazing subject and an effective tool. For example, the combination of the two potentially deadly elements sodium (Na) and chlorine (Cl) result in a white crystalline substance we put on our fries. Conversely, the lack of chemistry knowledge can prove to be dangerous. With knowledge comes safety.
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