Fireworks or fire-won’t-works? Chemistry dictates if sparks you will see! And metals determine what color they will be! Redox reactions put the work in fireworks and G – it’s pretty cool! So let’s see how redox & thermodynamics apply to the fireworks we see in the sky!
The chemicals in fireworks are made up of atoms, as are all chemicals, including water! (Remember “chemical” doesn’t mean bad!) Water is a chemical that is made up of 2 hydrogen atoms, and one oxygen atom (so dihydrogen monoxide or H₂O). The chemicals in fireworks involve things like potassium potassium perchlorate, KClO₄, which has one potassium atom, one chlorine atom, & 4 oxygens.
Atoms are made up of smaller parts called subatomic particles. There are 3 main ones – neutral neutrons and positively-charged protons hang out in a dense central core called the atomic nucleus. And whizzing around them in a cloud are negatively-charged particles called electrons.
The protons try their best to use the “opposite charges attract” thing to leash in the negatively-charged electrons whizzing around them. But they can only hold a certain number. And kinda like a dog walker trying to walk too many energetic dogs at once, the most energetic electrons, valence electrons, which are located furthest from the nucleus, are most likely to run away (or hook up with other molecules)
Redox reactions are chemical reactions in which molecules called reducing agents (reductants) give up some of their negatively-charged electrons (e⁻) & other molecules called oxidizing agents (oxidants) take them. To remember which is which, think OIL RIG: Oxidation Is Loss of electrons (e⁻) & Reduction is Gain of e⁻
We’ve seen some important but “small-scale” redox reactions – like the reduction & oxidative reformation of disulfide crosslinks in your hair in perms http://bit.ly/2FJUXPU or the reduction of water through electrolysis that creates the charge gradient needed for gel electrophoresis. http://bit.ly/2XMLMbG
Fireworks are also based on redox reactions, but ones w/MUCH greater release of free energy (G). Energy is the ability to do work, which we obviously need for fire-works! And change in free energy (ΔG) is like a measure of chemical “drive.”
I like to think of free energy as a sort of “couch shopping.” Say you’re sitting on a couch. It’s not a very comfortable couch, 😕 it’s cramped & hard so you’re squirming around a bit trying to get comfortable 😩 There’s a more comfortable couch across the room, and when you’re in that couch you sink right in & can relax & stop squirming. Aaahhh… 😌
BUT in order to get to the comfy couch you have to overcome your laziness & get up off the 1st couch 😒 Whether it’s “worth it” depends on how much comfier the 2nd couch & this depends on how much less squirmy you’ll be there & how much you’ll be able to spread out.
Similarly, whether a reaction will occur SPONTANEOUSLY depends on whether the products have more or less “free energy” than the reactants. We can think of this “free energy” (aka Gibbs free energy (G)) as a sort of overall “comfiness.” It takes into account squirminess (kinetic energy aka HEAT) in the ENTHALPY term (H) & spread-out-ability (randomness/freedom/disorder) in the ENTROPY term (S).
ΔG = ΔH-TΔS
Δ (delta) means “change in” so this equation looks at DIFFERENCES in H & S between reactants (couch 1) & products (couch 2).
Where’d that T come from? It stands for temperature & it takes into account the “mood.” ΔH & ΔS are “constant” because they’re calculated based on the reactants & products. But just like couch 1 & couch 2 don’t change, but when it’s hot, you care more about spreading out, at higher temps, ENTROPY becomes more & more important. (molecules want to spread out when they’re hot just like you!)
NEGATIVE ΔG means products have LESS FREE ENERGY than reactants, so the “2nd couch is comfier” & the reaction is likely to proceed spontaneously. We call such reactions EXERGONIC.
POSITIVE ΔG means products have MORE FREE ENERGY than reactants, so you’ll have to really bribe it to go… 🙄 We call such reactions ENDERGONIC
NEGATIVE ΔH means products have LESS HEAT than reactants. This can only happen if reactants give up heat to the “surroundings,” and such heat-releasing reactions are called EXOTHERMIC
POSITIVE ΔH means products have MORE HEAT than reactants. Which basically means reactants “stole” heat from “surroundings.” Such heat-absorbing reactions are called ENDOTHERMIC
NEGATIVE ΔS means the products have LESS FREEDOM/RANDOMNESS. This can come from having more and/or stronger bonds tethering the molecules together.
POSITIVE ΔS means the products have MORE FREEDOM/RANDOMNESS. This can come from having fewer and/or weaker bonds tethering the molecules together, allowing them to move around more, which molecules like, remember. What they really like (you get a large positive ΔS from) is if you can go from a liquid to a gas &/or break up a big thing (which has limited motion bc it has to move as a group) into lots of smaller things which can move separately.
So, to summarize this thermodynamic mumbo-jumbo, reactions are more favorable if they let off heat (have a ➕ ΔH) &/or give the molecules more freedom/randomness (have a ➕ ΔS). BUT it’s the combination of those 2 (& temp) that’s the ultimate decider of whether a reaction will occur spontaneously (have a ➖ ΔG).
Now that we’ve had this thermodynamic refresher, let’s get back to those fireworks with those concepts in mind: ΔG = ΔH – TΔS where ΔH is change in enthalpy (heat), T is temperature, & ΔS is change in entropy (disorder/randomness).
Firework reactions have a large (negative) ΔG (strong chemical “drive”) that’s coming from both enthalpy & entropy.
🔹fireworks give off LOTS of heat (are very EXOTHERMIC). They have a large ➖ΔH, which makes ΔG more ➖
🔹fireworks gain LOTS of ENTROPY when they explode (molecules get to go from confined in a shell to shooting off randomly) AND this ΔS is multiplied by high temperature (H). Which makes ΔG VERY ➖
Result: the fireworks reaction releases LOTS of free energy ( the reaction is very EXERGONIC). So, what *is* this reaction I’ve been teasing? Let’s look inside (one example, as there are many other chemicals that can be used)
REACTANTS (things you start with): FLASH POWDER. This is a solid OXIDIZER (e.g. potassium perchlorate, KClO₄) mixed w/metal powder (e.g. aluminum, Al)(REDUCTANT)(other common reductants are sulfur (S) or carbon (C))
When ignited, KClO₄ breaks down into potassium chloride (KCl), releasing oxygen. That oxygen reacts w/Al, getting Al to burn to aluminum oxide (Al₂O₃)
⭐️ overall (unbalanced) reaction: KClO₄ + Al -> KCl + Al₂O₃
If you’re a chemist, you might be cringing to see an unbalanced reaction, so here’s the balanced form: 3 KClO₄ + 8 Al -> 3 KCl + 4 Al₂O₃
⚠️ here metal acts as REDUCTANT but in other reactions, like the one we use in silver staining of proteins in gels, metals acts as OXIDIZERS. Metals “have a lot of hyper dogs to walk” so they can gain & lose e⁻ relatively easily, so they’re useful in both roles!
As I mentioned above the reaction is super exergonic. So if there’s such a large ➖ΔG why doesn’t the reaction start without ignition? The key is that our reaction is actually 2 reactions combined
(1) 3 KClO₄ -> 3 KCl + 6 O₂
(2) 6 O₂ + 8 Al -> 4 Al₂O₃
rxn (2) is EXOTHERMIC, BUT reaction (1) is ENDOTHERMIC. So it NEEDS heat to proceed
(1) 3 KClO₄ + heat -> 3 KCl + 6 O₂
(2) 6 O₂ + 8 Al -> 4 Al₂O₃ + heat
So we have to provide heat in order to get (1) to start. Without (1) there can’t be (2) (missing our reactants! 😬) so the reaction doesn’t start. BUT once started, (2) provides heat for (1) and the reaction goes until “all” reactants are converted to products.
Where does the light come from? We’ve explained where the heat from fireworks comes from but that’s not what you go to see! Where do the light & colors come from?
When molecules are heated up they start moving around & give off energy in the form of “electromagnetic (EM) radiation” – basically heat is being transferred from the molecules through the air in the form of waves. When these waves have a certain energy content we can see it, so we call it “visible light” more here: http://bit.ly/lightleafcolor
This THERMAL RADIATION is given off whenever an object is hotter than its surroundings BUT usually the radiation given off doesn’t have enough energy for us to see (it’s in the infrared range, so it can be detected by infrared sensors like spies use but not our eyes)
BUT if we get it hot enough (VERY hot) we reach the visible range, where the molecules start to “glow.” This is called INCANDESCENCE.
And the color? Color of light depends on wavelength which depends on the energy content (which follows ROYGBIV 🌈). Incandescence usually emits low-energy light (red to yellow). It’s hard to get enough energy for colors like blue & purple. So how do fireworks makers do it?
LUMINESCENCE! In this type of light emission, only the e⁻ have to get excited, so you don’t need as much heat
🔹 Fireworks makers add “coloring agents” which are chemicals (usually metals) that absorb some of the energy the reaction releases (remember that the reaction involved in fireworks is VERY EXERGONIC). This energy lets e⁻ in those chemicals get “promoted” up to “excited” states 🔆 but the excitement wears off 🔅 & they fall back down, releasing energy in the form of light
🔹🔹 different chemicals absorb & release different amounts of energy so they give off different color light. For example, sodium gives you yellows & oranges; copper & barium – greens & blues; calcium & strontium – red; etc.; firework makers can mix & match metals to get color combos (but have to make sure they’re using compatible forms of them)
Hope this post helps you better appreciate fireworks! And chemistry!
This post is part of my weekly “broadcasts from the bench” for The International Union of Biochemistry and Molecular Biology. Be sure to follow the IUBMB if you’re interested in biochemistry (now on Instagram un-hacked @the_iubmb)! They’re a really great international organization for biochemistry.⠀
more on thermodynamics: http://bit.ly/thermodynamicsfreeenergy
more on all sorts of things: #365DaysOfScience All (with topics listed) 👉 http://bit.ly/2OllAB0