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!
Fourth Of July conveniently happens to follow quite nicely with our discussions of redox reactions – 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⁻ 👍
Electrons are 1 of 3 main subatomic particles – atoms also have positively-charged protons & neutral neutrons in a dense central core. The protons try their best 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)
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 👉 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 is ⚖️ & this depends on how much less squirmy you’ll be there & how much you’ll be able to spread out 👇
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 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 👆 equation 👀 @ 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” bc 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 ⬆️ temps, ENTROPY becomes more & more important 👍
NEGATIVE ΔG means products have LESS FREE ENERGY than reactants, so “2nd couch is comfier” & rxn is likely to proceed spontaneously 👍 We call such rxns EXERGONIC
POSITIVE ΔG means products have MORE FREE ENERGY than reactants, so you’ll have to really bribe it to go… 🙄 We call such rxns ENDERGONIC
NEGATIVE ΔH means products have LESS HEAT than reactants 👉 this can only happen if reactants give up heat to the “surroundings” 👉 such heat-releasing rxns are called EXOTHERMIC
POSITIVE ΔH means products have MORE HEAT than reactants 👉means reactants “stole” heat from “surroundings” 👉 such heat-absorbing rxns are called ENDOTHERMIC
NEGATIVE ΔS means the products have LESS FREEDOM/RANDOMNESS 👉 can come from having more/stronger bonds tethering the molecules together ⛓
POSITIVE ΔS means the products have MORE FREEDOM/RANDOMNESS 👉 can come from having fewer/weaker bonds tethering the molecules together so they can move around more 👍 Especially 👍 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 rxns 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 rxn 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) 👉 have a large ➖ΔH 👉 makes ΔG more ➖ 👍
🔹fireworks gain LOTS of ENTROPY 🎰🎰 when they explode (molecules go from confined in shell 2 shooting off randomly)👍 AND this ΔS is multiplied by ⬆️T 👉 makes ΔG VERY ➖👍👍
Result 👉 🎆 rxn releases LOTS of free energy 👉 rxn is EXERGONIC 👍
What is this rxn? 🤷♀️ Let’s 👀 inside (one example, there are other chemicals that can be used)👇
REACTANTS: FLASH POWDER 👉 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) ⏩ releases oxygen ⏩ oxygen reacts w/Al ⏩ Al burns to aluminum oxide (Al₂O₃)
⭐️ overall (unbalanced) rxn 👉 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!
If there’s such a large ➖ΔG why doesn’t rxn start w/o ignition?🤷♀️ Our rxn’s actually 2 rxn combined👇
1️⃣ 3 KClO₄ ➡️ 3 KCl + 6 O₂
2️⃣ 6 O₂ + 8 Al ➡️ 4 Al₂O₃
rxn 2️⃣ is EXOTHERMIC, BUT rxn 1️⃣ is ENDOTHERMIC 👉 NEEDS heat ♨️ to proceed👇
1️⃣ 3 KClO₄ + ♨️ ➡️ 3 KCl + 6 O₂
2️⃣ 6 O₂ + 8 Al ➡️ 4 Al₂O₃ + ♨️
We have 2 provide ♨️ 2 get 1️⃣ to start 🏁 👉 w/o 1️⃣ there can’t be 2️⃣ (missing our reactants! 😬) so rxn doesn’t start 🖐
BUT once started, 2️⃣ provides ♨️ for 1️⃣ 👉 rxn goes until “all” reactants are converted 2 products
Where does💡come from? 🤷♀️ We’ve explained where the heat ♨️ from fireworks 🎆 comes from but that’s not what you go to 👀! 😴 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 👀 it, so we call it “visible light” 👍 more here: http://bit.ly/2CfaXbJ
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 👀 (it’s in the infrared range) 😴
BUT if we get it hot enough (VERY hot🌡🌡) we reach the visible range 👉 the molecules start to “glow” 👉 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) 👉 hard to get enough energy for colors like blue & purple. 😰 So how do fireworks makers do it? 🤷♀️ 👇
LUMINESCENCE 👉 in this type of 💡 emission, only the e⁻ have to get excited 👉 don’t need as much ♨️ 👍
🔹 Fireworks makers add “coloring agents” 👉 chemicals (usually metals) that absorb some of the energy the rxn releases (remember that the reaction involved in fireworks is VERY EXERGONIC) ⏩ e⁻ in those chemicals get “promoted” ⬆️ to “excited” states 🔆 but the excitement wears off 🔅 & they fall back ⬇️, releasing energy in the form of light 💡
🔹🔹 different chemicals absorb & release different amounts of energy so they give off different color light 🌈💡🤗
sodium gives you yellows & oranges; copper & barium – grens & 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)