The Dazzling Chemistry of Fireworks

Yesterday was the Fourth of July, and millions of people all over America gathered in parks and along waterways to witness one of the most dazzling examples of ancient science; fireworks. The invention of gunpowder in China ca. 600-900 CE resulted in the creation of the first fireworks, later buffed up to the colorful sky-flowers we know today by Italian fireworks makers in the 1830s [1]. While waiting for a fireworks display at the Philadelphia Museum of Art, I found myself wondering how is it that we humans have come to create this technology. The first step was to devise a method of controlling highly exothermic and expansive (due to gas formation) chemical reactions and initiate them on purpose through the use of chemical oxidizers (originally saltpeter, KNO3) that contribute oxygen to combustion reactions, allowing them to burn in containers without oxygen flow or even in vacuums. But the role of gunpowder in history has had many uses beyond fireworks and isn’t what makes this specific variety of chemical explosions so addicting. What gives fireworks their characteristic hues are the metal ions that exist along with the crafted gunpowder in fireworks shells. Italian fireworks makers discovered that the use of KClO3 rather than saltpeter could increase the temperature of the combustion reaction, allowing for the light-emitting potential of metal ions to be unleashed [2]. When calculating the standard enthalpy of reaction for the following decompositions;

1.     2KNO3 --> K2O + N2 + 2.5O2

2.     2KClO3 --> 2KCl + 3O2

it can be observed that the decomposition of KClO3 is more exothermic than that of KNO3 (625 kJ/mol for KNO3 and -89.6 kJ/mol for KClO3 based on standard enthalpies of formation found on Wikipedia), the former also generating 0.5 more moles of O2 gas than the latter. The extra oxygen gas contributes to the exothermic nature of the combustion through the formation of CO2 and SO2 gas (-393.5 kJ/mol for CO2 and -296.81 kJ/mol for SO2 based on Wikipedia values), the same source of energy from which coal power plants operate, according to the following equations:

3.     C + O2 --> CO2

4.     S + O2 --> SO2

The color that a given metal salt produces when heated is characteristic of the metal element and can be calculated based on the following equations [3,4];

5.     E=hv 

6.     c=lv (v=fl modified for light)

7.     E=hc/l (from equations 5. and 6.)

8.     E=-Rhc(1/(nf)^2-1/(ni)^2) (Rydberg Formula*)

*note: UC Davis source seems to consolidate the Rydberg Formula hc term into the Rydberg constant, as evidenced by their subsequent derivation eliminating the hc term against the value of R.

Fig. 1: Sodium electron orbital energy diagram (Whitman College)

From the above electron orbital energy diagram for sodium, it can be seen that the transition between the highest occupied molecular orbital (HOMO), the 3s orbital, and the lowest unoccupied molecular orbitals (LUMO), the 3p set, corresponds to an emission of light in the 589nm wavelength region, or yellow light. This fact is experimentally verified since the yellow color of fireworks is often produced by sodium salts. It is in this fashion that the colors each metal element produces can be understood. So why metals? Well, the emission of light in various visible colors is reliant on electron energy state transitions as we just discussed, and electron band structure properties for metals tell us that the Fermi level, or highest occupied energy level (HOMO energy level), lies in the middle of an electron band in metals, meaning that there are higher electron energy levels to which metal electrons may be excited [5]. So metals are used simply because in our universe these are the elements that are capable of light-producing electron transitions.

Of course other factors must go into fireworks to achieve such controlled chemical acrobatics as producing tailed explosions or glittery rain, but with bang and color you have the basic makings of this recreational technology that is also a marvel of modern chemistry. 

I plan to do experiments inspired by daily events or questions that arise, so comment if this post has brought up any topics you would like to hear more about or want to see experiments on. Thanks!