How Does Chemiluminescence Occur? Key Factors and Examples
During a chemical reaction, the emission of light (of any wavelength) is normal. However, if the reaction may also give off a sufficient quantity of heat. We say this type of reaction is chemiluminescence.
The word “chemiluminescence” has two words, i.e., chemi and luminescence. Here, chemo means chemical reaction and luminescence means something that gives off light.
So, chemiluminescence is also known as chemiluminescence. This is the process of emission of light as a result of the chemical reaction.
Therefore, given reactants A and B, with an exciting intermediate ◊, we have:
[A] + [B] → [◊] → [Products] + light
We will understand what is chemiluminescence with illustrative chemiluminescence examples in detail.
Chemiluminescence Definition
We define chemiluminescence as the light emission as a result of the chemical reaction.
During the product formation, light isn't necessarily the only form of energy released by a chemiluminescent reaction. Besides, heat may also release, making the reaction exothermic.
How Chemiluminescence Works?
During a chemical reaction, the reactant atoms/molecules/ions collide with each other. These particles interact with each other to form what we call a transition state.
From the transition state, product formation occurs.
The transition state is the state at which enthalpy stays maximum. However, the products usually have less energy than the reactants.
Further, a chemical reaction occurs when there is an increase in the stability/decrease in the energy of the molecules.
Thus chemical reactions that release energy as heat, the vibrational state of the product remains excited. The energy disperses through the product, makes it warmer, in short, the release of heat. The same happens in chemiluminescence.
Now, we will understand what chemiluminescence is.
What is Chemiluminescence?
The process of chemiluminescence is the same we discussed in the “how chemiluminescence works,” section.
In this process, electrons become excited. The excited state is the transition/intermediate state.
When excited electrons come back to the ground state, the energy is released in the form of chunks of energy called photons.
The decay to the ground state can occur through a quick release of light, like fluorescence or a forbidden transition (likewise phosphorescence).
In theoretical terms, each molecule that participates in a reaction releases one photon of light.
In reality, the production is much lower. For instance, non-enzymatic reactions possess around 1% quantum efficiency.
On adding a catalyst, a great increase in the brightness or luminescence can be seen in many reactions.
Chemiluminescence Examples
Example 1:
An H2 (hydrogen) atom in its ground state has a single electron. A single electron is in a shell, i.e., n = 1. Since each shell has its own energy level.
When the hydrogen atom absorbs a quantum (quantized) amount of energy, it reaches a higher energy level (shell n = 2).
When hydrogen reaches an excited state or a high-energy state. We make an asterisk (*) aside the molecule to indicate this.
The electron retraces to its original position, i.e., the ground state (shell n = 1).
In the process, a packet of energy (a photon) releases in the form of electromagnetic radiation.
The wavelength of the light emitted depends on the amount of energy.
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Furthermore, if the wavelength is within the range of visible light, the electron transition is perceived as the light of a particular colour. Therefore, the wavelength of light determines the colour.
Example 2:
A reaction between hydrogen (H2) and oxygen (O2) to form water (H2O) is an example of a chemical change. In this reaction, the H-H bond in H2and the O-O bonds in O2 break.
Here, new H-O bonds form to make H2O.
For the most part, when chemicals undergo a change in this manner, they either give off heat (exothermic) or absorb heat (endothermic). Hence H2 plus O2 reaction is exothermic.
We say that a few very interesting kinds of chemical reactions occur in which the energy produced is given off not as heat but as light.
These reactions are what we term chemiluminescent. In living organisms, we call it bioluminescent.
Examples of Chemiluminescence
C8H7N3O2 (luminol) when reacts with H2O2 (hydrogen peroxide) → gives → 3-APA (vibronic excited state) → 3-APA (decays to a lower energy level) + light (release)
Here, 3-APA is 3-Aminopthalalate.
Point To Note:
No difference is there in the chemical formula of the transition state. Wherefore, only the energy level of the electrons.
This happens because iron is one of the metal ions that catalyzes the reaction. Chemists use the luminol reaction to detect blood.
Iron from hemoglobin results in the chemical mixture glows brightly.
Another good example of chemiluminescence is the reaction occurring in glow sticks.
The color of the glow stick occurs from a fluorescent dye called a fluorophore, which absorbs the light from chemiluminescence and releases it as another color.
Point To Note:
Chemiluminescence not only occurs in liquids; however, in gases as well.
For instance, a gas-phase reaction between vaporized phosphorus and oxygen results in the green glow of white phosphorus in the damp air.
FAQs on Understanding Chemiluminescence in Chemistry
1. What is chemiluminescence in chemistry?
Chemiluminescence is the emission of light as a direct result of a chemical reaction. In this process, the reaction produces a molecule in an electronically excited state. This unstable molecule then releases its excess energy in the form of light (photons) as it returns to its stable, ground state. It is often called “cold light” because it produces very little heat.
2. What is the basic principle of the chemiluminescence phenomenon?
The fundamental principle of chemiluminescence involves a three-step process:
- Activation: A chemical reaction provides the energy to push a product molecule into an electronically excited state.
- Emission: The excited, high-energy molecule is unstable and seeks to return to a lower energy level, or ground state.
- Relaxation: To return to the ground state, the molecule releases the excess energy as a photon of light, which we observe as a glow.
3. What is the difference between chemiluminescence and bioluminescence?
Chemiluminescence is the broad term for light produced by any chemical reaction. Bioluminescence is a specific type of chemiluminescence that occurs within living organisms. For example, the light from a recreational glow stick is chemiluminescence, while the light produced by a firefly is bioluminescence, which involves specific enzymes like luciferase.
4. How does temperature affect a chemiluminescent reaction?
Temperature directly impacts the rate of a chemiluminescent reaction. Increasing the temperature speeds up the reaction, causing more light to be emitted per second, which makes the glow appear brighter. However, because the reactants are consumed faster, the light does not last as long. Conversely, lowering the temperature (e.g., putting a glow stick in a freezer) slows the reaction, making the glow dimmer but significantly extending its duration.
5. What are some common examples of chemiluminescence in daily life and science?
Chemiluminescence has several fascinating applications and examples:
- Glow Sticks: These use the reaction between a diphenyl oxalate ester and hydrogen peroxide to produce light for recreational or emergency use.
- Forensic Science: The luminol test is used at crime scenes to detect trace amounts of blood. The iron in haemoglobin catalyses a reaction that makes luminol glow blue.
- Analytical Chemistry: It is used to measure concentrations of pollutants like nitrogen oxides in the air.
- Bioluminescence: Found in nature, such as in fireflies, certain fungi, and many deep-sea creatures.
6. Why is chemiluminescence referred to as “cold light”?
Chemiluminescence is called “cold light” because the light is generated from the energy released during a chemical reaction at or near room temperature, without a significant increase in heat. This distinguishes it from incandescence, where light is produced by heating a substance to a very high temperature until it glows, such as the filament in a traditional light bulb.
7. How is chemiluminescence different from fluorescence and phosphorescence?
The key difference lies in the source of energy that creates the light:
- Chemiluminescence: The energy comes from a chemical reaction. The light is produced as long as the reactants are available.
- Fluorescence: The energy comes from absorbing light (usually UV light). The substance glows only while the external light source is on and stops immediately when it's removed.
- Phosphorescence: The energy is also from absorbing light, but the substance can store this energy and release it slowly. This allows it to continue glowing for some time after the external light source is turned off, as seen in glow-in-the-dark toys.
8. Can a chemiluminescent reaction, like in a glow stick, last forever? Explain why.
No, a chemiluminescent reaction cannot last forever. Like any chemical reaction, it relies on a finite amount of reactants. The light is produced as these reactants are converted into products. The glowing stops once one or all of the initial chemicals are completely used up. The brightness of the light gradually fades as the concentration of the reactants decreases over time.
9. What role do catalysts play in chemiluminescent reactions like the luminol test?
In many chemiluminescent reactions, a catalyst is crucial for producing a bright, visible glow. For instance, in the luminol test for detecting blood, the iron atom in the haemoglobin of blood acts as the catalyst. It significantly speeds up the rate of the oxidation reaction of luminol. Without the catalyst, the reaction would be too slow and the light produced would be too faint to be easily observed.
