Chemistry of Bleached Hair in Everyday Life
- Page ID
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)This Exemplar will teach the following concepts from the ACS Examinations Institute General Chemistry ACCM:
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V. A. 1. a. Chemical equations represent reactions symbolically, so they must be balanced and accurately portray reactants and products
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V. D. 2. a. Oxidation–reduction reactions are important to chemistry and are sometimes referred to as “redox” reactions.
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V. D. 2. b. Being able to identify an oxidation–reduction reaction often involves assigning oxidation numbers.
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V. D. 2. c. The use of oxidation–reduction reactions as a category includes the concepts of oxidizing and reducing agents.
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V. D. 2. e. The oxidation–reduction of metals (with each other or with acids) yields a measure of the activities of metals.
It might be surprising to learn that one of the most popular hair procedures is directly related to one of the fundamental topics in chemistry. Hair bleaching, or the process of lightening one's hair for cosmetic purposes, is only possible due to the fact that the pigments in the hair are able to go through a redox reaction. But what exactly is a redox reaction? How does it relate to hair? Today, we want to explain how seemingly abstract concepts in chemistry relate to our everyday lives.
Figure 1: An example of bleach being applied to hair with depigmentation occurring.
What is Hair Bleaching?
Hair bleaching is the process in which people can apply chemicals to their hair that cause it to lighten. Hair has color due to melanin, a natural pigment. During the hair bleaching process, ammonia is added to the hair. This causes melanin to break down, resulting in the hair becoming much lighter. Bleaching irreversibly changes the chemical composition of one's hair by decomposing melanin in this manner. This can be seen as a sign that a chemical reaction, specifically a reduction-oxidation reaction, has occurred. What is this type of reaction?
Redox Reactions
Oxidation is an increase in oxidation state, or a loss of electrons. An example of an oxidized atom is O2- to O. The oxygen atom begins with an oxidation number of -2, indicating that it has two more electrons than its standard state. Oxygen is oxidized because it loses electrons, specifically the two additional ones, causing the oxidation number to become 0. Oxidation does not mean getting the oxidation number to 0: it means increasing the oxidation number, indicating a loss of electrons, as was shown in this example.
Reduction is a decrease in the oxidation state of an atom, or a gain of electrons. An example of reducing an atom can be seen between O and O2-. In this instance, the oxygen atom gains two electrons to its valence shell, causing a decrease in oxidation number as it goes from 0 to -2 by picking up the additional negative charge.
In an oxidation-reduction reaction, or a redox reaction, there is a transfer of electrons between atoms and molecules, resulting in changes of oxidation numbers. At least one molecule acts as an oxidizing agent, and at least one molecule acts as a reducing agent. An oxidizing agent is a reactant that acts as an electron acceptor in the reaction, being reduced and allowing another molecule to be oxidized by losing those electrons gained by the oxidizing agent. Separately, a reducing agent acts as an electron donor by giving electrons to other atoms, itself being oxidized and allowing the other atom(s) to be reduced.
The sum of the oxidation numbers of an atom or molecule as a whole is equal to the overall charge of that molecule. Therefore, in most cases, this sum is equal to 0. For atoms or molecules in their naturally-occurring form, such as an individual atom or neutral species like H, O2 or F2, the oxidation number of each atom is 0. Hydrogen (H) has a standard oxidation number of +1 in compounds, Fluorine (F) has a standard oxidation number of -1 in compounds, and Oxygen (O) has a standard oxidation state of -2 in compounds. Other atoms, such as C in organic compounds, can have variable oxidation numbers based on the molecules they are in to allow for the correct overall charge on the molecule.
The most common example of a redox reaction is a combustion reaction, in which a large organic molecule reacts with oxygen (O2) to break down into carbon dioxide (CO2) and water (H2O). However, hair bleaching is also a representation of a redox reaction.
Sample Redox Reaction- Corrosion
Before diving into understanding the complex redox reaction behind bleaching hair, a different example of a redox reaction that results in a color change is corrosion, or rusting. Corrosion is the reaction between a metal and oxygen and water to form a hydrated metal oxide.
\[ Fe + O_2 + H_2O \rightarrow Fe_2O_3 * H_2O \]
In this reaction, we assigned a +1 to each of the hydrogen on both sides of the equation. We then assigned 0 to both the solid iron and the molecular oxygen, as they are in their free states. To make H2O neutral, we assign the typical -2 to the oxygen on the reactant side of the equation. For the hydrated iron oxide, we want it to remain neutral. Thus, with the typical +1 on the hydrogen and the -2 on the oxygens, to make sure the molecule remains neutral, we can assign a +3 to the iron in the oxide. From the above equation, we see that iron goes from an oxidation state of +0 to +3. This means that during the reaction, it loses electrons and, thus, is oxidized. Conversely, we can see that oxygen goes from +0 to -2. This means that during the reaction, it gains electrons and is reduced. Because of this, we can say that Fe is the reducing agent and oxygen is the oxidizing agent. In this specific example, rusting and corrosion result in a change in appearance of the compound, similar to how the redox reaction behind bleaching hair, explained hereafter, causes a change in appearance.
\[4 Fe (s) + 3 O_2 (g) + 2 H_2O (l) \rightarrow 2 Fe_2O_3*H_2O \]
The redox reaction between iron, oxygen, and water, produces a hydrated iron oxide. This causes the originally silver-gray iron to turn red-brown because of the hydrated iron oxide on its surface, exemplifying a change in appearance of a substance as a result of a redox reaction.
The Process of Bleaching Hair
In the context of our problem, there is a two-step reaction process that takes place that enables hair to be bleached. Melanin, one of the most dominant pigments in hair, is oxidized by hydrogen peroxide via ammonia, which is present in bleach. The two-step process is detailed below:
We can clearly see how the two steps of this reaction are both redox reactions, as the oxidation numbers change in both steps of the reaction change for certain elements. In the first reaction, +1 is assigned to each hydrogen on the reactant side. This forces a -1 oxidation number to be assigned to each oxygen in the H2O2 and a -3 oxidation number to be assigned to the nitrogen in the ammonia reactant. While oxygen atoms typically have an oxidation state of 2-, considering the rules of redox reactions, the +1 charge is assigned to the hydrogen first, meaning all other molecules come second to that. This leaves oxygen with a -1 oxidation number. On the product side, hydrogen is still +1 whereas oxygen has a new oxidation state of -2 in H2O. Because the oxidation number for hydrogen does not change, the nitrogen in ammonia on the product side still has an oxidation number of -3. This leaves the O2 to have an oxidation state of 0. Because the oxidation number of oxygen changed from -1 to -2 from H2O2 to H2O, this means the oxygen in H2O was reduced. Oxygen’s oxidation number also changed from -1 to 0 going from H2O2 to O2. This increase in oxidation state means the oxygen in O2 was oxidized.
\[NH_{3} + 2 H_2O_2 \rightarrow NH_3 + 2 H_2O + O_2\]
For step 2, Carbon is oxidized, as its oxidation number goes from 0 to +4. In step 2, oxygen is also being reduced, as it goes from 0 to -2 as the reaction progresses.
Because of this redox reaction, the melanin pigment is oxidized and broken down into smaller molecules like water, oxygen, and carbon dioxide. In other words, through this breaking down process, melanin becomes nonexistent. Given that the presence of melanin is responsible for the color and pigment of one's hair, the decomposition of melanin in this manner accounts for the change in hair color. This is why bleaching hair results in a loss of pigment as, through a set of redox reactions, pigments such as melanin eradicated entirely.
\[NH_3 + H_2O + 18 O_2 + C_{18}H_{10}N_2O_4 \rightarrow 3 NH_3 + 4 H_2O + 2 O_2 + 18 CO_2\]
The change in appearance seen in the oxidation of melanin parallels the change in color as a result of corrosion. The decomposition of melanin results in the color change of hair, just as the silver-gray iron turns red-brown when hydrated.
Additional Examples
The following is a sample redox reaction, specifically the combustion of Methanol (CH3OH):
\[CH_3OH(l) + O_2(g) \rightarrow CO_2(g) + H_2O(g)\]
How would you balance it?
Solution
To begin breaking down this equation, +1 is assigned to each hydrogen and -2 is assigned to each oxygen. This forces the carbon to be -2 for neutrality in the methanol reactant molecule and +4 in the carbon dioxide product, indicating an increase in oxidation state (loss of electrons, increase in oxidation number). Differently, the oxidation state of O2 in the reactants is 0, which then drops to -2 in the products. With these values in mind, for this redox reaction and others, the equation can be balanced such that the number of atoms of each element is the same on each side. For this example, the result is:
\[ 2 CH_3OH(l) + 3 O_2(g) \rightarrow 2 CO_2(g) + 4 H_2O(g)\]
Another sample redox reaction (non-combustion reaction) is between potassium iodide and hydrogen peroxide. The unbalanced equation is:
\[H_2O_2 + KI + H_2SO_4 \rightarrow K_2SO_4 + I_2 + 2 H_2O\]
How would you balance it?
Solution
In this reaction, +1 is assigned to each hydrogen (reactants and products). Potassium also has a +1 on both sides of the equation. Sulfur has an oxidation state of +6 as a product and as a reactant as well. Given these oxidation states, the oxygen in H2SO4 must have a -2 and the oxygen in $K_2SO_4$ must have a -2 oxidation state as well. The difference in oxidation states is that the reactant O2 in H2O2 has a -1 oxidation state but is reduced in the reaction to be -2 in H2O. The resultant balanced equation following the redox equation is:
\[H_2O_2 + 2KI + H_2SO_4 \rightarrow K_2SO_4 + I_2 + 2 H_2O\]
Another difference can be seen in iodine. On the reactant side, iodine has a -1 oxidation state when it is part of potassium iodide. On the reactant side and when it is in its gaseous state, I2, is oxidized and is assigned 0. This difference allows the reaction to be balanced and it can be seen in the equation above. The transfer of electrons between atoms demonstrates the basic principles of a redox reaction.
References
(1) Corrosion as a Redox Reaction https://spmchemistry.blog.onlinetuit...-reaction.html (accessed 2022-12-07)
(2) Oxidation & Reduction. Uhcl.edu. https://sceweb.sce.uhcl.edu/wang/bio...x/2_redox.html (accessed 2022-12-07)
(3) Redox Reactions in Organic Chemistry and Biochemistry Libretexts.org. https://chem.libretexts.org/Courses/...SC_Upstate%3A_ (accessed 2022-12-07).
(4) Yuen, P. K.; Lau, C. M. D. Application of Stoichiometric Hydrogen Atoms for Balancing Organic Combustion Reactions. Chemistry Teacher International 2021, 0 (0). https://doi.org/10.1515/cti-2020-0034 (accessed 2022-12-07)
(5) Stubbings, J. Redox titration of hypochlorite in bleach chemistry tutorial. Com.au. https://www.ausetute.com.au/redoxbleach.html (accessed 2022-12-07).