In this explainer, we will learn how to describe photochemical reactions and their role in processes such as photographic development, photosynthesis, and ozone destruction.
A photochemical reaction is any reaction that is triggered by light energy. Photochemical reactions can occur in a wide variety of contexts. Just as a combustion reaction can begin from a flame or a spark, photochemical reactions can begin from sunlight or laser. There are many different photochemical reactions that occur in everyday life.
Definition: Photochemical Reaction
A photochemical reaction is a chemical reaction initiated by the absorption of energy from light.
Before the invention of digital cameras, all photography was done with photosensitive chemicals on specially coated materials. Camera film was coated with a thin layer of solid silver bromide (). When a picture was taken, the film was exposed to light through the lens for a fraction of a second.
When light strikes the film, it decomposes the silver bromide into solid silver and liquid bromine. More silver will form in the regions more exposed to the light. When the film is developed, the difference between the transparent silver bromide regions and the dark, opaque solid silver regions will be visible. This difference in visibility allows film photographers to print photographs that replicate the pattern of light that was initially let into the camera.
The photochemical reaction that takes place here is a decomposition reaction. The compound silver bromide decomposes into the elements silver and bromine. This reaction is also a redox reaction.
In one half reaction, silver ions are reduced:
In the other half reaction, bromide ions are oxidized:
In the full reaction, silver bromide decomposes:
The formation of solid silver through this reaction or a similar one is the foundation of black-and-white film photography dating back to 1839.
Example 1: Identifying the Chemical Equation for a Photochemical Decomposition Reaction
In black-and-white photography, light results in the decomposition of tiny amounts of silver bromide on the photographic film. What is the chemical equation for this reaction, including state symbols?
This question is asking us to pick the chemical equation that describes the decomposition of silver bromide. To answer this question, we need to know the chemical formula of silver bromide as well as the chemical formulas and states of the products.
First, we can determine the chemical formula of silver bromide. As a halogen, bromine forms ions. Silver can form or ions, but the ion is much more common. The ionic compound that forms between silver ions and bromide ions will have one ion of each to keep the overall charge neutral. The chemical formula of silver bromide is therefore . Choices B, C, and D give the correct formula of silver bromide.
Next, we can determine what silver bromide decomposes into. We know that it can only decompose into its constituent parts of silver and bromine, but what states will the silver and bromine be in? Like the majority of metals, silver will form a solid at room temperature, so it will be . As a halogen, bromine will most likely form a diatomic molecule of . In bromine’s case, it will be a liquid at room temperature and so will have the chemical formula .
The chemical equation for this process will have as a reactant and and as products. The balanced equation is written below:
Looking at the answer choices, only choice B gives the correct combination of silver bromide as , solid silver, and diatomic liquid bromine. The correct answer is choice B.
Example 2: Identifying Oxidation and Reduction in the Photochemical Decomposition of Silver Bromide
The photochemical decomposition reaction of silver bromide is also an example of a redox reaction. Which ions will be reduced, and which ions will be oxidized during the reaction?
- ions will be reduced, and ions will be oxidized.
- ions will be reduced, and ions will be reduced.
- ions will be oxidized, and ions will be reduced.
- ions will be oxidized, and ions will be reduced.
- ions will be reduced, and ions will be oxidized.
This question is asking us to determine which ions are oxidized and which ions are reduced during the decomposition of silver bromide. During this reaction, the compound silver bromide decomposes into the elemental forms of its constituent atoms, solid silver and liquid bromine.
The chemical equation for this reaction is as follows:
The ionic compound silver bromide is composed of silver ions and bromide ions. In this reaction, silver and bromine begin as ions and end as neutral atoms.
For to become , it has to gain an electron, and so it gets reduced. For to become neutral, it has to lose an electron, and so it gets oxidized.
The correct answer is choice E: ions will be reduced, and ions will be oxidized.
Another example of a photochemical reaction is the formation of photochemical smog. Photochemical smog is a brown haze that primarily forms over densely populated, warm-weather cities. The primary pollutants that cause photochemical smog are the nitrogen oxides and . These pollutants are primarily produced by cars and coal power.
When released into the atmosphere in high concentrations, ultraviolet light from the sun can initiate reactions that form even more harmful pollutants such as nitric acid. Photochemical smog can negatively impact the respiratory health of a city’s residents.
Two of the major photochemical reactions that contribute to photochemical smog are the decomposition of nitrogen dioxide () and the formation of low-level ozone ().
Nitrogen dioxide absorbs light energy and decomposes into nitrogen oxide and an oxygen atom, according to the following chemical equation:
The nitric oxide and oxygen atom produced by this reaction can combine with many compounds in the air to produce a variety of pollutants.
Photochemical reactions in the atmosphere can also produce a reactive hydroxy species (). Nitrogen dioxide can then react with this hydroxy species to produce nitric acid.
The hydroxy species can also react with sulfur dioxide and eventually lead to the formation of sulfuric acid.
The presence of nitric and sulfuric acid in the atmosphere results in acid rain. Acid is a major environmental problem that can damage buildings and harm both plants and aquatic wildlife.
In sunlight, an oxygen atom can combine with an oxygen molecule to form ozone:
While ozone forms a useful protective layer high up in our atmosphere, at ground level it can be harmful to the respiratory systems of living things.
Photochemical smog is a hazard that must be managed by limiting the release of reactants that can cause the smog to form.
Example 3: Determining Oxidation or Reduction of a Photochemical Reaction
During the formation of photochemical smog, nitrogen dioxide can absorb light and undergo the following reaction:
Is nitrogen oxidized or reduced during this reaction?
This question is asking us to identify whether nitrogen is being oxidized or reduced during this photochemical reaction. An important definition to remember is that reduction involves gaining electrons, while oxidation involves losing electrons.
To help us determine whether nitrogen is being oxidized or reduced, we can calculate whether its oxidation number is increasing or decreasing. One important rule when finding oxidation numbers is that oxygen’s oxidation number in simple oxides is . Another important rule is that a neutral compound’s overall oxidation number will be zero. Given these two rules, we can calculate the oxidation number of nitrogen in the reactant nitrogen dioxide and in the product nitrogen oxide.
In nitrogen dioxide (), the two oxygen atoms have a combined oxidation number of . To keep the overall compound’s oxidation number at zero, the nitrogen atom must have an oxidation number of .
In nitrogen oxide (), the oxygen atom has an oxidation number of . To keep the overall compound’s oxidation number at zero, the nitrogen atom must have an oxidation number of .
Nitrogen’s oxidation number goes from to over the course of this reaction. The addition of electrons lowers the oxidation number, so nitrogen must be gaining electrons. A reduction involves gaining electrons, so nitrogen is being reduced. Choice C is the correct answer.
Photochemical reactions are important in the production and decomposition of the ozone that helps protect us from harmful ultraviolet radiation. Ozone () is formed when molecules photochemically split into individual oxygen atoms, which can then react with to form ozone. The ozone molecules that are created can also be split apart by sunlight. Without the protective ozone layer in the atmosphere, Earth will be exposed to more harmful ultraviolet radiation from the Sun that can photochemically damage DNA and potentially cause skin cancer.
The formation and destruction of ozone molecules proceed as follows. Ultraviolet light from the Sun can split an oxygen molecule into individual atoms:
The atoms of oxygen are very reactive and can combine with oxygen molecules to form ozone:
Sunlight can also trigger the reverse reaction and decompose ozone molecules:
The production and subsequent breakdown of ozone is called the “ozone–oxygen cycle.” About 400 million tonnes of ozone or about of the ozone layer is decomposed and regenerated each day using these photochemical reactions.
Example 4: Identifying the Chemical Equation for the Photochemical Decomposition of Ozone
In the ozone–oxygen cycle, molecules of ozone () absorb ultraviolet light to split into oxygen gas and an individual atom of oxygen. What is the chemical equation for this reaction?
This question is asking us to find the chemical equation that represents the photochemical decomposition of ozone as described. The correct answer will represent ozone molecules splitting into oxygen gas molecules and an individual atom of oxygen.
To answer this question, we need to know the chemical formulas of the substances involved. Ozone’s chemical formula is , molecular oxygen’s chemical formula is , and an atom of oxygen has the chemical symbol .
Only choices D and E include all three of these substances. Choice E unnecessarily includes an extra ozone molecule on either side of the equation. The chemical equation in choice D represents an ozone molecule breaking down into a molecule of oxygen gas and an oxygen atom. The correct answer is therefore choice D.
A very important and well-known example of a photochemical reaction is photosynthesis. Plants undergo photosynthesis to turn water and carbon dioxide, with the help of sunlight, into oxygen and sugar (glucose) as represented by the following reaction:
The photosensitive chemical in plants is called chlorophyll. Chlorophyll absorbs red and blue light, but reflects green light, giving leaves their green color. Chlorophyll is incredibly important in plants, as the light it absorbs is essential for photosynthesis.
Demonstration: Showing the Effect of Light on the Rate of Reaction of Photosynthesis
- Place a plant, such as pondweed, into a beaker containing a dilute solution of sodium hydrogen carbonate.
- Place a funnel and then a test tube over the pondweed.
- Place the lamp at a certain distance from the beaker and switch it on.
- Count the number of bubbles produced by the pondweed over a period of one minute.
- Repeat the experiment, but this time with the lamp being placed nearer to or further from the pondweed.
The number of bubbles produced per minute increases when the lamp is placed closer to the pondweed.
The chemical equation for photosynthesis is shown below:
The bubbles observed during the experiment are bubbles of oxygen gas produced by the plant as it photosynthesizes.
The sodium hydrogen carbonate decomposes to produce the carbon dioxide needed for the reaction, while the water is also present in the solution. As the lamp is placed closer to the pondweed, the intensity of the light reaching it increases. The increase in light intensity increases the rate of photosynthesis, causing more oxygen gas to be produced and, therefore, more bubbles are observed.
- The nearer the lamp is placed to the pondweed, the higher the rate of photosynthesis and the more the bubbles are produced.
- The rate of a photochemical reaction can be increased by increasing the intensity of light.
Example 5: Identifying the Reactants and Products of Photosynthesis in an Experimental Setup
An experimental setup to measure the effect of light intensity on photosynthesis is shown below.
- Which gas is the reason the bubbles are produced?
- Carbon dioxide
- Why is a dilute solution of sodium hydrogen carbonate required?
- To keep the water at a neutral pH level
- As it decomposes to form carbon dioxide, which plants require for photosynthesis
- To provide the pondweed with sodium to help it grow
- As it decomposes to form oxygen, which plants require for photosynthesis
- To provide glucose, which plants require for photosynthesis
This question is asking us to identify the gas responsible for the bubbles shown in the diagram. The question also indicates that the plant is undergoing photosynthesis. In other words, this question is asking us to identify the gas product of photosynthesis.
The chemical equation for photosynthesis is as follows.
The gas product of this equation is oxygen. The pondweed is absorbing light from the lamp and using it to photosynthesize to produce glucose and oxygen.
The correct answer is choice C, oxygen.
This part of the question is asking us to identify the purpose of the sodium hydrogen carbonate () solution. Since several of the answers mention what “plants require for photosynthesis,” let us take a look at the chemical equation for photosynthesis.
The three things needed for the reaction to proceed are carbon dioxide, water, and light. In this diagram, we can see that the lamp provides light and the container holds water. Where does the come from? The answer is from the decomposition of the sodium hydrogen carbonate. When it decomposes, it forms sodium carbonate, water, and carbon dioxide:
The correct answer is choice B, it decomposes to form carbon dioxide. To be thorough, let us ensure that the remaining answers are incorrect.
Choices D and E claim that plants require oxygen and glucose for photosynthesis, but oxygen and glucose are the products of photosynthesis, not the reactants. Choices D and E are incorrect.
Choice C claims that the sodium is necessary to help the plant grow. However, plants can still undergo photosynthesis in the absence of any growth additives.
Choice A claims that the sodium hydrogen carbonate will keep the solution at a neutral pH. However, plants can grow at a range of pH values. While plants do not grow well at extremes, a perfectly neutral solution is not necessary. In addition, the sodium hydrogen carbonate dissolves to form basic solutions. Choice A is incorrect.
- Photochemical reactions are chemical reactions that require light energy to proceed.
- The photochemical decomposition of silver bromide to form solid silver is a key reaction in film photography.
- Atmospheric pollutants can undergo photochemical reactions to form harmful photochemical smog.
- The ozone layer that protects our atmosphere from harmful radiation is constantly regenerated by the photochemical production and decomposition of ozone.
- Photosynthesis is perhaps the most well-known photochemical reaction. During photosynthesis, plants absorb carbon dioxide, water, and sunlight to create sugar and oxygen.
- The intensity of light can affect the rate of reaction.