Lesson Video: Photochemical Reactions Chemistry

In this video, we will learn how to describe photochemical reactions and their role in processes such as photographic development, photosynthesis, and ozone destruction.

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Video Transcript

In this video, we will learn how light plays an important role in certain chemical reactions, which we call photochemical reactions. We’ll learn how this works by looking at several processes: photograph development, photosynthesis, and ozone destruction.

All plants require sunlight in order to grow. Why is this? Well, plants are able to convert light energy from the sun into chemical energy that they can use to grow in a process known as photosynthesis. Photosynthesis occurs in the part of a plant called chloroplasts that contain a large, complicated molecule called chlorophyll. Chlorophyll is a pigment in plants that absorbs the red and blue light from the sun. The green light from the sun is not absorbed by plants. It’s reflected, which is why we perceive plants as being green. The green light that’s reflected by the plant is detected by your eye.

This red and blue light that the plant absorbs is able to provide the energy that’s needed for the plant to produce glucose, a sugar that the plant uses for food. The light from the sun plus carbon dioxide from the air and water is all the plant needs to create this food for itself plus the other product of photosynthesis, oxygen, which we breathe.

We can observe the process of photosynthesis ourselves in an experiment that involves pond weeds or the plant called elodea. If you keep this experimental setup in the dark, you won’t see much happening. And if you place it in low-light conditions, you’ll start to see some bubbles form, and some gas will collect in the test tube. And if you do this experiment in very high-light conditions, you’ll see more bubbles form, and you’ll be able to collect lots of gas in the test tube. This gas that’s being produced is oxygen, one of the products of photosynthesis. In the low-light conditions, we saw some oxygen being produced; in the high-light conditions, we saw lots of oxygen being produced. But when the plant was in the dark, we weren’t able to observe any oxygen being produced.

Photosynthesis is an example of a photochemical reaction, which is a process that’s initiated by absorbing light. You’ll notice that light is written above the reaction arrow in the chemical equation that describes the process of photosynthesis. We’ll see this for all photochemical reactions since they all require light in order to start. Sometimes, you might see the word photon or the symbol ℎ𝜈 instead of the word light. That’s because light is made up of elementary particles called photons, but we’ll stick with using the word light for this video. Now, let’s look at some more examples of photochemical reactions.

The next example we’ll look at is the photoreduction of silver chloride. Silver chloride or AgCl is a white powder, but if you shine a laser at it, it turns gray. So, let’s figure out what’s going on here. Silver chloride is an ionic substance, so that means that it’s composed of silver ions and chloride ions. The light from the laser causes the chloride ions to lose an electron and form chlorine atoms. The loss of electrons is called oxidation. So, this first step involves the light causing the chloride ions to be oxidized to form chlorine atoms. Of course, chlorine atoms can’t exist on their own very well, so they’ll combine with other chlorine atoms to form chlorine gas. Next, the silver ions gain the electron that the chloride ions lose, forming silver. The process of gaining an electron is called reduction.

So, the overall reaction for this process is silver chloride in the presence of light reacting to form silver plus chlorine gas. So, when we shine a laser on the silver chloride, the light causes the silver ions to be reduced into silver, which is the gray powder that we saw. That’s why this is called the photoreduction of silver chloride. The light causes the silver to be reduced.

This is how cameras that use film work. The film is made of a piece of plastic that’s coated in gelatin that’s mixed with silver chloride or another silver halide. When you take a picture with the camera, light enters the camera through the lens. And since the silver chloride changes color when exposed to light, when you take the picture, different parts of the film change color, depending on how much light they’re exposed to through the camera lens, which puts an image on the film. Then, the film can be developed to make it so it’s no longer sensitive to light preserving your picture.

The next reaction we’ll look at is a reaction that’s constantly occurring in Earth’s atmosphere. The molecule ozone is produced primarily in the part of Earth’s upper atmosphere, called the stratosphere. The formation of ozone starts with UV light from the sun hitting a molecule of oxygen in Earth’s atmosphere. This causes the oxygen molecule to be split apart into two individual oxygen atoms. These oxygen atoms will then collide with other oxygen molecules, which will produce ozone, or O3 molecules. So, the overall reaction for the formation of ozone is three molecules of oxygen reacting to form two molecules of ozone.

The UV light from the sun can also cause this reaction to go in the other direction. The UV light can break apart the molecule of ozone, creating one molecule of oxygen and an oxygen atom. This oxygen atom can then collide with another molecule of ozone, causing it to form two more molecules of oxygen. So, the overall reaction for the destruction of ozone is two molecules of ozone reacting to form three molecules of oxygen. The ozone that’s produced in Earth’s upper atmosphere as a result of UV light hitting molecules of oxygen to form ozone molecules is extremely important for life on Earth. The ozone filters out most of the UV light from the sun that has high frequencies, which can damage the DNA in our cells.

Though ozone is being constantly formed and destroyed in Earth’s upper atmosphere, the rate that these two reactions is occurring happens so that we maintain a good layer of ozone in Earth’s atmosphere to protect us from UV light. Unfortunately, certain chemicals that humans have put into the atmosphere, such as chlorofluorocarbons or CFCs that used to be used in refrigeration, aerosols, and air conditionings, have acted as a catalyst for the destruction of ozone but not the formation of ozone. This means that we no longer have the careful balance between the ozone formation and the ozone destruction that protects us from the UV light from the sun.

Because the destruction of ozone is catalyzed, but the formation of ozone isn’t, ozone is destroyed faster than it could be produced, which means more UV light from the sun is able to enter Earth’s atmosphere. CFCs and other chemicals that act as catalyst in the destruction of ozone have been banned in most countries for this reason. Luckily, these compounds don’t stay in the atmosphere forever. So because ozone is constantly being both destroyed and formed, ozone in the atmosphere will eventually be restored to normal levels.

The final example we’ll look at of a photochemical reaction is photochemical smog. Photochemical smog is a kind of air pollutant that’s commonly found in cities. It appears as a brown haze. This smog is produced because of a reaction that occurs from one of the byproducts of burning fossil fuels. When we burn fossil fuels in things like our vehicles, chemicals like nitrogen oxide are released. These chemicals that are released can react with light from the sun, which forms the smog. This smog is not only unappealing to look at, but it can also cause a number of respiratory issues for people that are exposed to it.

So now, we’ve learned about photochemical reactions and seen several examples of them. So, let’s look at some problems.

Which of the following statements correctly describes a photochemical reaction? (A) A photochemical reaction is a chemical reaction that occurs without the absorption of energy from light. (B) A photochemical reaction is a chemical reaction that releases bright flashes of light. (C) A photochemical reaction is a chemical reaction initiated by an increase in temperature. (D) A photochemical reaction is a chemical reaction that does not involve the transfer of electrons. (E) A photochemical reaction is a chemical reaction initiated by the absorption of energy from light.

A photochemical reaction is a reaction that’s initiated by absorbing light. We can identify these kinds of reactions because light will appear above the reaction arrow in the chemical equation. This definition clearly seems to match answer choice (E), but let’s take a quick look through our other answer choices.

Answer choice (A) says that a photochemical reaction occurs without the absorption of energy from light, which is clearly the opposite of what a photochemical reaction is. So, this isn’t correct. Our next answer choice says that a photochemical reaction releases bright flashes of light. There are plenty of examples of chemical reactions that produce light. There are certain species of jellyfish that are able to bioluminesce or glow for instance. But these types of reactions would essentially have light as a product; they wouldn’t require light in order to react.

The next answer choice is about an increase in temperature. There are plenty of examples of reactions that are initiated by absorbing energy from an increase in temperature, but we’re specifically talking about reactions that are initiated by absorbing energy from light. Our next answer choice says that photochemical reactions do not involve the transfer of electrons. Well, it turns out that there are plenty of examples of photochemical reactions that do involve the transfer of electrons. For example, when you shine a light on silver chloride, electrons are transferred from the chloride ions to the silver ions. But whether or not there is a transfer of electrons is not the definition of a photochemical reaction. Photochemical reactions are simply reactions that are initiated by absorbing energy from light.

In the atmosphere, ultraviolet light can decompose molecules of oxygen gas into individual atoms of oxygen. What is the chemical equation for this reaction?

In the reaction described in this question, ultraviolet light is causing oxygen gas, which has the chemical formula O2, to decompose into individual atoms of oxygen. So, if we start off with one molecule of oxygen gas, it would decompose into two atoms of oxygen. To complete our chemical equation, we shouldn’t forget to write light over the reaction arrow since this reaction requires UV light in order to start, which makes this particular reaction a photochemical reaction. This reaction described is the first step in the reaction of ozone formation in Earth’s atmosphere.

The next step of the reaction involves each of those individual atoms of oxygen that are produced striking another molecule of oxygen gas, which would each form a molecule of ozone or O3. This photochemical reaction initiated by UV light striking oxygen in the atmosphere is extremely important for life on Earth, as ozone shields us from the harmful effects of UV radiation. But this question was only asking us about the first step in this process, which is where UV light causes O2 gas to decompose into two oxygen atoms.

Now, let’s summarize what we learned in this video with the key points. A photochemical reaction is a chemical reaction that’s initiated by absorbing light. A photochemical reaction will have light written above the reaction arrow in a chemical equation. One common example of a photochemical reaction is the process of photosynthesis that plants use to create food as well as the formation and the destruction of ozone in Earth’s upper atmosphere.

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