Chemical Reactions: Study with Video Lessons, Practice Problems & Examples

Chemical reactions involve the making and breaking of chemical bonds, resulting in reactants transforming into products. Reactants are the starting materials, while products are the end results. Reactions can be categorized as endergonic, requiring energy input, or exergonic, releasing energy. Endergonic reactions build complex molecules, while exergonic reactions break them down. Understanding these concepts is crucial for grasping metabolic pathways and energy dynamics in biological systems.

Chemical Reactions

In this video, we're going to begin our lesson on chemical reactions. Chemical reactions consist of the making and/or breaking of chemical bonds leading to changes in matter. Every chemical reaction has reactants and products. Reactants are the starting material in a chemical reaction; you can think of the reactants as the ingredients for the reaction. Then, of course, the products are the ending material in a reaction. Let's take a look at our image down below, which shows a chemical reaction. On the left-hand side, notice that we have these building blocks that are broken apart into smaller individual pieces. They're broken down. On the right, notice that the building blocks are coming together to build a larger, more complex structure. The beginning of every chemical reaction starts with reactants, which are, once again, the starting material in a chemical reaction. Over here on the right-hand side, what we have is the ending material, which is the products. So, every chemical reaction is going to have reactants and a product. The reactant is always found at the very beginning of a chemical reaction arrow, and the product is found at the very end of a chemical reaction arrow. We'll be able to talk about different types of chemical reactions as we move forward in our course, but for now, this concludes our introduction to chemical reactions as well as the difference between reactants and products, and we'll be able to get practice as we move forward through our course. So, I'll see you all in our next video.

Types of Chemical Reactions

In this video, we're going to introduce 2 different types of chemical reactions. Chemical reactions are categorized into 2 groups based on their energy requirement. These 2 groups are listed down below. We also have 2 images to show these different groups of chemical reactions. The first group is endergonic reactions, which require an input of energy. You can think of "en" in endergonic reactions as for the "en" in entering the reaction because energy needs to enter the reaction for endergonic reactions to occur. Just like this person here is entering into the room, you can think that endergonic reactions require energy to enter the reaction. You can see the little symbol here on his shirt represents energy, and so the person coming into the room represents energy entering the reaction.

The second type of reaction that you all should know are exergonic reactions, which are practically the opposite of endergonic reactions. Instead of requiring an input of energy, they release energy into the environment. Exergonic reactions allow energy to exit the reaction. You can think the "ex" in exergonic reaction is for the "ex" in exit the reaction. It's just like this person here is exiting the room through this door right here. You can see the little energy symbol on his shirt, so that he's representing energy and he's exiting the room.

Let's take a look at our example down below to better understand the difference between endergonic and exergonic reactions. Notice that our image is broken up into two halves. On the left-hand side, we are showing you the endergonic larger and more structured molecules. When you take a look at our image down below, notice that it's showing the building blocks, the broken down building blocks on the left-hand side as the reactants, the starting material, or the ingredients for the reaction. By the end of the reaction, notice that those starting materials have been built up into a larger, more complex structure here that is more organized. This would be the product over here, and there is some building occurring here in this endergonic reaction. Because it's an endergonic reaction, you can see that energy has to enter this system. You can see the entering person here and the energy coming into the chemical reaction.

If we take a look at the little graph that we have down below, notice that it has a y-axis that has potential energy increasing from the bottom to the top and it also has the progress of the reaction on the bottom. Notice that we start off with the reactants over here on the left-hand side and the reactants have lower energy, here in comparison to the products over here, which notice they have higher energy since it's a higher bar. So they have higher energy. The reason the products have higher energy is that energy is entering the system here. It's entering into the product. Energy is required for endergonic reactions and for building up larger molecules.

On the right-hand side over here, we're showing you the complete opposite. We're showing you exergonic reactions, and exergonic reactions are going to be used to break down substances into their smaller components. Notice that this time we're starting the reaction with reactants that are larger, more complex, and built up. Then notice that by the end of the reaction, the molecules are being broken down into their smaller individual components. In this exergonic reaction, notice that energy is actually leaving the system. It is exiting the system. You can think the "ex" in exergonic is for energy exiting the system.

When we take a look at the graph down below, notice that the reactants this time have higher energy than the products, which are over here. Notice the products over here, they have lower energy. Because the products have lower energy, it means that the energy is exiting the system. It's leaving the system and going into the environment. Energy is being released into the environment because there is this difference in energy here where the reactants are higher and the products are lower in energy. You can see how endergonic and exergonic reactions are practically the opposite of each other. The cell can utilize both endergonic and exergonic reactions, and we'll be able to talk even more about these reactions as we move forward through our course. But for now, this concludes our introduction to endergonic and exergonic reactions, and I'll see you all in our next video.

Which of the following statements is true for all exergonic reactions?

a) The products have more total energy than the reactants.

b) The reaction proceeds with a net loss of free energy.

c) The reaction goes only in a forward direction:all reactants will be converted to products.

d) A net input of energy from the surroundings is required for the reactions to proceed.

• A. The products have more total energy than the reactants.
• B. The reaction proceeds with a net loss of free energy.
• C. The reaction goes only in a forward direction:all reactants will be converted to products.
• D. A net input of energy from the surroundings is required for the reactions to proceed.

Here’s what students ask on this topic:

What are the main differences between reactants and products in a chemical reaction?

Reactants are the starting materials in a chemical reaction, often referred to as the ingredients. They undergo chemical changes during the reaction. Products, on the other hand, are the substances formed as a result of the reaction. They are found at the end of the reaction process. In a chemical equation, reactants are typically placed on the left side of the arrow, while products are on the right side. Understanding the distinction between reactants and products is crucial for analyzing and predicting the outcomes of chemical reactions.

What is the difference between endergonic and exergonic reactions?

Endergonic reactions require an input of energy to proceed, meaning energy must enter the system. These reactions build larger, more complex molecules from smaller ones. In contrast, exergonic reactions release energy into the environment, meaning energy exits the system. These reactions break down larger molecules into smaller components. The energy dynamics of these reactions are essential for understanding metabolic pathways and energy flow in biological systems.

How do endergonic and exergonic reactions relate to metabolic pathways?

Endergonic and exergonic reactions are fundamental to metabolic pathways. Endergonic reactions, which require energy input, are often involved in anabolic pathways that build complex molecules like proteins and nucleic acids. Exergonic reactions, which release energy, are typically part of catabolic pathways that break down molecules to release energy. The balance and regulation of these reactions are crucial for maintaining cellular energy homeostasis and overall metabolic function.

Why is energy required for endergonic reactions?

Energy is required for endergonic reactions because these reactions involve the formation of new chemical bonds to build larger, more complex molecules from smaller ones. This process increases the potential energy of the system, necessitating an input of energy to drive the reaction forward. Without this energy input, the reaction would not proceed, as the reactants would not have enough energy to overcome the activation energy barrier and form the products.

Can you provide an example of an exergonic reaction?

An example of an exergonic reaction is cellular respiration. In this process, glucose (C₆H₁₂O₆) is broken down into carbon dioxide (CO₂) and water (H₂O), releasing energy in the form of ATP (adenosine triphosphate). The overall reaction can be summarized as:

$C_6{H}_{12}{O}_6+6O_2\to 6{\mathrm{CO}}_2+6H_2{O}_{+}$

This reaction releases energy, making it exergonic.