Chemical Reactions - Online Tutor, Practice Problems & Exam Prep

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 energy dynamics in biochemical processes, as they illustrate how energy is utilized and released during reactions.

Chemical Reactions

In this video, we're going to begin our lesson on chemical reactions. And so, chemical reactions consist of the making and/or breaking of chemical bonds, leading to changes in matter, and every single chemical reaction has reactants and products. Now reactants are going to be the starting material in a chemical reaction, and so you can think of the reactants as the ingredients for the reaction. And then, of course, the products are going to be the ending material in a reaction. And so let's take a look at our image down below, which notice is showing 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. And on the right, notice that the building blocks are coming together to build a larger, more complex structure. And so the beginning of every chemical reaction is going to start off with reactants. And so reactants are going to be once again the starting material in a chemical reaction. And over here, on the right-hand side, what we have is the ending material.

And so the ending material is going to be the products. And so every chemical reaction is going to have reactants and a product, and the reactant is always going to be found at the very beginning of a chemical reaction arrow and the product is going to be found at the very end of a chemical reaction arrow. And so 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 going to be categorized into 2 groups based on their energy requirements. These 2 groups are listed below, and we also have 2 images to show the different groups of chemical reactions. The very first group is endergonic reactions, which require an input of energy. You can think of the "en" in endergonic as for "entering" the reaction because energy needs to enter the reaction for endergonic reactions to occur.

Just like this person here is entering the room, you can think that endergonic reactions require energy to enter the reaction. You can see a little symbol on his shirt representing energy; this 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, allowing energy to exit the reaction.

Think of the "EX" in exergonic for "exit" the reaction. It's just like this person here is exiting the room through this door. You can see the little energy symbol on his shirt, showing that he's representing energy and he's exiting the room. Let's take a look at our example below to better understand the difference between endergonic and exergonic reactions. Notice that our image is broken up into 2 halves. On the left-hand side, we're showing the endergonic reactions, which are building up larger and more structured molecules.

Notice in our image that it shows 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, those starting materials have been built up into a larger, more complex structure here that is more organized. This would be the product, 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 below, this plot shows a y-axis with potential energy increasing from the bottom to the top, and it also has the progress of the reaction at the bottom. Notice that we start off with the reactants on the left-hand side, which have lower energy compared to the products on the right-hand side, which have higher energy. The reason the products have higher energy is that energy is entering the system here, entering into the product, and energy is required for endergonic reactions and for building up larger molecules.

On the right-hand side, we're showing you the complete opposite. We're demonstrating exergonic reactions, which are used to break down substances into their smaller components. This time, we're starting the reaction with reactants that are larger, more complex, and built up, and then, 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.

So, you can think the "EX" in exergonic is for energy exiting the system. When we look at the graph below, notice that the reactants this time have higher energy than the products, which 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.

Here’s what students ask on this topic:

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

In a chemical reaction, reactants are the starting materials that undergo a transformation, while products are the substances formed as a result of the reaction. Reactants are found at the beginning of the reaction arrow, and products are found at the end. For example, in the reaction A + B → C + D, A and B are the reactants, and C and D are the products. 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 enters 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. For example, photosynthesis is an endergonic reaction, while cellular respiration is an exergonic reaction. Understanding these types of reactions is essential for grasping energy dynamics in biochemical processes.

How do endergonic and exergonic reactions relate to energy changes in a system?

Endergonic reactions involve the absorption of energy, resulting in products with higher energy than the reactants. This energy input is necessary to build complex molecules. Conversely, exergonic reactions release energy, resulting in products with lower energy than the reactants. This energy release occurs as larger molecules are broken down into simpler ones. The energy changes in these reactions are crucial for understanding how cells manage energy for various biochemical processes.

Can you provide an example of an endergonic reaction and an exergonic reaction?

An example of an endergonic reaction is photosynthesis, where plants absorb sunlight (energy) to convert carbon dioxide and water into glucose and oxygen. The overall reaction is: 6CO₂ + 6H₂O + energy → C₆H₁₂O₆ + 6O₂. An example of an exergonic reaction is cellular respiration, where glucose is broken down to release energy, carbon dioxide, and water: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy. These examples illustrate how energy is either absorbed or released during chemical reactions.

Why is it important to understand the concepts of reactants and products in chemical reactions?

Understanding reactants and products is fundamental for analyzing chemical reactions. It allows you to predict the outcome of reactions, balance chemical equations, and understand the transformation of matter. This knowledge is crucial in fields like biochemistry, pharmacology, and environmental science, where chemical reactions play a vital role. For instance, knowing the reactants and products in metabolic pathways helps in understanding how cells generate energy and synthesize essential molecules.