Enthalpy: Energy Transfer in Physical and Chemical Processes

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  • 0:01 Enthalpy Change
  • 1:17 Energy, Heat and Termperature
  • 3:40 Exothermic and…
  • 4:55 Enthalpy Change Revisited
  • 7:35 Lesson Summary
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Lesson Transcript
Instructor: Nicola McDougal

Nicky has taught a variety of chemistry courses at college level. Nicky has a PhD in Physical Chemistry.

This video explores the relationship between chemistry and energy. We learn the general properties of energy and the concepts of temperature and heat. We will learn about energy flow and consider the enthalpy change during chemical reactions.

Enthalpy Change

Let us consider two common processes, a plant photosynthesizing and a candle burning. Both are chemical reactions, and so there is a change of reactants into products. Both processes also involve a change in energy. A change in energy accompanies every chemical reaction. Enthalpy change is the measure of energy change during a chemical reaction. So let us look at each process and see if we can see what is going on.

First the burning candle - the wax is reacting with oxygen, forming a flame and producing carbon dioxide and water. If you put your hand close to the flame, you will burn yourself because it is hot. The flame is giving off heat.

On the other hand, the plant is just sitting there absorbing the UV rays from the sun. This time the heat is coming from the sun and going into the leaf.

So can you tell what is different about these two processes in terms of energy or heat? Don't worry if you can't quite explain it. By the end of this lesson, you will be able to. This lesson is all about heat and energy transfer during chemical reactions.

Energy, Heat and Temperature

Energy is at the center of everything we do, but it is actually really difficult to precisely define it. I will simply define energy as the ability to do work or produce heat.

An important type of energy is potential energy. Potential energy is energy due to position or composition. Potential energy is present in the chemical bonds of molecules.

One of the most important characteristics of energy is that it is conserved. The Law of Conservation of Energy states that energy can neither be created nor destroyed. It is converted from one form to another. This law is also known as the First Law of Thermodynamics.

If we return to the candle and the plant, we can illustrate this. For the candle, the potential energy is contained in the candle wax, and the oxygen is released. It is then transferred into the potential energy of the products. The extra energy left over is transferred out as heat.

For the plant, the energy from both the sun's UV rays and carbon dioxide are transferred into the products, which are oxygen and glucose. This time, though, there is no excess energy. In fact, there is a deficit in energy, so heat from the sun is absorbed by the plant to make up this deficit. Heat transfers into the plant. For both reactions, energy is transferred from one form to another.

Let us now consider two very important concepts that often get confused: heat and temperature. Temperature is a measure of the random motion of particles. We can easily measure temperature by using a thermometer. The air around the candle flame is a much higher temperature than further away. Heat is the transfer of energy between two objects due to temperature difference. It is important to know that the transfer of heat is always from hot to cold and never the other way around. In both reactions, heat is either being transferred in or transferred out.

Exothermic and Endothermic Processes

Now we are going to consider the direction of heat transfer during a chemical reaction. To really focus in on a specific chemical reaction, we have to divide the universe into two. The system is the part of the universe we are interested in. The surroundings include everything else in the universe.

For a candle burning, the reactants and products are the system. The surroundings are the air in the room and anything else. When a reaction results in heat being given out, it is called exothermic. Energy, in the form of heat, flows out of the system. The candle is burning, and energy flows out of the system as heat.

Reactions that absorb heat energy from surroundings are called endothermic. Heat flows into the system. Our plant is a great example of an endothermic process; it is absorbing heat from the sun. It is important to realize that the transfer of heat into or out of the system equals the amount of heat lost or gained in the surroundings. Energy must be conserved.

Enthalpy Change Revisited

So far, we have learned that a change in energy accompanies every chemical reaction. We have also seen that reactions can give out heat or take in heat. But how do we measure change in energy of a reaction? This is where enthalpy change comes in. With our new knowledge, you can think of enthalpy change as the amount of heat absorbed or released by a chemical reaction. Let's look at the enthalpy changes for our two examples.

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