How to Draw & Label Enthalpy Diagrams

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  • 0:04 What Is an Enthalpy Diagram?
  • 0:52 Examining the Diagram
  • 1:50 Drawing From a Chain Reaction
  • 3:13 Diagram for…
  • 3:51 Lesson Summary
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Lesson Transcript
Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

An enthalpy diagram allows us to easily see details of a chemical reaction. By knowing how to draw and label an enthalpy diagram we can see what the starting energy level is, how much energy is needed to get the reaction going, and what energy is left at the end of a chemical reaction.

What Is an Enthalpy Diagram?

When we burn gasoline in an engine, we know that there is a transfer of energy that allows our vehicle to begin taking us to our destination. The chemical reaction between the spark of the ignition and the gasoline is a chemical reaction. During any chemical reaction, whether it be in a laboratory or unseen reaction occurring in your body or around you in nature, there is a change of energy. We may not see the energy, but anytime there is a chemical reaction, there is a transfer of energy.

The energy levels are tracked using an enthalpy diagram. An enthalpy diagram plots information about a chemical reaction such as the starting energy level, how much energy needs to be added to activate the reaction, and the ending energy. An enthalpy diagram is graphed with the enthalpy on the y-axis and the time, or reaction progress, on the x-axis.

Examining the Diagram

Let's look at the elements of this enthalpy diagram. First, as noted, the y-axis is labeled 'enthalpy' and the x-axis is labeled 'reaction progress.' Then we have the actual energy diagram plot. Typically the energy first goes up, and this is the activation energy or, as abbreviated, E a . Then it comes back down. The ending energy, delta H, could be lower than the starting or higher than the starting energy, this depends on the type of reaction that takes place which we will explain further later.

A complete enthalpy diagram will include starting energy, ending energy, and E a and delta H.

This enthalpy diagram has starting products, ending products, delta H, and activation energy labeled
Enthalpy labeled

There are two different types of energy changes in reactions, endothermic and exothermic. Endothermic is when delta H is positive; energy needs to be added overall for the reaction to occur, so the ending energy is higher than the starting energy. Exothermic is when delta H is negative; energy is removed overall, so the ending energy is lower than the starting energy.

In this endothermic diagram, the products have a higher enthalpy than the reactants
Endothermic diagram

In this exothermic diagram, the products have a lower enthalpy than the reactants
Basic enthalpy diagram

Drawing From a Chain Reaction

In drawing an enthalpy diagram we typically start out with the simplest part first, the change in energy. Let's say that we're looking at the chemical reaction of methane and oxygen burning into carbon dioxide and water, as you can see here:

Methane burning reaction

The delta H for this reaction is -890 kJ. Typically we start the reaction at an enthalpy of 0 kJ. So the starting material is at 0 kJ:

We start the diagram at 0 kJ
Starting energy enthalpy diagram

Then we draw the ending energy at -890 kJ. When we draw a line between these two points we get the delta H:

The products are drawn at -890 kJ which is equal to delta H
Enthalpy diagram ending

But how do we put in the activation energy? As this energy progresses there is a point where methane needs to share some of its hydrogens with oxygen. This is a high energy state. It isn't an actual intermediate (like in some reactions) because it all occurs simultaneously. But there is still a moment when the products are in a state of high energy.

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