Entropy
Entropy (S) is described as a measure of the disorder or randomness of a system. It can also be described as a system's tendency to change from a lower energy state to a higher energy state. Therefore, if there is more disorder in the system, there is more entropy. What does it mean to increase a systems disorder? Take a moment to think about the three states of matter. How are the atoms arranged differently in a solid, a liquid, and a gas? In the image below, you can observe that there is an obvious difference in the arrangement of the atoms. Hover your mouse over each state of matter to learn more about each state of matter’s entropy.
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For a system, the change in entropy, ΔS, is measured by finding the difference between the entropy of the products and of the reactants. Does this sound familiar? You may remember this process when you calculated the change in enthalpy. Standard entropies are determined in the lab and published in standard entropy tables, just like standard enthalpies. In fact, both standard enthalpy and standard entropy are sometimes listed on that same table. View a printable version of the Standard Thermodynamic Values Table.
Calculating the Change in Entropy
Calculating the change of entropy in a reaction requires basic mathematical skills. Since the standard entropies are determined in a laboratory setting, you will just need to access a Standard Thermodynamic Values Table and then plug each of the entropy values into a standard equation. In this activity, click on each of the questions to learn how to calculate change of entropy in a reaction.
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It is possible to predict a change in entropy positive or negative based on the numbers and natures of the reactants and products. Generally speaking, a reaction resulting in fewer products, or less variety in products, will be negative. A reaction resulting in more products or more variety of products will be positive. Remember, a negative change in entropy goes from a state of high disorder to a state of low disorder. A positive change in entropy indicates the opposite or disorder.
In the interactivity above, the first reaction depicts two moles of hydrogen gas and one mole of oxygen gas combining to make two moles of water. You can tell this is an ordering reaction based on a few observable characteristics. There is less of a variety of products, fewer molecules of the products, and the products' state of matter is more organized, since it is a liquid instead of gas. The entropy change is negative, indicating a decrease in randomness. In the second reaction, the two moles of potassium chlorate are decomposed into two moles of potassium chloride and three moles of oxygen. This is definitely a disordering reaction for the exact opposite reasons. There is more of a variety of products, more moles of the products, and the products' state of matter is less organized. The entropy change is positive, indicating an increase in randomness.
Other Factors Affecting Entropy
There are a number of factors that affect the entropy of a system. Please review the list below to learn some of the other factors that affect entropy.
- When a liquid is formed from a solid, or a gas is formed from a liquid, the disorder increases.
- Entropy increases when a solid or liquid dissolves in water (the molecules have more possible positions).
- Entropy decreases when a gas dissolves in water (the molecules have less possible positions).
- Disorder increases when a solid or liquid dissolves in water.
- Larger molecules have more disorder and smaller, simpler molecules have less disorder.
Entropy Review
Now that you have learned about entropy, complete this non-graded activity to check your knowledge. In this activity, review each reaction and decide whether have a positive or negative change in entropy. Then, select the appropriate answer and click SUBMIT to check your response. Click on the interactivity thumbnail, and then click NEXT to get started.