Second Law of Thermodynamics - Online Tutor, Practice Problems & Exam Prep

The second law of thermodynamics states that energy conversions are never 100% efficient, as some energy is lost as heat, increasing universal entropy. Spontaneous processes, characterized by a negative delta G, occur without external energy and are typically catabolic, breaking down substances and increasing disorder. In contrast, non-spontaneous processes, with a positive delta G, require energy input and are anabolic, building larger substances and decreasing local entropy. Both types of processes contribute to an overall increase in universal entropy, highlighting the balance between local and universal changes in energy systems.

2nd Law of Thermodynamics

In this video, we're going to review the second law of thermodynamics. The second law of thermodynamics can be phrased and stated in many different ways to mean the same thing. It simply states that 100% efficient energy conversion or transferring all of the energy of one form into another form is impossible because thermal energy or heat is lost with every energy transfer. This heat that's lost increases the universal entropy or the entropy of the surroundings. The second law of thermodynamics also states that all spontaneous processes increase universal entropy as they proceed towards a state of equilibrium or minimal potential energy. We'll discuss more about equilibrium in some of our later videos.

Recall from our previous videos that, even though the universe is increasing in entropy, it doesn't mean that local entropy cannot decrease. Local entropy, or the entropy of a system we're focusing on, can decrease as long as it's offset by an increase in universal entropy. This is important to remember so that you understand that the second law of thermodynamics is not broken.

Let's consider an example of the second law of thermodynamics. On the left, we have a large amount of energy represented by this large arrow. Notice that at the point of energy conversion, not all of the energy is transferred. Therefore, 100% efficient energy conversion is not possible, and there's a smaller amount of energy transferred at the point of energy conversion, represented by this dotted line. The energy that is not transferred is lost in the form of heat. This heat that's lost, or this thermal energy that's lost, increases the universal entropy. All spontaneous processes release heat and increase the universal entropy. We'll talk more about this as we discuss our exergonic and endergonic processes in our next video. I'll see you guys then.

Exergonic vs. Endergonic Processes

So recall from your previous chemistry courses that spontaneous processes are exergonic processes with a negative ∆G value. All spontaneous processes occur without outside intervention, meaning that there is no need for an energy input for these processes to occur. Be careful not to confuse spontaneous with fast. Spontaneous does not mean fast, and there are spontaneous processes that occur very, very slowly. Again, recall that spontaneous just means that they will eventually occur without any outside intervention. Spontaneous processes are associated with catabolic processes, and catabolic processes break down materials. They take a single larger substance and break it down into multiple smaller substances, increasing the disorder and increasing the entropy. Because there is an increase in entropy, these are thermodynamically favorable processes.

Non-spontaneous processes are the complete opposite of spontaneous processes. Non-spontaneous processes, notice that they are endergonic; they have a positive ∆G value and require outside intervention, so they need some kind of energy input in order to proceed. Non-spontaneous processes are associated with anabolic processes or processes that build up materials, so they take multiple smaller substances and link them together to create a single larger substance, which decreases the local entropy. Because there is a decrease in the local entropy, these are thermodynamically unfavorable, so they are not favorable. As we mentioned several times in our previous videos, anabolic processes and non-spontaneous processes, although they decrease the local entropy, they are still accompanied by an overall increase in the universal entropy. Universal entropy is going to increase with both exergonic and endergonic processes.

Let's take a look at our example below to clear that up. We have the classic graphs for exergonic and endergonic reactions that you guys are very familiar with. On the left, we have our exergonic reactions and notice that the reactants have a higher free energy than the products, and they have a negative ∆G value. So the change in Gibbs free energy from reactants to products, there is a decrease in that free energy. As a result, there is energy that is released. Exergonic reactions release energy into the environment and have a negative ∆G value.

Now, over here with our endergonic reaction, notice that the reactants have lower energy than the products. Because of that, the products have more energy and there was an increase in the change in energy. There is a positive ∆G value, and that means that the energy was absorbed from the environment. You should also notice that we have these three other arrows here, double arrows, which represent essentially alternate Y-axis. Notice that the reactants up here, which have higher energy, are associated with an unstable system, low local entropy, and an ordered system, as are the products of the endergonic reaction. Then down here, the products of exergonic reactions which have lower energy, so lower energy is associated with a more stable system, higher local entropy, and more disordered system, as are the reactants of the endergonic reaction. This is how the associations occur.

With an exergonic process, notice that we are going from up high to down low. So for the exergonic process, we are going in this direction, an arrow from top to bottom. Exergonic processes increase the stability of the system, they increase the entropy, the local entropy, and they increase disorder. Now, notice that we are going from low-energy reactants to high-energy products for endergonic processes. So, we are going in the opposite direction, in this direction for endergonic processes. Endergonic processes create an unstable system with their products. They decrease the local entropy and increase the order of the system. Even though there is a decrease in the local entropy with endergonic reactions, we know that the overall universal entropy is still going to increase. Exergonic and endergonic processes are both still associated with an increase in the universal entropy. This is important to keep in mind as we move forward in our biochemistry course. This is a good summary of the differences between exergonic and endergonic processes, and I'll see you guys in our practice videos.

Which of the following statements is false?

• A. All endergonic reactions have a +ΔG.
• B. All endergonic reactions are catabolic.
• C. All exergonic reactions have a -ΔG.
• D. All exergonic reactions are spontaneous.
• E. All of the above are true.

Which of the following statements is true?

• A. Spontaneous reactions are fast reactions.
• B. Endergonic reactions are spontaneous.
• C. Endergonic reactions tend to decrease local entropy.
• D. All exergonic reactions decrease universal entropy.
• E. All of the above are false.

Here’s what students ask on this topic:

What is the second law of thermodynamics?

The second law of thermodynamics states that energy conversions are never 100% efficient because some energy is always lost as heat. This lost heat increases the universal entropy. In simpler terms, it means that every energy transfer or transformation increases the overall disorder of the universe. This law also implies that all spontaneous processes, which occur without external energy input, increase universal entropy as they move towards equilibrium or minimal potential energy.

How does the second law of thermodynamics relate to entropy?

The second law of thermodynamics is fundamentally about entropy, which is a measure of disorder or randomness. It states that in any energy transfer or transformation, some energy is lost as heat, increasing the universal entropy. This means that the total entropy of an isolated system can never decrease over time. While local entropy within a system can decrease, it must be offset by an increase in the entropy of the surroundings, ensuring that the universal entropy always increases.

What is the difference between spontaneous and non-spontaneous processes in thermodynamics?

Spontaneous processes occur without external energy input and are characterized by a negative ΔG (Gibbs free energy). These processes are typically exergonic, releasing energy and increasing entropy. Non-spontaneous processes, on the other hand, require external energy input and have a positive ΔG. These are usually endergonic processes, which absorb energy and decrease local entropy. Despite the local decrease in entropy, the universal entropy still increases in both types of processes.

Why are energy conversions never 100% efficient according to the second law of thermodynamics?

According to the second law of thermodynamics, energy conversions are never 100% efficient because some energy is always lost as heat during the process. This lost heat contributes to an increase in universal entropy. The inefficiency arises because it is impossible to convert all the energy from one form to another without some loss. This inherent inefficiency ensures that the total entropy of the universe always increases, aligning with the second law.

How do exergonic and endergonic reactions relate to the second law of thermodynamics?

Exergonic reactions are spontaneous processes with a negative ΔG, releasing energy and increasing entropy. These reactions align with the second law of thermodynamics as they contribute to the increase in universal entropy. Endergonic reactions, on the other hand, are non-spontaneous with a positive ΔG, requiring energy input and decreasing local entropy. Despite the local decrease, the overall universal entropy still increases, adhering to the second law. Both types of reactions illustrate the balance between local and universal changes in energy systems.