Lesson A is going to be on spontaneous reactions, which is an honors topic. In this lesson, we'll talk about what our spontaneous processes will go over the difference between reversible and irreversible chemical reactions, and we'll talk about the contributions of entropy and enthalpy. So to begin, a spontaneous process is any of those that can occur without outside stimulus, which means that they can occur without a scientist or some other sort of outside force causing the reaction to occur. For example, that would be like lighting paper on fire. Normally, when we talk about spontaneous processes, we think of diffusion and effusion. As you spontaneously make the molecules move from one chamber into the other, as we say, we eventually will get into a equilibrium. But we'll have to understand that sometimes when a spontaneous reaction occurs, the reverse of it is non-spontaneous. For example, if you look in the picture, it would be not spontaneous, meaning you need to put energy into it if you wanted to get the bottom jars of gas completely evacuated on one side. So when we talk about spontaneous processes or chemical reactions, when you are spontaneous in one direction, this means that you are non-spontaneous in the reverse. So if we think about this picture, we have iron nails in the bottom left, and we also have iron nails that have been rusted, which are going to be the rusty red objects we say. So for the iron to rust, it is a spontaneous reaction because this occurs naturally. But for the reverse, for that rusty iron nail to go back into the original form, that would be a non-spontaneous reaction because this is not a curve without outside stimulus. So when we think about this, when a regular iron nail rusts, that is a spontaneous reaction. But no rusty nails ever spontaneously reformed themselves into pure iron samples. So temperature also has a major effect on determining if reactions can be spontaneous or non-spontaneous. When we think about water, we know that above zero degrees Celsius, it's spontaneous for ice to melt. Therefore, the reverse is, if you're below zero degrees Celsius, it is spontaneous for water to freeze. Water has a major effect on whether or not a reaction will take place spontaneously or will require energy to spontaneously react. So a reversible reaction is any type of chemical change that the system in surroundings can be put back into the original states by exactly reversing the process. Before in the earlier units we talked about, hydrated ionic compounds. If you take a hydrated ionic compound, for example, like copper sulfate, pentahydrate, which is a blue crystal, and you endothermically add heat to it, which means that you're going to be evaporating it, you can actually remove the water out of the crystal lattice structure of the anhydrous copper sulfate. And the opposite can occur. You can add water to the anhydrous copper sulfate, and you'll go back into your original blue crystals. The fact that you can go back and forth with the hydration process means that the reaction is reversible. However, irreversible reactions cannot be undone by exactly reversing the changes to the system. So all spontaneous reactions are irreversible. For example, if you were to blow up anything like gasoline or any type of fuel, you can't reverse that reaction. So some contributions to spontaneuity are changes in enthalpy, which we know is whether or not heat is being absorbed or released. We also understand that a change in entropy, which is a measure of randomness, will also contribute to whether or not a reaction becomes spontaneous. Also temperature, kinetic energy provides motion of the particles, which in turn allows chemicals and different compounds to react with one another. So many spontaneous reactions are exothermic, which means they release heat. That means you're going to have a negative delta H and a negative delta T. Endothermic reactions are non-spontaneous at room temperatures. This means they require more heat or energy, which makes them at a higher temperature, to make them become spontaneous. So some reactions will never occur in a cold environment, but once you warm up that sample, then the reaction can spontaneously occur. So if we think of that, limestone is a common basic type of building material. Limestone is also called calcium carbonate, and calcium carbonate can decompose into calcium oxide, which is a solid, and also releases carbon dioxide gas. This process is endothermic and requires about 17,000 kilojoules for this to reaction to occur. However, limestone only breaks down at 1100 Kelvin, which is an incredibly hot temperature. This means that unless the temperature is at 1100 Kelvin, that limestone will not naturally decompose on its own. So that would be called a non-spontaneous decomposition. But if you got that limestone sample to be super-heated by giving it all that energy that it requires since it's endothermic, then it will decompose. So when we think about entropy and how this relates into spontaneative reactions, we have to take into effect what the different laws of thermodynamics are. The first thermodynamic states that energy in the universe is constant, therefore energy cannot be created or in order to destroy it. The second law of thermodynamic states that randomness or disorder in the universe is constantly increasing. Whenever you have a solid sample, it will always, always, always break down into smaller and smaller pieces, because randomness is something that should always be increasing in the universe. So one way to increase chaos on a molecular level is by adding more particles into a reaction. More particles means more collisions, which means more random motion. If you add more energy, which is heat, then therefore your particles will be colliding with each other at a faster rate. And if we increase the volume, particles are allowed to move more. Therefore they can range and they can spread out into farther distances. We also understand that entropy increases with the freedom of motion in the molecules. Therefore, the sign of entropy being s of a gas is the greatest, while the entropy of a liquid is in between, and the entropy of a solid is going to be the lowest. We also understand that when you dissolve any type of ionic compound into water and it dissolves, the entropy of the ions are going to be increasing. Why? Because when the sample is being broken down into now two different ionic compounds, they are also freely floating in a 3D space of water. The last law of thermodynamic states that it absolute zero, which is zero degrees Kelvin, the coldest possible temperature. Any pure substance exists as a perfect crystal with zero motion. Therefore the entropy of a pure crystal in substance at absolute zero is what we refer to as zero. Therefore there will be no motion.