In this video, we'll take a look at how to calculate the entropy of our system, which represents our chemical reaction. Here we're going to say that each substance has a standard molar entropy, represented by X0 associated with it. Now, these values will always be provided in some way. That's because there's so many compounds, so many substances, each with their own unique molar entropy. There's no way you can memorize all of them.
Now this is important. Unlike standard molar enthalpies, which number RΔH0 for substances in their natural state, we're going to say that your entropy, standard molar entropy, does not equal 0. It's always going to be a value greater than 0. Now here we have the entropy of reaction formula. Here it says that the change in the standard entropy of our reaction, which is our system, equals. So when we talk about standard entropy of our reaction, that's typically in units of joules per Kelvin.
Here we have Σ which is summation and this would be N times standard entropy of our products minus Σ mole standard entropy of our reactants. Now remember, Σ just stands for sum of. So we're taking all the products, lumping them together, taking all the reactants, lumping them together. And here represents the moles of substance. And then we're going to say here entropy with not equals your standard mode entropy of a substance, and this will be in joules over moles times K.
When we talk about standard conditions, remember that we're talking about a temperature of 25°C and a pressure of one atmosphere. So basically we're saying here that the entropy, standard entropy of our reaction is equal to products minus reactants. We're going to utilize this formula to help us solve for the entropy of our system in the following questions. So let's pay attention and see how we utilize this formula.