There are two important concepts that will help us to understand the reaction yield. First, we need to understand that there is an equilibrium constant and that not all reactions will necessarily go to completion. Sometimes the equlibrium consists of some amount of reactantsand products coexisting. In favorable cases where the equilibrium constant is large (which is actually quite common) we can expect that products will be formed to such a great extent that we will not detect the reactants at all. However, this dicussion so far assumes that the reacting species are all present in precisely the correct stoichiometric ratios to react with out any leftover material. If there is an excess of one reagent then that may determine the reaction yield (as measured relative to the initial mass of the reacting species). IN such cases we need to determine a limiting reagent. The question is which reagent is present in less than the stoichiometric ratio. This reactant will determine the maximum yield of the products. At this stage in the course we are not yet studying the equilibrium cosntant so we will assume that the processes are 100% reacting to rpdocuts. We will instead forcus on the limiting reagent.
We start with an example of application of limiting reagent that has no real practical value, but is an amusing project. Bottle rockets are relatively safe home made projectives that use CO2 as the propellent. In this example problem the amount of propelletn is limited by one of the reactants.
Photosynthesis is an example of a reaction that occurs in nature and uses two natural inputs, H2O and CO2. These reagents are inherently present in vastly different amounts in nature. Thus, one of them will be limiting for the overall process. Of course, in life there are other possible limiting reagents that we will not consider here. For example, Fe is often limiting for growth since iron hydoxides are not very soluble. Therefore, Fe is not abundant in many aquatic environments.
Nitrogen is a limiting reagent in the soil. WIthout sufficient nitrogen plants cannot grow and agriculture as we know it would cease. The nitrogen that we use as fertilizer comes mainly from a process for artificial nitrogen fixation known as the Baher process. This process was invented over 100 years ago as the soltion to a critical problem that was limiting for development of agriculture. While the social, economic and enivronmental aspects of a limiting reagent are beyond the scope of a chemistry class it is worth understanding that the concept has a connection. Here, we consider the nature of the Haber process itself, which is the production of ammonia at high temperature and pressure.
The carbon cycle has both organic (photosynthesis) and inorganic mechanisms of carbon capture. The inorganic mechanism involves formation of limestone in the oceans. This is a slow process, but over the billions of years of the earth's history the original CO2 atmosphere was consumed and replaced by an oxidizing atmosphere. Where did the CO2 go? Scientists believe that it was taken up by the oceans and ended up as CaCO3, which is limestone. In this segment we consider the chemistry for that conversion (or some of the concisderations of that chemistry) in a quantitative way.
Today we use limestone as the basis for the cement industry. Cement is created by driving off the CO2 using heat. The following segment considers the quantitative aspects of this problem. It is a surprisingly important part of our economy since cement is used extensively for construction. It is also an important energy use.
The final consideration is a retrospective look at how attitudes toward safety in the chemistry laboratory have changed. The pratices described in this video (that I experienced as a student) would be illegal today. It is also a case of a limiting reagent, but in a reaction that you would not want to run outside of a fume hood.
