(c) How many grams of Al are left over in part b? Consider the reaction:ĭetermine the limiting reagent and the theoretical yield of the product if one starts with: The second technique will make its first appearance in Example #6.Įxample #1: Here's a nice limiting reagent problem we will use for discussion. The first technique is discussed as part of the solution to the first example. There are two techniques for determine the limiting reagent in chemical problems. So, which "reactant" is limiting and which is in excess? The test tubes are limiting (they ran out first) and the stoppers are in excess (we have some left over when the limiting reagent ran out). And we also have 20 test tubes with stoppers firmly inserted. So when the test tubes are used up, we have 10 stoppers sitting there unused. Seems obvious, doesn't it? We had 20 test tubes, but we had 30 stoppers. ![]() Suddenly, we run out of one of the "reactants." Which one? We run out of test tubes first. Step by step we use up stoppers and test tubes (the amounts go down) and make stoppered test tubes (the amount goes up). So now we let them "react." The first stopper goes in, the second goes in and so on. ![]() Test tube plus stopper gives stoppered test tube. It seems to be a simple concept, but it does cause people problems. It is simply the substance in a chemical reaction that runs out first. Second comment before starting: What is the Limiting Reagent? This is a part of many limiting reagent problems and it causes difficult with students. To answer this problem, a subtraction will be involved. of B react, how much of the excess compound remains. Obviously (I hope), the other compound is seen to be in excess. Which is in excess? What we will do below is find out which substance runs out first (called the limiting reagent). ![]() (b) 20 grams of A and 20 grams of B react. Since it is "in excess," this means there is more than enough of it. In this case, mentally set compound B aside for the moment. (a) Compound A reacts with an excess of compound B. You will see the word "excess" used in this section and in the problems. Figuring out which substance is the limiting reagent is an area that many students struggle with. I certainly hope it is something you pay attention to and remember. Just a bit below, I'm going to tell you (several times) how to determine the limiting reagent in a chemistry problem. Limiting Reagent Examples Limiting Reagent Problems #1-10 Problems solved using dimensional analysis only Limiting Reagent Problems #11-20 Stoichiometry Menu ChemTeam: Stoichiometry: Limiting Reagent Examples
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