Brief Summary
This video explains the concept of limiting reagents in chemical reactions, emphasizing how one reactant limits the reaction based on stoichiometric calculations. It also discusses the concept of theoretical and actual yields and how to calculate the percent yield of a reaction.
- The limiting reagent is the substance that runs out first and stops the reaction.
- Percent yield is calculated as actual yield divided by theoretical yield, multiplied by 100.
Professor Dave introduces the topic of limiting reagents, explaining that in chemical reactions, it is rare to have reactants in the exact amounts required for complete reactions. One reagent will be in excess while the other will be the limiting reagent, which is the one that will run out first, halting the reaction.
When determining which reactant is limiting, it isn’t always the one present in the smaller amount. For example, in the analogy of making bologna sandwiches, having seven slices of bologna and ten slices of bread shows that bread is the limiting reagent as it is needed in larger quantities per sandwich.
In a practical example involving urea production, Dave uses mass to identify the limiting reagent. He converts the mass of each reactant into moles, pointing out that one reactant, carbon dioxide (CO2), cannot produce as much product as ammonia, thus CO2 is the limiting reagent.
The discussion moves to the concept of theoretical yield, which represents the expected amount of product if the reaction occurs perfectly. In reality, reactions yield less than the theoretical yield, leading to the calculation of percent yield as (actual yield/theoretical yield) × 100. For example, if 10 grams of product were expected but only 8.4 grams were obtained, the percent yield would be 84%.
Percent yield is a vital measurement of a chemical reaction's efficiency and proves invaluable in planning organic compound syntheses. Professor Dave wraps up by reinforcing the concept of limiting reagents and the significance of understanding yields in chemistry.
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Transcript
Full transcript of “Limiting Reagents and Percent Yield”. Click a timestamp to open the video at that moment. Need another video? Use the YouTube transcript generator.
0:00 professor dave here, let's talk about limiting reagents when we react two substances it's essentially impossible to have them present in precisely the correct amounts for both of them to react completely. one of them will be the reagent in excess and one of them will be the limiting reagent the limiting reagent is the one that runs out first which makes reacting further impossible. the other reagent or reagents will then be said to be in excess because there will be some left over in order to tell which is the limiting reagent we have to see which quantity will limit the reaction stoichiometrically and it won't automatically be the substance that is present in the lesser amount. for example
0:48 if we are making bologna sandwiches we might use the following equation: the balanced equation shows that we need one baloney and two bread to make one sandwich but say we have seven baloney and 10 bread, how many sandwiches can we make? 7 baloney could potentially make 7 sandwiches but 10 bread can only make five because we need two per sandwich so even though there's more bread than Bologna it is bread that is the limiting reagent in this example. the bread will react fully and leave an excess of two Bologna so baloney is in access let's look at a real reaction. urea is produced this way if we start with 10 grams of each reactant which one limits the reaction?
1:34 well the first thing we have to do is convert to moles because mass doesn't tell us anything we need numbers so we convert both to moles using their respective molar masses we can now look at these numbers a few different ways. one way is to see how many moles of urea each has the potential to produce. use the stoichiometric ratios to see that CO2 can't produce as much so that must be the limiting reagent you could also see how much of the other reactant each could react with this many moles of ammonia could react with this many moles of CO2 which we don't have this many moles of CO2 could react with this many moles of ammonia which we do have
2:21 so CO2 will react completely and limit the reaction once we identify the limiting reagent we just use the data for that substance to calculate how much product we will get since that substance will get used up if we used the other one we would get the wrong answer because not all of the reagent in excess will go towards generating product another real-life limitation of chemistry is that the amount of product we calculate that we should get is actually just the theoretical yield. this is the amount of product we can expect if every molecule of reactant converts flawlessly into product in reality this does not occur and the actual yield will always be some fraction of the theoretical yield which we call
3:07 the percent yield. percent yield is actual yield over theoretical yield times a hundred. if you expected 10 grams of product from your stoichiometric calculations but after doing the experiment only collected 8.4 grams of product your reaction had an eighty-four percent yield. percent yield is an important measure of the efficiency of a chemical reaction or technique and is very useful in planning syntheses organic compounds. let's check comprehension
4:06 thanks for watching guys subscribe to my channel for more tutorials and as always feel free to email me
