May 9, 2017
Limiting reagent and theoretical yield
It’s a classic conundrum: We have five hot dogs and four hot dog buns. How many complete hot dogs can we make?
A reaction with five hot dogs and four hot dog buns reacting to give four complete hot dogs and one leftover hot dog. The hot dog buns are the limiting reagent, and the leftover single hot dog is the excess reagent. The four complete hot dogs are the theoretical yield.
Assuming the hot dogs and buns combine in a one-to-one ratio, we will be limited by the number of hot dog buns we have since we will run out of buns first. In this less than ideal situation, we would call the hot dog buns the limiting reagent or limiting reactant.
In a chemical reaction, the limiting reagent is the reactant that determines how much of the products are made. The other reactants are sometimes referred to as being in excess, since there will be some leftover after the limiting reagent is completely used up. The maximum amount of product that can be produced is called the theoretical yield. In the case of the hot dogs and hot dog buns, our theoretical yield is four complete hot dogs, since we have four hot dog buns. Enough about hot dogs, though! In the following example we will identify the limiting reagent and calculate the theoretical yield for an actual chemical reaction.
Problem solving tip: The first and most important step for any stoichiometric calculation—such as finding the limiting reagent or theoretical yield—is to start with a balanced reaction! Since our calculations use ratios based on the stoichiometric coefficients, our answers will be incorrect if the stoichiometric coefficients are not right.
How to Calculate Percent Yield in Chemistry ?
1.Start with a balanced chemical equation. A chemical equation describes the reactants (on the left side) reacting to form products (on the right side). Some problems will give you this equation, while others ask you to write it out yourself. Since atoms are not created or destroyed during a chemical reaction, each element should have the same number of atoms on the left and right side.[1]
For example, oxygen and glucose can react to form carbon dioxide and oxygen: →
Each side has exactly 6 carbon (C) atoms, 12 hydrogen (H) atoms, and 18 oxygen (O) atoms. The equation is balanced.
Read this guide if you are asked to balance an equation yourself
2.Calculate the molar mass of each reactant. Look up the molar mass of each atom in the compound, then add them together to find the molar mass of that compound. Do this for a single molecule of the compound.
For example, one molecule of oxygen () contains two oxygen atoms.
Oxygn's molar mass is about 16 g/mol. (You can find a more precise value on a periodic table.)
2 oxygen atoms x 16 g/mol per atom = 32 g/mol of .
The other reactant, glucose () has a molar mass of (6 atoms C x 12 g C/mol) + (12 atoms H x 1 g H/mol) + (6 atoms O x 16 g O/mol) = 180 g/mol.
3.Convert the amount of each reactant from grams to moles. Now it's time to look at the specific experiment you are studying. Write down the amounts of each reactant in grams. Divide this value by that compound's molar mass to convert the amount to moles.[2]
For example, say you started with 40 grams of oxygen and 25 grams of glucose.
40 g / (32 g/mol) = 1.25 moles of oxygen.
25g / (180 g/mol) = about 0.139 moles of glucose.
Image titled Calculate Percent Yield in Chemistry Step 4
4.Find the ratio of your reactions. Remember, a mole is just a large number that chemists use to "count" molecules. You now know how many molecules of each reactant you started with. Divide the moles of one reactant with the moles of the other to find the ratio of the two molecules.
You started with 1.25 moles of oxygen and 0.139 moles of glucose. The ratio of oxygen to glucose molecules is 1.25 / 0.139 = 9.0. This means you started with 9 molecules of oxygen for every 1 molecule of glucose.
5.Find the ideal ratio for the reaction. Go back to the balanced equation you wrote down earlier. This balanced equation tells you the ideal ratio of molecules: if you use this ratio, both reactants will be used up at the same time.
The left side of the equation is . The coefficients tell you there are 6 oxygen molecules and 1 glucose molecule. The ideal ratio for this reaction is 6 oxygen / 1 glucose = 6.0.
Make sure you list the reactants in the same order you did for the other ratio. If you use oxygen/glucose for one and glucose/oxygen for the other, your next result will be wrong.
6.Compare the ratios to find the limiting reactant. In a chemical reaction, one of the reactants gets used up before the others. This limiting reactant determines how long the chemical reaction can take place. Compare the two ratios you calculated to identify the limiting reactant:[3]
If the actual ratio is greater than the ideal ratio, then you have more of the top reactant than you need. The bottom reactant in the ratio is the limiting reactant.
If the actual ratio is smaller than the ideal ratio, you don't have enough of the top reactant, so it is the limiting reactant.
In the example above, the actual ratio of oxygen/glucose (9.0) is greater than the ideal ratio (6.0). The bottom reactant, glucose, must be the limiting reactant.
Summary
The limiting reagent is the reactant that gets used up first during the reaction and also determines how much product can be made. We can find the limiting reagent using the stoichiometric ratios from the balanced chemical reaction along with one of the many nifty methods in Example 1.
Once we know the limiting reagent, we can calculate the maximum amount of product possible, which is called the theoretical yield. Since the actual amount of product is often less than the theoretical yield, chemists also calculate the percent yield using the ratio between the experimental and theoretical yield.
What do you mean limiting reagent?
ReplyDeleteThe limiting reagent is the reactant that is completely used up in a reaction, and thus determines when the reaction stops. From the reaction stoichiometry, the exact amount of reactant needed to react with another element can be calculated. If the reactants are not mixed in the correct stoichiometric proportions (as indicated by the balanced chemical equation), then one of the reactants will be entirely consumed while another will be left over. The limiting reagent is the one that is totally consumed; it limits the reaction from continuing because there is none left to react with the in-excess reactant.
DeleteAt what point, the concept of reagents can be applied?
ReplyDeleteIn reactions involving more than one reactant, one has to identify first, of all the reactant,
DeleteWhat cause can make teorytical result and practice result are different when we calculate rendement ?
ReplyDeletehi al thanks for your question , Experimental error is unavoidable during the conduct of any experiment, mainly because of the falsifiability principle of the scientific method.
DeleteScientific experiments involve a different type of error analysis than a statistical experiment. In science, experimental errors may be caused due to human inaccuracies like a wrong experimental setup in a science experiment or choosing the wrong set of people for a social experiment.
Systematic error refers to that error which is inherent in the system of experimentation. For example, if you want to calculate the value of acceleration due to gravity by swinging a pendulum, then your result will invariably be affected by air resistance, friction at the point of suspension and finite mass of the thread.
Random errors occur because it is impossible to practically achieve infinite precision. Since the value is higher or lower in a random fashion, averaging several readings will reduce random errors.
how to Compare the two ratios you calculated to identify the limiting reactant
ReplyDeletein this article already explain about that
Deleteread carefully and you will get the answer
thank you ^^