Have you ever noticed that hot dogs are sold in packages of 10, whilethe buns come in packages of eight? In this scenario, the buns are thelimiting reactant in the sense that they limit the hot dog preparationto eight. The limiting reactant or reagent is the one that is consumedfirst in the chemical reaction, and its consumption halts the pro
gressof the forward reaction.
When answering questions about limitingreagents on the exam, your first step should always be to convert allthe masses you were given into moles. You should set up your table asyou did before, only now you’ll have
two amounts and thus two numbers of moles to get you started.
Let’s look at a specific question,involving the Haber process. Basically, this is the process of makingammonia from the reaction of nitrogen and hydrogen gases. The reactionis shown below:
| Molar mass | (28.02) | (2.02) | (17.04) |
| Balanced equation | N2 + | 3H2  | 2NH3 |
| No. of moles |
|
|
|
| Amount |
|
|
|
无忧
托福社区门户 k*|)j0`:N H"_[B Suppose you have a total of 25.0 kg of nitrogen to react with a totalof 5.00 kg of hydrogen. What mass of ammonia can be produced? Whichreactant is the limiting reactant? What is the mass of the reactantthat’s in excess? Insert the masses in the correct rows and find thenumber of moles of
both.
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| Molar mass | (28.02) | (2.02) | (17.04) |
| Balanced equation | N2 + | 3H2  | 2NH3 |
| No. of moles | 892 mol | 2475 mol |
|
| Amount | 25,000 g | 5000 g |
|
Start with nitrogen. You have 892 moles ofit available, and in order for the nitrogen to react completely withhydrogen, you’d need 3(892 mol) = 2676 moles of hydrogen, which youdon’t have. Therefore, hydrogen is the limiting reagent. Now let’sanswer the other parts of the question. The mass of ammonia that can beproduced is limited by the amount of hydrogen, so do your calculationsbased on the number of moles of hydrogen available. Your chart shouldlook like the one below:
| Molar mass | (14.02) | (2.02) | (17.04) |
| Balanced equation | N2 + | 3H2  | 2NH3 |
| No. of moles | 825 mol used | 2475 mol used | therefore 1650 mol produced |
| Amount | 892 mol  23,117 g used 25,000g  | 5000 g | 1650 mol (17.04) = 28,116 g produced |
