ammonia (g)
Hello, Sarah,
I tried to explain the details, so this is a little long. Sorry.
We'll start with writing, and balancing, the equation for this reaction. Nitrogen reacts with hydrogen to produce ammonia. Write the molecular formulas in the form of an equation:
N2 + H2 = NH3
Note that we write both the nitrogen and hydrogen as molecules of N2 and H2 since individual atoms of N and H are not normally found in nature. Now that we have the correct molecular formalus, we need to balance the atoms on both sides. We can see that we've written that 1 N2 molecule will make 1 NH3 molecule. But we have 2 N atoms in 1N2, and only 1 N atom on the product side. The equation is unbalanced. It would help to make it 2NH3, since that will balance the N atoms: 2 on each side.
N2 + H2 = 2NH3
The H atoms are unbalanced: 2 on the left and 6 (2*3) on the right. We're lucky - if we add a "3" in front of the H2 and the reactant side, it would be 6 H atoms on both sides:
N2 + 3H2 = 2NH3
Check to see if everything now balanced:
Reactants Products
N 2 2
H 6 6
The equation is balanced.
We want to know how many grams of ammonia are produced with 21.5 g H2 and excess N2 ("excess" means we'll get what we want when we need it - The Aladdin's Lamp of chemistry problems).
The balanced equation only tells us we'll get 2 NH3 molecules for every 3 molecules of H2. It does not say grams, only molecules. The concept of the mole was developed for just this conundrum: it relates the number of molecules, or atoms, to mass. The word mole, on it's own, only means Avogadro's number, 6.02x1023. If I have a mole of math problems, it means I have 6.02x1023 homework problems, nothing more. Put whatever noun you want after mole (e.g., one mole of: paperclips, presidents, dollars, fleas, etc)
But in science, the mole has a relationship with the atomic masses on the periodic table. It is a quite remarkable discovery. A mole of any element will produce a mass that is equal to that element's atomic mass. [What??].
Example: Gold, Au, has an atomic mass of 197 AMU (aomic mass units). Because of the specuial nature of Avogadro's number, we can also say that 197 grams of gold has 6.02x1023 gold atoms. All we had to do is replace AMU with a new unit, grams/mole (g/mol), also known as the "molar mass."
The same goes for molecules. Add the individual atomic masses for each individual atom (element) to determine the molar mass of that compound. For this reaction, we find:
N2 28 grams/mole (14 + 14)
H2 2 grams/mole (1 + 1)
NH3 17 grams/mole (14 + 1 + 1 + 1)
This unit is extremely useful in that it now allows us to count and measure grams on a laboratory scale in place of of counting individual molecules, a process that is not only impossible, but would also be extremely painful and slow. [Quick, count to 602,300,000,000,000,000,000,000. If you can count 1 number per second, it would take 1.91x1016 years]. But if we want one mole of N2 atoms, just measure 28 grams and you'll have it.
Sorry for the diversion. We can now interpret the balanced equation differently. Use the term mole istead of molecule:
"One moleculemole of nitrogen reacts with 3 moleculemoles of hydrogen to make 2 moleculemoles of ammonia,"
We will now use the mole to answer the question. Convert 21.5 grams of H2 into moles H2 by dividing the grams by hydrogen's molar mass of 2 grams/mole:
(21.5 grams H2)/(2 grams H2/mole H2) = 10.75 moles H2.
Yhe balanced equation says we'll get 2 moles ammonia for every 3 moles of hydrogen, a molar ratio of 2/3 or 0.666667, Thus, if we started with 10.75 moles of H2 , we would obtain
(10.75 moles H2 )*(0.66667 moles NH3/moles H2) = 7.17 moles NH3.
Now convert moles ammonia to grams ammonia by using the molar mass of ammonia, 17 grams/mole.
(7.17 moles NH3)*(17 grams/mole NH3) = 122 grams NH3
Bob
