Stoichiometry, every chemistry student’s nightmare! But we got you!
Stoichiometry, every chemistry student’s nightmare! But we’ve got you! The episode reviews the mole concept and describes the meaning of coefficients in a balanced chemical equation (1:03). Using sandwiches as an analogy, it shows how to relate the amount of reactants to products (2:42) and uses the same concept for the synthesis of ammonia (5:08). With a second reactant, we recap the concept of limiting and excess reactants for the sandwich (5:50) as well as the production of ammonia (6:24). The episode closes with a recap of how to use molar mass to convert moles to grams (6:58).
Question:(8:26) Which day is the chemist’s favorite holiday - mole day?
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Hi and welcome to the APsolute Recap: Chemistry Edition. Today’s episode will recap Stoichiometry
Lets Zoom Out:
Unit 4 - Chemical Reactions
Topic - 4.5
Big idea - Scale, Proportion and Quantity
Stoichiometry - what a WEIRD word. And often a component of every chemistry student's nightmares. But, fear no more! You’ve got us! Here at the APsolute RecAP, we are here to help you and recap how we are MEASURING ELEMENTS - this is the literal translation of stoichiometry, in case you don’t know any ancient greek (and who actually does?!)
Let’s zoom in:
We’ve actually already started talking about the foundation of stoichiometry when introducing the mole concept in episode 12. But let’s recap: the mole is like a dozen. One mole of any substance contains 6.022 times 10 to the 23rd elementary units, like a dozen contains 12. So mole is pretty much an expression of how many particles, atoms, ions, molecules, … you have.
We also already know about the law of conservation of mass, which states that mass can neither be created nor destroyed. For our chemical reactions that means that each atom, each particle, that is a reactant also has to show up on the product side. At the same time, we have to stick with our chemical formulas. Therefore, to fulfill both, we use coefficients to balance chemical equations.
Coefficients can be interpreted in different ways: I can, for example, say: 2 diatomic molecules of hydrogen react with 1 diatomic molecule of oxygen to form 2 molecules of dihydrogen oxide - aka water. On the reactant side I have 4 hydrogen atoms, on the product side I have 4 hydrogen atoms. On the reactant side, I have 2 atoms of oxygen and there are two atoms of oxygen on the product side. I can make the same statement and replace it with moles: 2 moles of diatomic hydrogen react with 1 mole of diatomic oxygen to form 2 moles of water. Coefficients give us information about the relationship between the amounts of reactants and products. We can therefore use coefficients as conversion factors.
Let’s talk about this a bit more and use an analogy: sandwiches. I don’t know how you like your sandwich, but let’s keep it simple: 2 slices of bread, 3 slices of ham, 1 slice of cheese - really no need for lettuce or tomatoes. Let’s write this as a chemical equation:
2 bread + 3 ham + 1 cheese yields 1 sandwich.
Let’s say we have 4 slices of bread - how many sandwiches can you make? You might now say - simple TOO EASY - 2 sandwiches. But stoichiometry is not much more complicated! Let’s look at the steps that you automatically took in your head:
I have 4 slices of bread. I know that 1 sandwich needs 2 slices.
My conversion factor therefore is: 1 sandwich / 2 slices of bread.
If I set it up using dimensional analysis: 4 slices of bread x 1 sandwich over 2 slices of bread. The slices of bread are cancelling out, I am left with the unit “sandwich” - and doing the math, 2 of them. Easy - peasy! Dimensional analysis is A-MA-ZING! And if you have a hard time with setting up the conversion factor: Follow the principle “unknown over known”. If you have given slices of bread, the slices of bread are “known” from the balanced equation and the sandwiches are the “unknown”. what you are calculating. That will automatically cancel out the units.
Let’s build a chemistry sandwich: 1 mole of diatomic nitrogen reacts with 3 moles of diatomic hydrogen to form 2 moles of ammonia, NH3. If I now have 12 moles of hydrogen, how many moles of ammonia can I produce? My conversion factor according to the balanced chemical reaction is: 2 moles of ammonia for each 3 moles of hydrogen.
Therefore, 12 moles of hydrogen x 2 moles of ammonia over 3 moles of hydrogen equals 8 moles of ammonia. I can produce 8 moles ammonia. Yeehaw! Only downside: It is a toxic gas, so… please don’t eat it.
New scenario: What if I for my 2 bread, 3 ham, 1 cheese sandwich have: 4 slices of bread, 5 slices of ham and 2 slices of cheese? We already know that we get two sandwiches from the bread. 5 slices of ham - with a conversion factor of 1 sandwich needs 3 slices - will, however, only give us 1.67 sandwiches. Argh! I can still make 1 complete sandwich, because I am short ONE slice of ham - it was probably the cat who stole it. Ham is, in this case, our limiting reactant - the reactant that we are running out of.
Similar things apply to chemistry: if I have my 12 moles of hydrogen and 1 mole of nitrogen, how much ammonia can I make? Well, I already know that I can make 8 moles of ammonia from hydrogen. Let’s look at nitrogen. 2 mole of nitrogen - with a conversion factor that is 2 moles of NH3 out of 1 mole of ammonia - I will only be able to produce 4 moles of ammonia before I run out of nitrogen. And my leftover hydrogen is the excess reactant - the reactant where we have leftovers.
In a laboratory setting, we cannot measure moles directly, because we measure our reactants and products in grams. Now what was the connection between moles and amount in grams? - RIIIIGHT: Molar Mass. Using the periodic table, we know that 2 moles of diatomic hydrogen weigh 2 x 2.016 g. So, we can go from grams to moles of the reactant via Molar Mass, use the conversion factor to determine the amount of moles of the product and can then use the Molar Mass of the product to determine how many grams we are producing! And, to make it even more beautiful: 1) We can also figure out how much of a second reactant we need if we have a first reactant - to make sure the second one is not limiting. AND: 2) Since we are following the law of conservation of mass the amount of reactants and amount of products has to be equal. Therefore, we can use the amount of reactant to calculate the amount of product and vice versa.
To recap:
Coefficients in the balanced chemical equation relate the amount of reactants and products. They can represent moles of a substance and can therefore be used as conversion factors between reactants and products. The reactant that limits the amount of product that can be formed is called the limiting reactant. The reactant which is not completely used up is the excess reactant. Using molar mass we can convert the moles to grams.
Coming up next on the APsolute RecAP Chemistry Edition: Types of chemical reactions
Today’s Question of the day is about the mole concept.
Question: When is mole day? Every chemist's favorite holiday.