Reading Notes for Chapter 6

These are Dr. Bodwin's reading notes for Chapter 6 of "Introduction to Chemistry". I am using a local .pdf copy that was downloaded in August 2020.

Chapter Summary:

To really understand chemical reactions, we need to be able to count individual "pieces" in the reaction so we can understand the relationship between products and reactants. Because those "pieces" are so small, it's difficult and impractical to think about counting pieces on the macroscale, so we need a better way to measure macroscopic quantities of these microscopic pieces.

Grouping Units:

We use grouping units all the time. Some are more standardized than others, but every trip to the grocery store involves grouping units. You're throwing a big parts and you need 80 hot dog buns. At the store, the hot dog buns you want come in packages of 8. How many packages do you need? A "package" is a grouping unit. A case of peaches contains 60 peaches. How many "cases" do you need if you want 300 peaches for canning? A "sleeve" of golf balls is 3 golf balls. These are all grouping units.
Grouping units allow us to specify a (relatively) large quantity with a smaller number, and are usually defined as 1 "grouping unit" is equal to {some number larger than 1} individual pieces.

The Mole:

Atoms are tiny and very difficult to measure individually. We can solve that problem by using a grouping unit to describe a large number of individual pieces. Because atoms are so small, the grouping unit we use is large. VERY large.
1 mole = 6.022x10²³ pieces
For perspective...

1 mole of seconds is 1.9x10¹⁶ years
1 mole of millimeters is 2 billion trips to the Sun and back
1 mole of grains of sand would occupy a cube 157 miles wide

So it's safe to say that a mole is big. But how small is an atom?

1 mole of iron atoms is a cube just under 2cm wide
1 mole of water molecules is 18 mL, a little over a tablespoon

The Mole - Moles as Grouping Units
Moles - Moles from grams

Atomic Mass Units and Molar Mass:

The mass listed on the Periodic Table does not have units because the units depend upon how you're measuring mass. If it's microscale, then the unit is "Atomic Mass Units", or "amu". If you're doing a macroscale measurement, then the unit is "grams per mole", or "g/mol".

Stoichiometry:

We looked at mole-to-mole stoichiometry in Chapter 5 (although we didn't exactly call it that...) That's step 3 of the process described here: a 4-step approach
For stoichiometry problems, we follow the same 4-step approach:

Write a balanced chemical equation
Find moles of something you have a lot of information about
Use the mole-to-mole relationship from the balanced equation to find moles of what you're interested in
Use additional information from the problem to convert "moles of interest" into whatever quantity you're looking for

There are a LOT of problems in chemistry that can be approached with these same 4 steps, it takes practice to recognize them. A couple of the more common and obvious types of problems are described next...

Yield and Percent Yield:

"Yield" is how much stuff a reaction makes. The most common types of yield are:
Yield - This is just how much the reaction made. Often this is called "actual yield" to be very clear.
Theoretical Yield - This is how much the reaction could make if the reaction is complete and we don't lose any products
Percent Yield - This is the ratio of how much was made to how much could have been made; Actual yield divided by theoretical yield times 100. *NOTE* Don't make percent yield a new, hard problem, it's just a percent. If you take a quiz that's worth 40 points (the "theoretical yield" of the quiz) and you score 32 points (the "actual yield"), you know how to calculate the "percent yield" on the quiz. Theoretical yields in chemical equations are exactly the same concept, just applied to a different system.
Actual yield is a measurement that you make (or a number that is given in a problem) - the mass of products, the volume of products, etc
Theoretical yield is a quantity you calculate based upon the amount of reactant(s) used in the reaction

Limiting and Excess Reagents:

For a reaction that has two reactants, A & B, there is a relationship between the moles of A & the moles of B that react. What if the actual amounts of A & B do not match that relationship?
If I'm making cheese sandwiches and I have 2 slices of cheese and 20 slices of bread, the number of sandwiches I cn make is limited by the number of cheese slices and I will have excess bread. (Unless you like very unusual sandwiches...)
Same thing in a chemical reaction; there is almost always a Limiting Reagent (also called a Limiting Reactant) and an Excess Reagent. Finding the limiting and excess reagents is a stoichiometry problem.

Return to ChemBits Fundamentals of Chemistry Index.

All information on this page is produced by Jeffrey Bodwin, Copper Sun Creations, or curated from the attributed source.

This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.