Reading Notes for Chapter 4


These are Dr. Bodwin's reading notes for Chapter 4 of "Chemistry 2e" from OpenStax. I am using a local .pdf copy that was downloaded in May 2020.

Chapter Summary:

Stoichiometry (stow-ick-ee-AHM-it-tree) is a sometimes cumbersome word, but it is probably the biggest fundamental concept in all of chemistry. The etymology of the word is from the Greek and it means the study of or measuring of elements. We've already seen a little glimmer of it in the previous chapters when we were writing balanced chemical formulas, and "balance" is really the key here. Nature does not allow random dangling bits. When writing balanced chemical formulas, that means that every positive charge must have a corresponding negative charge to balance it. In this chapter, we are going to move on to chemical reactions, and once again, we need to avoid the random dangling bits! One way we can frame stoichiometry is by the Law of Conservation of Matter... one way to say this is "matter cannot be created or destroyed", but another way to state this that is more specific to stoichiometry is "everything that goes in to a reaction, must come out, and everything that comes out must have gone in".It might seem like a pretty simple concept, but "simple" doesn't always mean "easy". Balancing chemical equations and working through stoichiometry take practice. I like to focus on using a consistent process because even though every problem is just a little bit different, the process to solve those problems is usually very close to the same.

Writing Balanced Chemical Equations:

There is some basic vocabulary on page 176 (reactants, products, coefficients) that we will be using constantly. Make sure you read over these definitions and cement them into your vocabulary. You will be seeing and hearing them often enough that they will become "normal" words, but I want to make sure everyone has these in place.
Back to the Conservation of Matter concept, every element that appears on the reactant side of an equation (the "left" side) must appear in equal number on the product side of an equation (the "right" side).
NOTE - In a nuclear reaction, the rules change a little bit, but at this point we are not studying nuclear equations, so let's keep it simple. Nuclear reactions are important, but the VAST majority of what we call "chemical reactions" take place in the electron cloud outside the nucleus, so at this point we can consider the nucleus to be constant and unchanging.
A chemical equation connects the microscopic and the macroscopic world, and can be used to describe both, depending upon context. For example, the balanced chemical equation:
2 Na + Cl2  -->  2 NaCl
tells us that two sodium atoms react with one chlorine molecule (under "normal" conditions, chlorine and the other halogens exist as diatomic species) to produce 2 sodium chlorides. But we could also say that 20 sodium atoms react with 10 chlorine molecules to form 20 sodium chlorides. The balanced chemical equation shows us the smallest whole-number ratio of reactants and products, but it's a ratio, so as long as the realtive relationship is maintained, it is still valid. This is where the mole concept is important... remember a "mole" is just a grouping unit, so we can say that 2 moles of sodium atoms react with 1 mole of chlorine molecules to form 2 moles of sodium chlorides. "Atoms" and "molecules" are descriptions in the microscopic world, "moles" are descriptions in the macroscopic world. The ratios established by the balanced chemical equation are equally correct in both worlds!
When we are working in the macroscopic domain, it is usually considered good practice to include state labels in our balanced chemical equations. Unless you have reason to believe otherwise, always assume that you are under "normal" conditions of temperature and pressue (around room temperature and around 1 atmosphere of pressue). Under these "normal" conditions, sodium is a metallic solid, chlorine is a gas, and sodium chloride is a solid. Adding these extra bits of information into the above equation...
2 Na(s) + Cl2(g)  -->  2 NaCl(s)
There are also liquids, (l), and aqueous solutions, (aq).

Writing correct balanced chemical equations takes practice. Take every opportunity to look at balanced equations and practice balancing them yourself. If someone handed you a bassoon and asked you to play an F major scale, you wouldn't expect to be able to do it without significant practice, and the F major scale isn't even a song. {Shout out to any bassoon players!} Balancing chemical equations is similar... you have to practice to become proficient.

Reaction Classes:

There are a few different ways to classify chemical reactions to make them easier to recognize and understand. One method looks at what type of reactants and products the reaction involves.
Combination - two (or more) elements combine to form an ionic or molecular product
Decomposition - The opposite of "combination"; an ionic or molecular reactant breaks apart into elements
Displacement/Replacement - An element reacts with (usually) an ionic compound to product a different element and ionic compound
Double displacement/Double replacement/Metathesis - Two ionic or molecular reactants "switch partners" to produce two new ionic or molecular products.
These are one way to start thinking about chemical reactions, and I include them because their terminology comes up in a number of other places. There are also a number of reaction types that also help see commonalities between chemical reactions...

Reaction Type - Precipitation:

Precipitation reactions are those in which 2 or more soluble reactants combine to form products, at least one of which is not soluble. For convenience at this point, we will limit ourselves to aqueous solutions so water is our solvent. Precipitation reactions are metathesis reactions.
Key review topics - writing balanced chemical formulas, molarity and concentration
Table 4.1 summarizes the solubility rules we will use. Pair these up with your polyatomic ion memorization practice. If you use physical flashcards for the polyatomic ions, you can even add some additional notes to those flashcards with solubility information.
These solubility rules are a good example of why you will see nitrate so often in sample problems (and acetate, chlorate, and perchlorate)... nitrate salts are soluble. When I'm writing a question and I need a soluble salt, I go with nitrate because there are no significant exceptions to the "All nitrate salts are soluble" rule.
Net Ionic Equations:
A chemical reaction is a shorthand way to describe chemical change. It can also be a "recipe" for a chemical reaction. If we are most interested in describing chemical change, then the reaction can be simplified by eliminating anything that is not undergoing chemical change. Let's start with a precipitation reaction, the reaction of calcium nitrate solution with sodium phosphate solution:
3 Ca(NO3)2(aq) + 2 Na3PO4(aq) -->  Ca3(PO4)2(s) + 6 NaNO3(aq)
This is the "full" or "molecular" equation. It contains all the species present in the reaction, and it would be a very good version of the reaction if you were trying to prepare these reagents from bottles of solids that you might find on a shelf. If we want to explore the actual chemical changes that are happening in this reaction, we might first want to more correctly represent what is present in solution. "Ca(NO3)2(aq)" is not actually intact calcium nitrate units floating around in solution, it is actually calcium ions in solution and nitrate ions in solution. The same is true of the other species in this reaction that are labelled "aq". The "Ca3(PO4)2(s)" actually is intact calcium phospahte units, now as part of a solid designated by the "(s)". With this in mind, we can re-write the balanced equation as a "full ionic" or "complete ionic" equation:
3 Ca+2(aq) + 6 NO3-1(aq) + 6 Na+1(aq) + 2 PO4-3(aq) -->  Ca3(PO4)2(s) + 6 Na+1(aq) + 6 NO3-1(aq)
Looking at that reaction, there are a number of ions that appear to just be sitting around watching the chemical change take place. They are spectators. If we want a version of the balanced chemical reaction that only describes the chemical change that is taking place, then we can eliminate those spectators, and we are left with the "net ionic" equation:
3 Ca+2(aq) + 2 PO4-3(aq) -->  Ca3(PO4)2(s)
This net ionic equation describes the chemical change that is taking place. For any molecular equation, there is only one net ionic equation, but what about the reverse of that statement? Think about solubility rules... I need a soluble sourse of Ca+2(aq) ions for my reaction, so I could choose a few different options, for example, calcium chlorate. I also need a soluble source of PO4-3(aq) ions; that could be potassium phosphate or lithium phosphate. Any combination of a soluble calcium salt with a soluble phosphate salt should give the exact same net ionic equation because the net ionic equation is only describing the chemical change that is taking place in the system.

Reaction Type - Acid-Base:

Acid-base reactions are usually metathesis reactions. The easiest way to recognize and acid-base reaction is to be able to recognize common acids and bases. There are a number of definitions of acids and bases, let's start with these:
NOTE: Just because a reaction has as acid or a base doesn't make it an Acid-Base reaction.
Acids - substances which when dissolved in water increase the concentration of H+1 {or H3O+1}. An H+1-donor.
Bases - substances which when dissolved in water increase the concentration of OH-1. An OH-1-donor.
For an acid or base to be considered "strong", it must "completely dissociate in water". That means it breaks up into its component ions completely. Weak acids and bases tend to "stick" together to varying extent.
For our purposes, the acids in Table 4.2 are "strong". If an acid is not in that table (and you aren't specifically told otherwise), then you can assume that the acid is weak.
For our purposes, soluble hydroxides (as noted in the "exceptions" column of Table 4.1) are "strong". Unless you are specifically told otherwise, all other bases are "weak".
For strong acids and strong hydroxide bases, the net ionic equation is always the same... H+1(aq) + OH-1(aq) -->  H2O(l)
If either the acid or base is weak, then that species has to appear in the net ionic equation because it is not completely dissociated.

Reaction Type - Redox:

This topic has its own chapter later on, so let's try to focus on the basics here.
Let's add one more "number" to our toolbox of numbers that keep track of subatomic particles. "Oxidation number" is the number of protons minus the number of electrons. Yes, I know, that sounds just like charge, BUT oxidation number works for individual ions as well as atoms that are part of a polyatomic ion or molecule.
Redox reactions are reactions in which reduction and oxidation are both occuring. If you have reduction, you MUST have oxidation and vice-versa.
Oxidation Is Losing electrons (OIL)
Reduction Is Gaining electrons (RIG)
So OIL-RIG might help you remember. If that doesn't click with you...
Losing Electrons is Oxidation (LEO)
Gaining Electrons is Reductions (GER)
So LEO the lion says GER might work better for you. Either way, they both work.
For monoatomic ions, oxidation number of equal to charge.
There are a couple other "rules" that can help us at this point.
1.   For uncombined, uncharged elements, the Oxidation Number os zero
2.   For monoatomic ions, the Oxidation Number is equal to the charge
3.   The sum of the Oxidation Numbers for a polyatomic ion or molecule is equal to the charge on the polyatomic ion or is equal to zero for a neutral molecule
4.   Oxygen is almost always Oxidation Numebr -2, unless it is molecular oxygen (Ox# = 0) or a peroxide (Ox# = -1). For our purposes, a peroxide will have contain either "peroxide" or "peroxo" in its name.
5.   Hydrogen is almsot always Oxidation Number +1, unless it is molecular hydrogen (Ox# = 0) or a hydride (Ox# = -1). For our purposes, a hydride will have the word "hydride" in its name.
Redox reactions can be single displacements, combination, decomposition, and might even look like metathesis reactions. One way to quickly recognize redox reactions is to look for uncombined elements as either reactants or products or both; not every redox reaction has an uncombined element, but every reaction that includes an uncombined element is a redox reaction. {Well, that may not be absolutely true, but it's a pretty reliable "rule" for now...}
Your textbook goes through a very nice process for balancing redox reactions using the half-cell method. Take a look at it, but don't worry too much about this process at this point. Many (most?) of the redox reactions we will use in this course can be balanced by inspection rather than the half-cell method. Just remember, similar to atoms in the Law of Conservation of Matter, charge must balance in a correctly balanced chemical equation. Nature doesn't like random charges dangling about.
Balancing by inspection:
Zinc metal reacts with copper(II) ions to form copper metal and zinc(II) ions
Zn(s) + Cu+2(aq) --> Cu(s) + Zn+2(aq)
Magic! It's already balanced! One zinc on each side, one copper on each side, +2 total charge on both sides... balanced!!
Zinc metal reacts with iron(III) ions to form iron metal and zinc(II) ions
Zn(s) + Fe+3(aq) --> Fe(s) + Zn+2(aq)
One zinc on each side, one iron on each side... um, but there's a charge problem. +3 vs +2... We can get +3 up to +6 by doubling it, and +2 up to +6 by tripling it, so
3 Zn(s) + 2 Fe+3(aq)  -->  2 Fe(s) + 3 Zn+2(aq)

Reaction Stoichiometry:

Stoichiometry problems all have the same 4 steps and they're all about the moles:
  1. Write a balanced chemical equation.
  2. Find the number of moles of whatever reactant(s) or product(s) for which you have enough information to do so.
  3. Use the mole-to-mole ratio(s) in the balanced chemical equation to find the moles of whatever you're interested in.
  4. Once you have moles of what you're looking for, find whatever quantity you're interested in finding.
This is nicely shown in the flowchart in Figure 4.11.
There are certainly shortcuts, but don't take them. Figure out how to go through the 4-step process, and just do it. Explicitly write out mathematical expressions and keep track of the units, make sure the units cancel. Example 4.9 has a nice example of writing out an expression and cancelling units. So does Example 4.10. And Example 4.11.
To start thinking about reaction stoichiometry, think about what you would have to do to plan a chemical reaction. If we want to use 22.99g of sodium metal to react with water to form sodium hydroxide and hydrogen gas, how much water do we need to use?
2 Na(s) + 2 H2O(l)  -->  2 NaOH(s) + H2(g)
22.99g of sodium metal is 1 mol of Na(s). 2 moles of Na(s) react with 2 moles of water... or 1 mol of Na(s) reacts with 1 mol of water. We just did stoichiometry.

Yield:

Whenever we perform a chemical reaction, it's nice to evaluate how successful that reaction was. This is "yield". Yield is how much we made.
Theoretical yield is the amount of product that could be made if all of the limiting reactant reacted perfectly to produce product.
To find the limiting reactant, do a stoichiometry problem. If I use all of reactant A to make product, how much product can I make? If I use all of reactand B to make product, how much product can I make? Whichever reactant makes less product is the limiting reactant, and the amount of product you just calculated for the limiting reactant is the theoretical yield.
"Limiting reactant" and "limting reagent" are slightly different terms for the exact same thing. They are fully interchangeable.
Don't make percent yield into something harder than it is. You have enough cookie dough to make 28 cookies. You only make 23 cookies because you accidentally ate some of the cookie dough while you were baking. Your percent yield is (23/28)*100% = 82%.

Application - Titration:

Titration is a specific type of stoichiometry problem. Treat it like a stoichiometry problem. Don't make it into a whole new and different thing.

Memorization Notes:

Polyatomic ions
Diatomic elements - nitrogen, oxygen, halogens
Solubility Rules
Common acids & bases



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