Reading Notes for Chapter 5

These are Dr. Bodwin's reading notes for Chapter 5 of "Introduction to Chemistry". I am using a local .pdf copy that was downloaded in August 2020.

Chapter Summary:

When we write a chemical reaction, it is important that we follow a number of fundamental rules, the most obvious of which is the Law of COnservation of Matter. Just like when you balance your checkbook, the deposits and withdrawals have to be balanced, but in the case of chemical reactions there can be no balance carried forward. Balancing chemical reactions takes practice both in the balancing and in recogizing different reaction types to make it easier to predict the products of those reactions.

Conservation of Matter:

In science, a law is a statement developed after making repeated observations of a phenomenon that is considered to be reliably true.
As additional observations are made, laws are either supported or modified. A law is based upon a LOT of observations, so although it can be modified or restricted based upon additional observations, something that has risen to the level of a "law" is rarely just discarded as incorrect.
The Law of Conservation of Matter is the basis for many of the things we do in chemistry, and it is stated in a number of ways. Your textbook has a very good formal definition of the Law of Conservation of Matter, but a more common way to express it is "matter cannot be created or destroyed". That means we can change matter, but the total amount of matter that goes into a chemical reaction must be equal to the total amount of matter that comes out of a chemical reaction.

Chemical Equations:

A chemical equation is a representation of the matter that is reacting and the products that are formed.
Chemical equations can be "shown" in a number of different ways... descriptively in sentence & paragraph form, pictorially, symbolically, etc
Writing chemical equations requires that the ionic and covalent formulas are written correctly, so make sure you practice those!

Balancing Chemical Equations:

When we "balance" a chemical equation, we are using the Law of Conservation of Matter - everything that goes in, must come out, but we can rearrange it.
Balancing chemical equations is an exercise in "atomic accounting" - every atom that goes in must come out. Just like with money, we can't spend more than we earn.
Chemical reactions are balanced by putting whole number coefficients in front of each reactant and product so that the total matter going in matches the total matter coming out of the reaction. Let's say you're making a simple bicycle. You need a frame, a seat, handlebars, and wheels.

Frame + Seat + Handlebars + Wheel --> Bicycle

Is that "equation" balanced? Not quite... Let's add coefficients:

1 Frame + 1 Seat + 1 Handlebars + 2 Wheel --> Bicycle

Now it's balanced! In chemical equations, we usually don't explicitly include coefficients of "1", but I included them here to be explicit. This "equation" is balanced because all the matter going in (on the left side of the arrow) is coming out (on the right side of the arrow).
Unless the reaction specifically requires it, do not break up polyatomic ions when balancing chemical reactions. Keeping them together will make balancing much easier!

Molecular Stoichiometry from Chemical Reactions:

Why do we bother balancing chemical equations?? A balanced chemical equation describes the relationship between different reactants and products at the atomic/molecular level. We might know that propane reacts with oxygen to form carbon dioxide and water, but how many oxygen molecules react with each propane molecule in that reaction? That's what the balanced chemical equation tells us.
Stoichiometry is the relationship between the atoms and molecules in a chemical reaction and is often expressed as a ratio of the coefficients in the balanced chemical equation. Using the bicycle example above, the "stoichiometry" of frames to seats is 1:1. If I have 7 frames, I need 7 seats. If I have 14 wheels, I need 7 handlebars.
Let's say you work at a restaurant and are serving meals that consist of 2 baked potatoes, 6 asparagus spears, and 5 baked mushroom caps.

2 baked potatoes + 6 asparagus spears + 5 mushroom caps + 1 plate --> 1 meal

The relationships between those ingredients is the stoichiometry of your meal.

Reaction Types:

The purpose of studying reaction types is to explore broader trends in chemical reactivity. There are billions of different combination of substances that we can call chemical reactions, so being able to group them into general types of reactions is a helpful way to understand many different combinations without having to memorize each one.
Combination Reactions - also called "composition" reactions. Two or more substances combine to form a single product.

Reaction Type	Other Names	What is it?	How to recognize	Example
Combination	Composition	Two or more substances combine to form a single product	Only one product	K(s) + Cl2(g) -> 2 KCl(s) "One product" doesn't have to mean the coefficient is 1.
Decomposition		One substance breaks down into two or more products. This is the reverse/opposite of a "combination" reaction	Only one reactant	CaCO3(s) -> CaO(s) + CO2(g)
Displacement		A substance reacts with an element to form a different substance and a different element	Look for uncombined neutral elements on both sides of the equation	AgNO3(aq) + Cu(s) -> Cu(NO3)2(aq) + Ag(s)
Combustion	Burning	One substance reacts with oxygen {O2(g)} to form oxygen-containing products	Oxygen in a reactant and all products contain oxygen. Hydrocarbons are often involved in combustion reactions, but hydrocarbons are not the only substance that can burn.	CH4(g) + 2 O2(g) -> CO2(g) + 2 H2O(g)
Oxidation-Reduction	Redox	Two (or more) substances exchange electrons to form products	If one (or more) reactant or product is an uncombined neutral element, the reaction will involve oxidation and reduction Oxidation cannot happen without reduction and reduction cannot happen without oxidation. If you find one, the other must be there.	Na(s) + H2O(l) -> NaOH(s) + H2(g)
Metathesis	Exchange, Double replacement, Double displacement	Two ionic reactants exchange partners There is also an organic reaction type that is called "metathesis" which is similar but not the same.	If both reactants are ionic compounds, try mixing up the pairs and see if there is a reason for the reaction to occur.	NaCl(aq) + AgNO3(aq) -> AgCl(s) + NaNO3(aq)
Precipitation		Two ionic reactants react to form an insoluble substance This is a type of Metathesis reaction	Look for a cation-anion combination that would result in an insoluble salt	Ca(NO3)2(aq) + K2SO4(aq) -> CaSO4(s) + 2 KNO3(aq)
Neutralization	Acid-base	An acid and a base react with one another. This is a type of Metathesis reaction	Most of the common acids and bases we will use in this class will form H2O(l) molecules from H+1(aq) + OH-1(aq)	HCl(aq) + NaOH(aq) -> H2O(l) + NaCl(aq)
Gas-forming		An aqueous reaction produces a gase (either directly or by decomposition of as unstable product). This is a type of Metathesis reaction.	Look for carbonates, sulfites, sulfides, and acetates reacting with acid or ammonium ions reacting with a base.	Na2SO3(aq) + HCl(aq) -> H2O(l) + SO2(g) + 2 NaCl(aq) {H2SO3(aq) decomposes to form H2O(l) and SO2(g)}

Notice that a reaction can fall into multiple categories! Combustion reactions and displacement reaction are all redox reaction. Composition and decomposition reactions can be redox, but they don't have to be. Some acid-base reactions also form a precipitate. The best way to see these patterns is to look at every chemical reaction you come across and try to identify the type(s).

Oxidation & Reduction:

Redox reactions involve a change in the oxidation number of elements in the reaction. Oxidation number is similar to charge, but where charge refers to monoatomic ions and groupings of atomcs that are combined into a molecule or polyatomic ion, oxidation number lets us look at the electrons assigned to each atom in a molecule or polyatomic ion. There are a couple rules for assigning oxidation numbers:

Assigning Oxidation Numbers (Rules):

For neutral elements uncombined with other elements, the oxidation number is zero
For monoatomic ions, the oxidation number is equal to the charge
Oxygen is almost always oxidation number -2 except for O₂ (see rule 1) and peroxides (Ox# = -1)
Hydrogen is almost always oxidation number +1 except for H₂ (see rule 1) and hydrides (Ox# = -1)
The sum of the oxidation numbers in a polyatomic ion or molecules must equal the charge on the polyatomic ion or molecule.

The rules work for most ionic compounds, and they are relatively easy to use. Sodium sulfate is 2 sodium ions and a sulfate ion. The sodium ions are monoatomic, so the oxidation number on each sodium ion is +1 (see Rule 2). The sulfate ion has an overall -2 charge. If each of the four oxygens has oxidation number -2 (Rule 3) for a sum of -8, then the sulfur must have oxidation number +6 because the sulfate polyatomic ion has a total charge of -2 (Rule 5).

When a redox reaction occurs, oxidation numbers change. Again, when trying to identify redox reactions, finding an uncombined neutral element is a great indicator of redox because of Rule #1 - the oxidation number of uncombined neutral elements is zero and the oxidation number an element in just about any other context is not zero... so there had to be a redox reaction!

Redox reactions are all around in nature. Most biological processes involve some redox chemistry: digestion, respiration, replication... Everything is chemistry, and a LOT of that chemistry is redox!

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