Introduction to Reactions:

In a simplified definition, specific interactions with molecules result in the rearrangement of atoms to create new
molecules.
The following are observations that are evidence of a chemical change, and therefore, a chemical
reaction taking place:
a) Heat or light - combustion reactions, as we'll discuss below, are evident with fire.
b) Formation of gas (and maybe an odor) - you may see bubbles.
c) Precipitation - this refers to the formation of a solid when two solutions mix
d) Color change - using indicators, or chemicals that change color based on pH.

An example of a synthesis reaction is 2Na + Cl2 → 2NaCl, as it sees two reactants, Na and Cl2, become one
product: NaCl. The synthesis of water from hydrogen and oxygen is another example of a synthesis reaction:
2H2 + O2 → 2H2O.
Decomposition reaction is the complete opposite of a synthesis reaction. For example, hydrogen
peroxide decomposes into hydrogen gas and water in the reaction 2H2O2 → 2H2O + H2. You can also reverse
the synthesis of water in the last section to represent the decomposition of water: 2H2O → 2H2 + O2.
Combustion reactions are a special type of decomposition reaction where hydrocarbons, molecules
made of only carbon and hydrogen, burn in the presence of oxygen. Combustion reactions always follow this
format:

Hydrocarbon + Oxygen → Carbon Dioxide + Water,

and they release a ton of energy.

Single replacement reactions occur when you have a compound reacting with an element. The general
form is AB + C → AC + B. The most common form of a single replacement reaction is one called a redox
reaction, where electrons are transferred between atoms. An example is 3Mg + 2AlCl3 → 3MgCl2 + 2Al.
Single = one, replacement = a switch. These reactions involve the replacement of one element in a compound by
another element. Only one switch takes place.
The most common type of reaction is a double replacement reaction, with the general form AB + CD
→ AD + CB. For example, if you mix an acid and a base, you get a reaction that forms water and some type of
salt. A formula example would be HCl + NaOH → NaCl + H2O. These reactions are the majority of the
reactions you'll see in AP Chemistry, so get used to them! Double = two, replacement = switch. These reactions
involve the exchange of ions between two compounds to form two new compounds. Here, two switches take
place.

Precipitation occurs when one of the products is insoluble. The Rule of Solubility follows the table
below.

A net ionic equation is a chemical equation that shows only the species participating in a chemical
reaction and omits the spectator ions. Spectator ions are ions that appear on both sides of the arrow in a
chemical equation but do not actually participate in the reaction.
Let's see these concepts in action with our previous reaction:

2𝐾𝐼 (𝑎𝑞) + 𝑃𝑏(𝑁𝑂₃)₂ (𝑎𝑞) → 2𝐾𝑁𝑂₃ (𝑎𝑞) + 𝑃𝑏𝐼₂ (𝑠)

If we write this reaction out with all of the ions, we get

2𝐾⁺ (𝑎𝑞) + 2𝐼⁻ (𝑎𝑞) + 𝑃𝑏²⁺ (𝑎𝑞) + 2𝑁𝑂₃⁻ (𝑎𝑞) → 2𝐾⁺ (𝑎𝑞) + 2𝑁𝑂₃⁻ (𝑎𝑞) + 𝑃𝑏𝐼₂ (𝑠)
 This was obtained by dissociating, or separating, each soluble compound into its constituent parts.
Note that PbI₂ does NOT dissociate, since it is insoluble. Be on the lookout for this! Only dissociate soluble
compounds. Also, don't dissociate weak acids and bases 🍊.
This equation, with the ions written out, is called the complete ionic equation or the total ionic
equation. It shows every ion in the solution that is present during the reaction, including spectator ions.
Now let's take the spectator ions out of the reaction. When looking at the total ionic equation, we see
that potassium and nitrate go in and come out the same way, so we can cross them out and cancel them.
Potassium and nitrate were the spectator ions 👓!
Once we eliminate the spectator ions, we are left with

2𝐼⁻ (𝑎𝑞) + 𝑃𝑏²⁺ (𝑎𝑞) → 𝑃𝑏𝐼₂(𝑠).

This is what we call the net ionic equation for the reaction, and it tells us what really happens.
Potassium and nitrate were in the flask, but they really didn't do much for us in terms of the formation of the
insoluble compound. They could have been any suitable ion and the reaction would have had the same
outcome.

General Steps
Now that you have an idea of what a net ionic equation is, it is time to learn how to write one out! First,
you must know the basics of representing a chemical reaction. Make sure you review those in the last study guide
before moving on.
To work through and learn this process, here are the general steps to writing a net ionic equation:
1. Figure out which compounds are soluble and insoluble using solubility rules
2. Balance the given chemical equation. It may already be balanced, but it also may not, so you
always have to check.
3. Write the complete ionic equation by dissociating soluble compounds into ions.
4. Omit the spectator ions and write the final net ionic equation of the given reaction. Make sure
you include the phase of matter each compound is in.
Example #2
Try writing the net ionic equation for the following reaction:
𝐾𝑂𝐻 + 𝐹𝑒(𝑁𝑂₃)₃ → 𝐾𝑁𝑂₃ + 𝐹𝑒(𝑂𝐻)₃
Using solubility rules, you should find that: Fe(OH)3 is the solid or the precipitate formed. Add the
respective states of matter to each compound in the chemical equation.
𝐾𝑂𝐻 (𝑎𝑞) + 𝐹𝑒(𝑁𝑂₃)₃ (𝑎𝑞) → 𝐾𝑁𝑂₃ (𝑎𝑞) + 𝐹𝑒(𝑂𝐻)₃ (𝑠)
You should notice that there is only 1 OH on the left and 3 OH on the right. The same goes for NO3,
but vice versa. This means we have to balance the equation.
3𝐾𝑂𝐻 (𝑎𝑞) + 𝐹𝑒(𝑁𝑂₃)₃ (𝑎𝑞) → 3𝐾𝑁𝑂₃ (𝑎𝑞) + 𝐹𝑒(𝑂𝐻)₃ (𝑠)
 Write out the complete ionic equation by dissociating all soluble compounds into their constituent
ions. If you're ever unsure of what you should and shouldn't dissociate, remember that every compound in the
aqueous solution will dissociate and the solid will not.
3𝐾⁺ (𝑎𝑞) + 3𝑂𝐻⁻ (𝑎𝑞) + 𝐹𝑒⁺³ (𝑎𝑞) + 3𝑁𝑂₃⁻ (𝑎𝑞) → 3𝐾⁺ (𝑎𝑞) + 3𝑁𝑂₃⁻ (𝑎𝑞) + 𝐹𝑒(𝑂𝐻)₃ (𝑠)
Identify the spectator ions by taking a look at what ions look the same before and after the chemical
reaction took place or identifying which ions are not in the formed precipitate:
Write out your final answer and net ionic equation:
3𝑂𝐻⁻ (𝑎𝑞) + 𝐹𝑒⁺³ (𝑎𝑞) → 𝐹𝑒(𝑂𝐻)₃ (𝑠)

The law of conservation of mass is a fundamental principle in chemistry that states that the amount
of matter stays constant in a closed system. In short, matter cannot be neither created nor destroyed in thin air.
This makes it necessary for us to make sure that our chemical equations are balanced; that is, we need to make
sure that the number of elements in the reactants is the same as those in the products, even if the products are
different from what we started with. We'll do lots of practice balancing equations so that you're a master at it!
Steps to Balance Equations
1. Make sure the equation isn’t already balanced
2. Find the elements that are only in one compound on both sides and have the same number of
atoms on both sides. These must have equal coefficients.
3. Look at the elements that are only in one compound on both sides and have different numbers
of atoms on both sides. Balance them.
4. Look at the elements that are in more than one compound. Balance them.
5. Finally, double-check your answer and make sure that the number of element X on the
reactants side is the same as on the products side. The key to this is satisfying the law of
conservation of mass.
 Titrations are an experimental method 🧪 used to determine the unknown concentration of a
chemical solution. The titrant refers to the solution of known concentration, and it is added to the analyte, the
solution of unknown concentration.
At some point during the acid-base titration, a color change is observed. This is called the endpoint
of a titration. The equivalence point is the point at which the number of moles of titrant added is equal to the
number of moles of the analyte.
From here, you'll begin to learn about the specifics of three major types of reactions:
1) precipitation reactions,
2) acid-base reactions, and
3) oxidation-reduction reactions.
Acid-base reactions are chemical reactions that involve the transfer of a proton from one molecule to
another. Oxidation-reduction reactions, also known as redox reactions, involve the transfer of electrons.
Precipitation reactions, though, are completely different. They are those in which two or more soluble reactants
combine to form an insoluble product.
In this course, you'll use the Brønsted-Lowry definitions: focuses on the transfer of a proton, so acids are
proton donors while bases are proton acceptors. What you basically see happen is a hydrogen ion being transferred
from a substance, denoted as the acid, to another substance, denoted as the base.

Last but not least, redox reactions! These reactions deal with the transferring️ of electrons, which causes molecules
to change oxidation states. An oxidation state is a measure of the degree of oxidation of an atom in a chemical
compound. It is represented by a positive or negative number that expresses the number of electrons that an
atom has gained or lost in a compound relative to its elemental state on the periodic table.
When a molecule loses an electron, it’s oxidized, and its oxidation number increases. When a molecule
gains an electron, it’s reduced, and its oxidation number decreases. Electrons travel from the oxidized species to
the reduced species. (LEO the lion GER.)