Chemistry AP Study Notes 3 (Aqueous Solutions)

Thus far:
1. The Basics
2. Basic Chemical Reactions

Chapter 4
Reactions in Aqueous Solutions

• electrolyte (conducts electricity when dissolved in water); non-electrolytes
• cations ("cats are positive") and anions (negative)
• hydration - when substances dissolve into ions that are surrounded in a certain way by water; H₂O is sometimes written over the reaction arrow.
• reversible reactions (double arrow in both directions)
• Some specific types of reactions: 1) precipitation reactions, 2) acid-base reactions, 3) redox reactions (oxidation-reduction). See the five broader types of reaction in the notes on the previous chapter.

Precipitation Reactions

• Precipitation reactions (solid comes out of the reaction) is a double displacement reaction (also called a metathesis reaction).
• solubility - how much solute will dissolve in solvent at a certain temperature
• molecular equation versus ionic equation
• Ions not involved in the overall reaction are called "spectator ions."
• A "net ionic equation" only shows the species that actually take place in the reaction. 

Acid-Base Reactions

• Acid-base reactions. Acids have sour taste (like vinegar). Bases have a bitter taste.
• Arrhenius defined an acid as a substance that produces an H⁺ ion in solution and a base as a substance that produces an OH^- ion in solution.
• Bronsted defined an acid as a proton donor and a base as a proton acceptor.
• the hydronium ion (H₃O⁺)
• monoprotic, diprotic, and triprotic acids
• acid-base neutralization, usually yielding a salt and water. 
• titration is a method for determining the concentration of an acid or base. For example, an acid is prepared with an "indicator" like phenolphthalein (which turns pink in a basic solution--7+ on the ph scale--but is otherwise clear--7- on the ph scale). Drops of a known concentration of a base like NaOH is dripped into the acid until it reaches the endpoint (turns pink), which is the equivalence point (ph of 7).

Oxidation-Reduction Reactions

• Oxidation-reduction reactions include combustion reactions, single replacement reactions, and many double replacement and decomposition reactions.
• In a "redox" reaction, the "oxidized" element loses electrons ("ox-loss") and the "reduced" element gains electrons. The reduced element is called the oxidizing agent, and the oxidized element is called the reducing agent.
• In a sense, you have an oxidation reaction and a reduction reaction taking place simultaneously. For example, solid zinc may oxidize into Zn²⁺ as it dissolves in a solution with copper +2 ions. It is losing electrons so it is being oxidized.
• At the same time, a reduction reaction also takes place. The Cu²⁺ ion becomes solid copper. It is gaining electrons so it is being reduced.
• An "activity series for metals" lists metals in an order of decreasing ease of oxidation (so the metal at the top is most easily oxidized). If a higher metal is put in a solution of a lower metal, there will be a single replacement reaction. The opposite attempt would not react.

Oxidation Numbers

• A tool for balancing redox equations. Don't confuse them with charge.
• The oxidation number of a neutral element is 0.
• The oxidation number on a monoatomic ion is its charge (write the plus or minus after the number).
• The sum of all oxidation numbers in a neutral molecule is zero.
• Alkalis (+1), Alkalines (+2), Halogens (-1), oxygen (-2)

Other Tidbits

• All sodium, potassium, ammonium, and nitrate salts are soluble in water.
• Strong acids include HCl, HBr, HI, HNO₃, HCLO₃, HCLO₄, H₂SO₄
• Cu²⁺ ions are normally blue.
• Bromine solutions tend to be reddish.
• Iodine solutions tend to be brownish in water.
• Carbonates (CO₃^2-) produce carbon dioxide in the presence of an acid.
• Many hydrides (e.g., NaH) react with water to form the hydroxide ion (OH^-) and hydrogen gas.
• Charge is written with number first. Oxidation numbers put the charge first.
• Electrolysis is sometimes used to cause decomposition reactions.
• A complex ion is a metal ion bonded usually with water (called a ligand). So the chromium in chromium nitrate bonds with six water molecules in water. Coordination numbers are the numbers of water (or other) molecules that serve as such ligands. The most common number is 6, but also found are 2 and 4.