AP Chemistry Unit 4: Chemical Reactions

4.1: Introduction to Reactions

Chemical versus Physical Changes

Physical: Change in properties but not in composition; Change in appearance or state
- Ex. phase changes, Ice to water to gas, separating a mixture
Chemical: transformation into new substances with different compositions
- Ex. Chemical rxn

Indicators of Chemical Reactions

Temperature change
Light
Formation of gas (Bubbling)
Precipitate — a solid formed from the addition of 2 solutions
Color change
Change in smell

Indicators in Physical Changes

Change in shape or size — tearing, folding, moving.
Phase changes — melting, boiling, freezing, etc.

4.2: Net Ionic Equations

While many ionic compounds dissolve in water to form separate ions, not all do. The dissolution of ionic compounds in water depends on several factors including lattice energy of the compound and the solvation energy provided by the water. Some ionic compounds, like sodium chloride, readily dissolve in water and dissociate into separate ions (Na+ and Cl-). However, other ionic compounds such as barium sulfate or lead (II) sulfate have low solubility in water due to their high lattice energy, which makes them less likely to dissociate into separate ions.

Solubility rules

Solubility rules can provide a general guideline on which ionic compounds are expected to be soluble or insoluble in water.

All nitrates, bromides, halides, and iodides are SOLUBLE except for compounds containing Ag+, Pb2+, and Hg2 +2. All sulfates are soluble except for Ag2SO4, PbSO4, HgSO2, CaSO4, and SrSO4. Alkali salts (group 1A) and NH4+ are soluble. Strong acids are soluble. Acetate salts are also soluble

All phosphates, carbonates, chromates, and hydroxides are INSOLUBLE except for Ammonia. OH- salts are insoluble except for group 1 hydroxides and Br2+, Sr2+, and Ca2+.