Exam 2 Concept Guide: General Chemistry

Empirical and Molecular Formulas

Understanding how to determine empirical and molecular formulas is essential for analyzing chemical compounds. The empirical formula represents the simplest whole-number ratio of elements in a compound, while the molecular formula shows the actual number of each type of atom in a molecule.

• Empirical Formula: Simplest ratio of atoms in a compound.

• Molecular Formula: Actual number of atoms of each element in a molecule.

• Calculation Steps:

  1. Convert mass percentages to grams (assume 100 g sample if percentages are given).

  2. Convert grams to moles for each element.

  3. Divide by the smallest number of moles to get the simplest ratio.

  4. If necessary, multiply ratios to get whole numbers.

• Relationship: , where

• Example: A compound contains 40% C, 6.7% H, and 53.3% O. The empirical formula is CH2O.

Types of Chemical Reactions

Recognizing different types of reactions is crucial for predicting products and understanding chemical processes.

• Precipitation Reactions: Reactions where an insoluble solid (precipitate) forms when two solutions are mixed.

• Neutralization Reactions: Acid-base reactions that produce water and a salt.

• Redox Reactions: Reactions involving the transfer of electrons between species.

• Example: Mixing solutions of AgNO3 and NaCl forms a white precipitate of AgCl.

Molecular, Ionic, and Net-Ionic Equations

Chemical equations can be written in different forms to show varying levels of detail about the species involved.

• Molecular Equation: Shows all reactants and products as compounds.

• Ionic Equation: Shows all strong electrolytes as ions.

• Net-Ionic Equation: Shows only the species that actually change during the reaction.

• Example:

  • Molecular:

  • Ionic:

  • Net-Ionic:

Naming and Writing Chemical Formulas of Acids

Acids are compounds that release hydrogen ions (H+) in solution. Naming depends on the anion present.

• Binary Acids: Contain hydrogen and one other element. Named as "hydro-root-ic acid" (e.g., HCl: hydrochloric acid).

• Oxoacids: Contain hydrogen, oxygen, and another element. Naming depends on the polyatomic ion:

  • -ate → -ic acid (e.g., H2SO4: sulfuric acid)

  • -ite → -ous acid (e.g., H2SO3: sulfurous acid)

• Example: HNO3 is nitric acid; HNO2 is nitrous acid.

Oxidation Numbers and Redox Identification

Assigning oxidation numbers helps identify which elements are oxidized or reduced in a reaction.

• Oxidation Number: The hypothetical charge an atom would have if all bonds were ionic.

• Rules:

  1. Elements in their standard state: 0

  2. Monatomic ions: charge of the ion

  3. Oxygen: usually -2

  4. Hydrogen: +1 with nonmetals, -1 with metals

  5. Sum of oxidation numbers equals the charge of the molecule/ion

• Redox Terms:

  • Oxidized: Species that loses electrons (increase in oxidation number)

  • Reduced: Species that gains electrons (decrease in oxidation number)

  • Oxidizing Agent: Causes oxidation; is itself reduced

  • Reducing Agent: Causes reduction; is itself oxidized

• Example: In , Na is oxidized, Cl2 is reduced.

Activity Series and Predicting Redox Reactions

The activity series ranks metals by their ability to be oxidized. A metal higher in the series will displace a metal ion lower in the series from solution.

• Use: Predict if a single displacement reaction will occur.

• Example: Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s) (reaction occurs because Zn is above Cu).

Molarity, Dilution, and Titration Calculations

Molarity (M) is a measure of concentration, defined as moles of solute per liter of solution.

• Formula:

• Dilution:

• Titration: Use stoichiometry to relate volumes and concentrations of reactants.

• Example: To prepare 250 mL of 0.5 M NaCl from 2.0 M NaCl: mL

Wavelength and Frequency Calculations

Light exhibits wave properties, and its wavelength () and frequency () are related by the speed of light ().

• Relationship:

• Where:

  • = speed of light ( m/s)

  • = wavelength (m)

  • = frequency (Hz)

• Example: If nm, Hz

Quantum Numbers

Quantum numbers describe the properties of atomic orbitals and the electrons in them.

• Principal Quantum Number (): Energy level (n = 1, 2, 3, ...)

• Angular Momentum Quantum Number (): Shape of orbital (l = 0 to n-1)

• Magnetic Quantum Number (): Orientation of orbital ( to )

• Spin Quantum Number (): Electron spin ( or )

• Example: For a 3p electron: , , , or

Additional info: Some topics (wavelength/frequency, quantum numbers) are noted as "to be discussed" and may require further study from the textbook or lecture notes for full mastery.