General Chemistry Exam 2 Study Guide

Electrolytes and Solution Chemistry

Strong Electrolytes

Electrolytes are substances that dissociate into ions when dissolved in water, allowing the solution to conduct electricity. Strong electrolytes completely dissociate in solution.

• Examples of strong electrolytes: Strong acids (e.g., HCl), strong bases (e.g., NaOH), and most soluble salts (e.g., NaCl).

• Weak electrolytes only partially dissociate (e.g., acetic acid).

• Nonelectrolytes do not dissociate (e.g., sugar, ethanol).

Application: Identifying strong electrolytes is important for predicting solution conductivity and reaction types.

Solution Concentration Calculations

To determine the concentration of a solution after dilution or reaction, use the relationship:

$M_1V_1 = M_2V_2$

• Molarity (M): Moles of solute per liter of solution.

• Example: If 25.0 mL of 0.500 M NaOH neutralizes 250.0 mL of HCl, the concentration of HCl can be found using stoichiometry and the above formula.

Redox Reactions and Activity Series

Redox (Reduction-Oxidation) Reactions

Redox reactions involve the transfer of electrons between species. Oxidation is the loss of electrons, while reduction is the gain of electrons.

• Oxidizing agent: Species that is reduced (gains electrons).

• Reducing agent: Species that is oxidized (loses electrons).

Example: In the reaction $K_2Cr_2O_7 + H_2SO_4 + SO_2 \rightarrow Cr_2(SO_4)_3 + K_2SO_4 + H_2O$, identify which element is reduced by tracking changes in oxidation numbers.

Activity Series of Metals

The activity series ranks metals by their tendency to lose electrons (be oxidized). A metal higher in the series will displace a metal lower in the series from solution.

Metal	Symbol	Ion Formed	Electrons Lost
Lithium	Li(s)	Li+(aq)	1e-
Potassium	K(s)	K+(aq)	1e-
Calcium	Ca(s)	Ca2+(aq)	2e-
Magnesium	Mg(s)	Mg2+(aq)	2e-
Aluminum	Al(s)	Al3+(aq)	3e-
Hydrogen	H2(g)	2H+(aq)	2e-
Copper	Cu(s)	Cu2+(aq)	2e-
Silver	Ag(s)	Ag+(aq)	1e-
Gold	Au(s)	Au3+(aq)	3e-

Additional info: The full table includes more metals; those higher up are more easily oxidized.

Thermochemistry

Enthalpy, Heat, and Calorimetry

Enthalpy ($\Delta H$) is the heat content of a system at constant pressure. Calorimetry measures heat changes in chemical reactions.

• Heat ($q$): $q = m c \Delta T$ where $m$ is mass, $c$ is specific heat, and $\Delta T$ is temperature change.

• Phase changes: $q = m L_f$ for melting (fusion), where $L_f$ is the heat of fusion.

• Combustion reactions: Use standard enthalpies of formation: $\Delta H_{rxn} = \sum \Delta H_f^{\circ} (products) - \sum \Delta H_f^{\circ} (reactants)$

Example: Calculating the enthalpy of combustion for hexane using standard enthalpy values.

Gases and Gas Laws

Gas Law Calculations

The behavior of gases is described by several laws. The Ideal Gas Law is:

$PV = nRT$

• P: Pressure (atm)

• V: Volume (L)

• n: Moles of gas

• R: Gas constant (0.0821 L·atm/mol·K)

• T: Temperature (K)

Application: Used to calculate changes in gas volume, pressure, or temperature under different conditions.

Atomic Structure and Quantum Numbers

Quantum Numbers

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

• Principal quantum number (n): Energy level (n = 1, 2, 3, ...)

• Angular momentum quantum number (l): Shape of orbital (l = 0 to n-1)

• Magnetic quantum number (ml): Orientation (ml = -l to +l)

• Spin quantum number (ms): Electron spin (+1/2 or -1/2)

Example: For a 4d electron: n = 4, l = 2, ml = -2 to +2, ms = ±1/2.

Electron Configurations

Electron configuration shows the distribution of electrons among orbitals. Use the Aufbau principle, Hund's rule, and the Pauli exclusion principle.

• Example: Vanadium (V): [Ar]4s23d3

• Unpaired electrons: Count the number of electrons in singly-occupied orbitals.

• Filling energy levels: The fourth energy level (n=4) can hold up to 32 electrons (2 in 4s, 6 in 4p, 10 in 4d, 14 in 4f).

Light and Electromagnetic Radiation

Properties of Light

Light exhibits both wave and particle properties. The energy of a photon is given by:

$E = h \nu = \dfrac{hc}{\lambda}$

• h: Planck's constant ($6.626 \times 10^{-34}$ J·s)

• c: Speed of light ($3.00 \times 10^8$ m/s)

• \nu: Frequency (Hz)

• \lambda: Wavelength (m)

Electromagnetic spectrum: Ranges from radio waves (lowest frequency, longest wavelength) to gamma rays (highest frequency, shortest wavelength).

• Visible light: 400–700 nm

• Order of increasing frequency: Radio < Infrared < Visible < Ultraviolet < X-rays < Gamma rays

Atomic Orbitals and Nodal Planes

Atomic Orbitals

Atomic orbitals are regions in space where electrons are likely to be found. Each type of orbital has a characteristic shape and number of nodal planes.

• 2s and 3s orbitals: Spherical, with (n-1) radial nodes.

• 2p orbitals: Dumbbell-shaped, with one nodal plane.

• 3d orbitals: Four have cloverleaf shapes, one is donut-shaped; each has two nodal planes.

Coordinate system: Used to show orientation of orbitals in space.

Summary Table: Key Concepts

Concept	Key Formula/Fact	Example/Application
Electrolytes	Strong: Complete dissociation	NaCl in water
Gas Law	$PV = nRT$	Calculate gas volume at STP
Enthalpy	$\Delta H_{rxn} = \sum \Delta H_f^{\circ} (products) - \sum \Delta H_f^{\circ} (reactants)$	Combustion of hexane
Photon Energy	$E = h\nu = \dfrac{hc}{\lambda}$	Photoelectric effect
Electron Configuration	Aufbau principle	[Ar]4s23d3 for V