General Chemistry Midterm Exam Study Guide

Gases and Gas Laws

Calculating Moles of Gas Using the Ideal Gas Law

The Ideal Gas Law relates the pressure, volume, temperature, and number of moles of a gas. It is commonly used to determine the amount of gas present in a sample.

• Formula: $PV = nRT$

• Variables: P = pressure (in kPa or atm), V = volume (in L), n = moles, R = gas constant (8.314 J/mol·K), T = temperature (in K)

• Example: To find the moles of NO(g) in a 275.0 mL container at 32.5°C and 75 kPa, first convert volume to liters and temperature to Kelvin, then solve for n.

Partial Pressures in Gas Mixtures

In a mixture of gases, each gas exerts a pressure called its partial pressure. Dalton's Law states that the total pressure is the sum of the partial pressures of each gas.

• Formula: $P_{i} = \frac{n_{i}RT}{V}$

• Example: For a vessel containing 2.0 mol H2 and 1.0 mol N2 in 22.4 L at 298 K, calculate each gas's partial pressure using the formula above.

Intermolecular Forces

Types of Intermolecular Interactions

Intermolecular forces are attractions between molecules that affect physical properties like boiling and melting points.

• Hydrogen Bonding: Strong interaction when H is bonded to N, O, or F (e.g., H2O, NH2OH).

• Dipole-Dipole: Occurs between polar molecules (e.g., NH2OH).

• London Dispersion: Present in all molecules, especially nonpolar ones (e.g., CBr4).

Thermochemistry

Calculating Heat Required for Temperature Change

The amount of heat needed to change the temperature of a substance is calculated using its heat capacity.

• Formula: $q = mC\Delta T$

• Variables: q = heat (J), m = mass (kg), C = heat capacity (J/kg·K), $\Delta T$ = change in temperature (K)

• Example: To heat 300.0 g of water from 20.0°C to 100.0°C, use the formula above.

Enthalpy of Formation and Reaction Enthalpy

The enthalpy of formation is the heat change when one mole of a compound forms from its elements. The reaction enthalpy can be used to assess fuel efficiency.

• Formula: $\Delta H_{rxn} = \sum \Delta H_{f,products} - \sum \Delta H_{f,reactants}$

• Application: Used to evaluate TNT as a rocket fuel.

Spontaneity and Thermodynamics

Criteria for Spontaneous Reactions

A reaction is spontaneous at constant pressure and temperature if the change in Gibbs free energy is negative.

• Formula: $\Delta G = \Delta H - T\Delta S$

• Criteria: $\Delta G < 0$ for spontaneity

Estimating Temperature for Spontaneity

To determine the temperature at which a reaction becomes spontaneous, set $\Delta G = 0$ and solve for T.

• Formula: $T = \frac{\Delta H}{\Delta S}$

Phase Changes and Vapor Pressure

Clapeyron-Clausius Equation

The Clapeyron-Clausius equation relates vapor pressure and temperature, useful for finding boiling points.

• Formula: $\ln \frac{P_2}{P_1} = -\frac{\Delta H_{vap}}{R} \left( \frac{1}{T_2} - \frac{1}{T_1} \right)$

• Application: Used to estimate the boiling point of dimethyl ether at 1 atm.

Solutions and Solubility

Solvent Choice and Solubility

The solubility of a substance depends on the nature of the solvent. "Like dissolves like" is a guiding principle.

• Water: Polar solvent, dissolves ionic and polar substances (e.g., KCl, CH3COOH).

• Benzene: Nonpolar solvent, dissolves nonpolar substances (e.g., CCl4).

Molality Calculation

Molality is a concentration unit defined as moles of solute per kilogram of solvent.

• Formula: $m = \frac{\text{moles of solute}}{\text{kg of solvent}}$

• Example: Dissolving 25.0 g NaCl in 500.0 g water.

Bonus: Rocket Fuel Thermodynamics

Balancing Chemical Equations and Thermodynamic Feasibility

Balancing equations is essential for stoichiometric calculations. Thermodynamic data at high temperatures can be used to assess fuel viability.

• Balance the equation: Ensure equal numbers of atoms on both sides.

• Assess spontaneity: Use $\Delta G$ at 2000 K to determine if the reaction is spontaneous.

Reference: Periodic Table of the Elements

Periodic Table Overview

The periodic table organizes elements by atomic number, properties, and groups. It is essential for understanding chemical behavior and predicting reactions.

• Groups: Columns with similar properties (e.g., alkali metals, halogens).

• Periods: Rows indicating energy levels.

• States: Elements are classified as solid, liquid, or gas at room temperature.