General Chemistry Exam 2 Study Guide: Chemical Reactions, Gases, Stoichiometry, and Solution Chemistry

Chemical Reactions and Stoichiometry

Types of Chemical Reactions

Chemical reactions involve the transformation of reactants into products, often accompanied by energy changes. Understanding reaction types and balancing equations is fundamental in general chemistry.

• Combustion Reaction: A reaction where a substance reacts with oxygen to produce energy, typically forming CO2 and H2O. Example: $2C_8H_{18} + 25O_2 \rightarrow 16CO_2 + 18H_2O$

• Precipitation Reaction: Occurs when two aqueous solutions combine to form an insoluble solid (precipitate). Example: $Pb(NO_3)_2(aq) + 2KI(aq) \rightarrow PbI_2(s) + 2KNO_3(aq)$

• Redox Reaction: Involves the transfer of electrons between species, changing their oxidation states. Example: $2H_2O_2(aq) \rightarrow 2H_2O(l) + O_2(g)$

Balancing Chemical Equations: Ensure the same number of atoms of each element on both sides of the equation.

Stoichiometry and Mole Relationships

Stoichiometry is the quantitative study of reactants and products in chemical reactions. It uses balanced equations to relate moles, masses, and volumes.

• Mole Ratio: The ratio of moles of reactants and products as indicated by the coefficients in a balanced equation.

• Limiting Reactant: The reactant that is completely consumed first, limiting the amount of product formed.

• Percent Yield: $\text{Percent Yield} = \frac{\text{Actual Yield}}{\text{Theoretical Yield}} \times 100$

Example: If 20.0 g of PbI2 is obtained and the theoretical yield is 23.0 g, percent yield is $\frac{20.0}{23.0} \times 100 = 87.0\%$.

Gases and Gas Laws

Properties of Gases

Gases are characterized by their ability to expand, compress, and fill containers. Their behavior is described by several laws.

• Boyle's Law: At constant temperature, the pressure and volume of a gas are inversely related. $P_1V_1 = P_2V_2$

• Dalton's Law of Partial Pressures: The total pressure of a mixture of gases is the sum of the partial pressures of each gas. $P_{\text{total}} = P_1 + P_2 + P_3 + \ldots$

• Ideal Gas Law: Relates pressure, volume, temperature, and moles of a gas. $PV = nRT$

Example: Calculating the partial pressure of a gas in a mixture using mole fractions: $P_i = X_i \times P_{\text{total}}$

Solutions and Aqueous Chemistry

Concentration Units

Concentration expresses the amount of solute in a given amount of solvent or solution.

• Molarity (M): $M = \frac{\text{moles of solute}}{\text{liters of solution}}$

• Preparation of Solutions: Dilution calculations use $M_1V_1 = M_2V_2$

Example: To prepare 200.0 mL of 0.120 M HCl from 1.20 M HCl, use $V_1 = \frac{M_2V_2}{M_1}$

Solubility and Precipitation

Solubility rules determine which compounds dissolve in water. Insoluble compounds form precipitates.

• Insoluble in Water: Example: AgNO3 is soluble, but PbI2 is insoluble.

• Net Ionic Equation: Shows only the species that participate in the reaction. Example: $Pb^{2+}(aq) + 2I^-(aq) \rightarrow PbI_2(s)$

Atomic Structure and Periodic Table

Empirical and Molecular Formulas

Formulas represent the composition of compounds.

• Empirical Formula: Shows the simplest whole-number ratio of atoms in a compound.

• Molecular Formula: Shows the actual number of atoms of each element in a molecule.

Periodic Table and Activity Series

The periodic table organizes elements by atomic number and properties. The activity series ranks metals by their reactivity.

• Periodic Table: Used to identify elements, atomic numbers, and chemical symbols.

• Activity Series: Predicts which metals will displace others in reactions.

Physical Constants and Reference Data

Selected Physical Constants

Physical constants are used in calculations throughout chemistry.

Constant	Symbol	Value
Atomic mass constant	mu	1.660 539 021 × 10−27 kg
Avogadro's constant	NA	6.022 141 29 × 1023 mol−1
Boltzmann constant	k	1.380 6488 × 10−23 J K−1
Ideal Gas Constant	R	0.082 057 L atm mol−1 K−1
Molar volume, ideal gas (0°C, 1 atm)	Vm	22.414 L mol−1

Lab Techniques and Calculations

Identifying Spectator Ions and Writing Ionic Equations

In precipitation and redox reactions, it is important to identify spectator ions and write net ionic equations.

• Spectator Ions: Ions that do not participate directly in the chemical change.

• Net Ionic Equation: Shows only the ions and molecules directly involved in the reaction.

Calculating Moles and Limiting Reactants

Use stoichiometry to determine the amount of product formed and identify the limiting reactant.

• Example: For $2N_2O_5(g) \rightarrow 4NO_2(g) + O_2(g)$, calculate moles of NO2 formed from a given mass of N2O5.

Summary Table: Key Concepts and Formulas

Concept	Formula/Definition	Example/Application
Boyle's Law	$P_1V_1 = P_2V_2$	Pressure-volume relationship
Ideal Gas Law	$PV = nRT$	Calculate moles or volume of a gas
Molarity	$M = \frac{\text{mol solute}}{\text{L solution}}$	Solution preparation
Percent Yield	$\frac{\text{Actual Yield}}{\text{Theoretical Yield}} \times 100$	Lab calculations
Empirical Formula	Simplest ratio of atoms	CH2O for glucose

Additional info: These notes synthesize exam questions, worked problems, and reference tables from the provided file, covering topics from chemical reactions, stoichiometry, gas laws, solution chemistry, and atomic structure, all of which are core to a General Chemistry college course.