CHM1045 Exam 2 Study Guide: Chemical Reactions, Solutions, and Gases

Stoichiometry and Chemical Reactions

Stoichiometry and Mole Ratios

Stoichiometry is the quantitative relationship between reactants and products in a chemical reaction. It allows chemists to predict the amounts of substances consumed and produced.

• Mole ratios are derived from the coefficients in a balanced chemical equation and are used to convert between grams, moles, and particles of reactants and products.

• Typical conversion sequence: grams A → moles A → moles B → grams B.

• Example: In the reaction $2H_2 + O_2 \rightarrow 2H_2O$, the mole ratio between $H_2$ and $H_2O$ is 1:1.

Limiting Reactant, Theoretical Yield, and Percent Yield

The limiting reactant is the substance that is completely consumed first, thus determining the maximum amount of product formed.

• Theoretical yield is the maximum amount of product that can be formed from the limiting reactant.

• Percent yield is calculated as:

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

• Example: If the theoretical yield is 10 g and the actual yield is 8 g, percent yield is $\frac{8}{10} \times 100\% = 80\%$.

Solutions and Aqueous Chemistry

Solute vs. Solvent and Aqueous Solutions

A solution is a homogeneous mixture of two or more substances. The solute is the substance dissolved, and the solvent is the substance doing the dissolving.

• Aqueous solution: A solution where water is the solvent.

• Example: Salt water is an aqueous solution with NaCl as the solute and water as the solvent.

Molarity and Solution Calculations

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

• $M = \frac{\text{mol}}{\text{L}}$

• Used as a conversion factor in stoichiometry and dilution calculations.

• Dilution equation: $M_1V_1 = M_2V_2$

• Example: To dilute 1.0 L of 2.0 M solution to 0.5 M, use $2.0 \times 1.0 = 0.5 \times V_2 \Rightarrow V_2 = 4.0$ L.

Electrolytes and Solubility Rules

Electrolytes are substances that dissociate into ions in solution.

• Strong electrolytes: Completely dissociate (e.g., NaCl, strong acids/bases).

• Weak electrolytes: Partially dissociate (e.g., weak acids/bases).

• Nonelectrolytes: Do not dissociate (e.g., sugar).

• Solubility rules help predict whether a compound will dissolve in water.

Precipitation Reactions and Ionic Equations

Precipitation reactions occur when two solutions are mixed and an insoluble product (precipitate) forms.

• Molecular equation: Shows all compounds as intact molecules.

• Complete ionic equation: Shows all strong electrolytes as ions.

• Net ionic equation: Shows only the ions and molecules directly involved in the reaction.

• Example: Mixing $AgNO_3$ and $NaCl$ forms $AgCl$ precipitate.

Acid-Base Reactions and Titrations

Acid-base reactions involve the transfer of a proton ($H^+$) from an acid to a base.

• Neutralization: Acid reacts with base to form water and a salt.

• Titration: A technique to determine the concentration of an acid or base using a solution of known concentration.

• Example: $HCl + NaOH \rightarrow NaCl + H_2O$

Gas Evolution and Redox Reactions

Gas evolution reactions produce a gas as a product. Redox reactions involve the transfer of electrons.

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• Assign oxidation numbers using rules (see below).

• Identify oxidizing and reducing agents.

• Example: Combustion of methane: $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$

Properties and Laws of Gases

Gas Properties and Laws

Gases have unique properties and obey several fundamental laws.

• Boyle's Law: $P_1V_1 = P_2V_2$ (pressure and volume inversely related)

• Charles's Law: $\frac{V_1}{T_1} = \frac{V_2}{T_2}$ (volume and temperature directly related)

• Avogadro's Law: $\frac{V_1}{n_1} = \frac{V_2}{n_2}$ (volume and moles directly related)

• Ideal Gas Law: $PV = nRT$

• Gas constant: $R = 0.0821 \text{ L} \cdot \text{atm} / \text{mol} \cdot \text{K}$

• STP: Standard Temperature and Pressure (0°C, 1 atm); 1 mol gas = 22.4 L

Gas Density and Molar Mass

The density of a gas at STP can be calculated using its molar mass and molar volume.

• $\text{Density} = \frac{\text{Molar Mass}}{\text{Molar Volume}}$

• Example: $\text{Density of O}_2$ at STP: $\frac{32.00 \text{ g/mol}}{22.4 \text{ L}} = 1.43 \text{ g/L}$

Mixtures of Gases and Partial Pressures

In a mixture, each gas exerts a partial pressure proportional to its mole fraction.

• Dalton's Law: $P_{\text{total}} = P_1 + P_2 + P_3 + \ldots$

• Mole fraction: $\chi_a = \frac{n_a}{n_{\text{total}}}$

• Partial pressure: $P_a = \chi_a P_{\text{total}}$

Kinetic Molecular Theory

This theory explains the behavior of gases based on the motion of their particles.

• Gas particles are in constant, random motion.

• Collisions are elastic; energy is conserved.

• Pressure results from collisions with container walls.

Reference Tables and Rules

Solubility Rules

Solubility rules help predict whether a compound will dissolve in water.

• Group IA salts, $NH_4^+$, $NO_3^-$, $C_2H_3O_2^-$ are soluble.

• $Cl^-$, $Br^-$, $I^-$ salts are soluble except with $Ag^+$, $Pb^{2+}$, $Hg_2^{2+}$.

• $SO_4^{2-}$ salts are soluble except with $Ba^{2+}$, $Pb^{2+}$, $Ca^{2+}$, $Sr^{2+}$.

• $CO_3^{2-}$, $S^{2-}$, $PO_4^{3-}$, $OH^-$ salts are insoluble except with Group IA, $NH_4^+$, $NO_3^-$, $C_2H_3O_2^-$.

Rules for Assigning Oxidation Numbers

Oxidation numbers are assigned to atoms in compounds to track electron transfer in redox reactions.

• Sum of oxidation numbers equals the charge of the molecule or ion.

• Free elements: 0

• Group 1A, 2A, Al: +1, +2, +3

• H: +1; F: -1

• O: -2

• Group 7A: -1; Group 6A: -2; Group 5A: -3

• When rules conflict, use the lower number and ignore the conflicting rule.

Periodic Table of the Elements

The periodic table organizes elements by increasing atomic number and groups elements with similar properties together.

• Groups (columns) indicate similar chemical properties.

• Periods (rows) indicate increasing atomic number.

• Metals, nonmetals, and metalloids are classified based on their properties.

Common Polyatomic Ions

Polyatomic ions are ions composed of two or more atoms covalently bonded, carrying a net charge. They are frequently encountered in chemical reactions and formulas.

Name	Formula	Name	Formula
Acetate	C2H3O2-	Hypochlorite	ClO-
Carbonate	CO32-	Chlorite	ClO2-
Hydrogen carbonate (bicarbonate)	HCO3-	Chlorate	ClO3-
Hydroxide	OH-	Perchlorate	ClO4-
Nitrite	NO2-	Permanaganate	MnO4-
Nitrate	NO3-	Sulfite	SO32-
Chromate	CrO42-	Hydrogen sulfite (bisulfite)	HSO3-
Dichromate	Cr2O72-	Sulfate	SO42-
Phosphate	PO43-	Hydrogen sulfate (bisulfate)	HSO4-
Hydrogen phosphate	HPO42-	Cyanide	CN-
Dihydrogen phosphate	H2PO4-	Peroxide	O22-
Ammonium	NH4+