Stoichiometry and Chemical Equations

Balancing Chemical Equations

Balancing chemical equations is essential for representing chemical reactions accurately. Each side of the equation must have the same number of atoms of each element.

• Balanced Equation: Shows the reactants and products with coefficients indicating the number of molecules or moles.

• Example: Combustion of ethanol:

• Sum of Coefficients: Add all coefficients in the balanced equation for certain problems.

Types of Chemical Reactions

Chemical reactions can be classified into several types, including decomposition, synthesis, single replacement, and double replacement.

• Decomposition Reaction: A single compound breaks down into two or more simpler substances.

• Example: (decomposition of hydrogen peroxide)

Stoichiometric Calculations

Stoichiometry involves calculations based on the relationships between reactants and products in a chemical reaction.

• Mole-to-Mass Conversion:

• Atoms in a Sample: Use Avogadro's number ( atoms/mol) to convert between moles and number of atoms.

• Example: How many gold atoms are in 0.002582 g of gold?

Molecular and Empirical Formulas

Empirical vs. Molecular Formula

The empirical formula shows the simplest whole-number ratio of elements in a compound, while the molecular formula shows the actual number of atoms of each element in a molecule.

• Empirical Formula: Simplest ratio (e.g., CH2O for glucose)

• Molecular Formula: Actual composition (e.g., C6H12O6 for glucose)

• Calculation: , where

Atoms, Molecules, and Moles

Avogadro's Number and Mole Concept

The mole is a fundamental unit in chemistry representing entities (atoms, molecules, ions).

• Number of Molecules:

• Mass of a Sample:

• Example: How much does 0.729 mol of caffeine weigh?

Limiting Reactant and Yield

Limiting Reactant

The limiting reactant is the reactant that is completely consumed first, limiting the amount of product formed.

• Calculation: Compare the mole ratio of reactants to determine which is limiting.

• Theoretical Yield: Maximum amount of product possible from the limiting reactant.

• Percent Yield:

Reactions in Aqueous Solution

Electrolytes and Solution Types

Substances dissolved in water can be classified as strong electrolytes, weak electrolytes, or non-electrolytes based on their ability to conduct electricity.

• Strong Electrolyte: Completely dissociates in water (e.g., NaCl, HCl)

• Weak Electrolyte: Partially dissociates (e.g., CH3COOH)

• Non-Electrolyte: Does not dissociate (e.g., sugar)

Solubility Guidelines

Solubility rules help predict whether an ionic compound will dissolve in water.

Net Ionic Equations and Precipitation

Net Ionic Equations

Net ionic equations show only the species that participate in the reaction, omitting spectator ions.

• Steps:

  1. Write the balanced molecular equation.

  2. Write the complete ionic equation.

  3. Cancel spectator ions to get the net ionic equation.

• Example: Reaction between Hg2(NO3)2 and Sr(OH)2

Oxidation-Reduction (Redox) Reactions

Oxidation Numbers

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

• Rules:

  • Elemental form: 0

  • Oxygen: usually -2

  • Hydrogen: +1 (with nonmetals), -1 (with metals)

  • Alkali metals: +1

  • Alkaline earth metals: +2

• Example: Oxidation number of O in O2 is 0; C in Na2CO3 is +4

Concentration and Solution Calculations

Molarity

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

• Formula:

• Example: Calculate the concentration after dilution.

Titration

Titration is a technique to determine the concentration of a solution by reacting it with a solution of known concentration.

• Equivalence Point: The point at which the reaction is complete.

• Calculation: (for monoprotic acids/bases)

• Example: Titration of phosphoric acid with NaOH.

Periodic Table and Atomic Masses

Periodic Table Usage

The periodic table provides atomic numbers, symbols, and atomic masses for elements, which are essential for stoichiometric calculations.

• Atomic Mass: Used to calculate molar mass for conversions between grams and moles.

• Example: Molar mass of Fe is 55.85 g/mol.

Fundamental Constants and Conversion Factors

Key Constants

Several constants are frequently used in general chemistry calculations.

Sample Problems and Applications

Representative Problems

• Calculate the mass of a given number of moles of a substance.

• Determine the number of atoms or molecules in a sample.

• Identify the limiting reactant and calculate theoretical yield.

• Write and balance chemical equations for reactions.

• Classify substances as strong, weak, or non-electrolytes.

• Use solubility rules to predict precipitation reactions.

• Calculate molarity and perform titration calculations.

• Assign oxidation numbers and identify redox reactions.

Additional info: These study notes are based on a set of general chemistry exam and homework questions, including supplemental tables and periodic table data. All content is relevant to introductory college-level general chemistry.