Chapter 3: Chemical Reactions and Reaction Stoichiometry

3.1 Chemical Equations

Chemical equations are symbolic representations of chemical reactions, showing the transformation of reactants into products. They are fundamental tools for chemists to communicate and analyze chemical changes.

• Reactants and Products: Reactants are the starting materials (left side), and products are the substances formed (right side). An arrow (→) separates them.

• Plus Sign (+): Used to separate multiple reactants or products.

• Balancing Equations: Follows the Law of Conservation of Mass. Balance by adjusting coefficients (numbers in front of formulas), not subscripts (numbers within formulas).

• Phases: Indicated in parentheses: (s) for solid, (l) for liquid, (g) for gas, (aq) for aqueous solution.

Example: The reaction of hydrogen and oxygen to form water:

3.2 Simple Patterns of Chemical Reactivity

Chemical reactions can be classified into several types based on their patterns. Understanding these helps predict products and write equations.

• Combination Reactions: Two or more substances combine to form one product. General form:

• Decomposition Reactions: One substance breaks down into two or more products. General form:

• Combustion Reactions: Rapid reactions with oxygen that produce a flame. Hydrocarbons typically yield CO2 and H2O. General form:

Example: Combustion of methane:

3.3 Formula Weight and Percent Composition

Formula weight and molecular weight are measures of the mass of a chemical formula or molecule, respectively. Percent composition quantifies the mass percentage of each element in a compound.

• Formula Weight (FW): Sum of atomic weights of all atoms in a formula (in amu).

• Molecular Weight (MW): For molecules, the sum of atomic weights of all atoms in the molecule.

• Percent Composition:

Example: For glucose (), MW = amu.

3.4 Avogadro’s Number and the Mole

The mole is a counting unit for atoms, molecules, or ions, defined by Avogadro’s number. Molar mass connects the microscopic and macroscopic worlds.

• Avogadro’s Number: particles per mole.

• Molar Mass: Mass of 1 mole of a substance (g/mol), numerically equal to formula weight in amu.

• Mole Relationships: The number of atoms/molecules in a sample can be calculated using Avogadro’s number.

Example: 3 grams of copper () contains atoms.

3.5 Empirical and Molecular Formulas

Empirical formulas show the simplest whole-number ratio of elements in a compound. Molecular formulas show the actual number of atoms of each element in a molecule.

• Finding Empirical Formula:

  1. Convert percent composition to grams (assume 100 g sample).

  2. Convert grams to moles for each element.

  3. Divide by the smallest number of moles to get the ratio.

• Molecular Formula: , where

Example: If empirical formula is CH and molar mass is 78 g/mol, molecular formula is .

Combustion Analysis

Combustion analysis is used to determine the empirical formula of compounds containing C, H, and O by burning a known mass and measuring the masses of CO2 and H2O produced.

• Mass of C: Determined from mass of CO2 produced.

• Mass of H: Determined from mass of H2O produced.

• Mass of O: Determined by subtracting the masses of C and H from the total mass of the sample.

3.6 Quantitative Information from Balanced Equations

Balanced equations provide the mole ratios needed for stoichiometric calculations, allowing conversion between masses of reactants and products.

• Stoichiometry: The calculation of quantities of reactants and products using balanced equations.

• Mole Ratio: Derived from coefficients in the balanced equation.

Example: To find grams of water produced from 1.00 g glucose, convert grams to moles, use the mole ratio, then convert moles to grams.

Heat and Stoichiometry

Heat changes in reactions are indicated by the Greek symbol delta (Δ) over the reaction arrow. The amount of heat is related to the stoichiometry of the reaction, but is not shown in the balanced equation.

3.7 Limiting Reactants and Yield

In reactions, the limiting reactant is the one that is completely consumed first, thus limiting the amount of product formed. Theoretical yield is the maximum possible amount of product, while actual yield is what is actually obtained.

• Limiting Reactant: The reactant present in the smallest stoichiometric amount.

• Excess Reactant: The reactant that remains after the reaction is complete.

• Theoretical Yield: Maximum amount of product predicted by stoichiometry.

• Percent Yield: