Study Notes: Chemical Reactions and Quantities (General Chemistry)

Chapter 7: Chemical Reactions and Quantities

Overview

This chapter focuses on the fundamental concepts of chemical reactions and the quantitative relationships involved. Students will learn to write and balance chemical equations, classify reaction types, and perform calculations involving moles, masses, and energy changes.

Writing and Balancing Chemical Reactions

Writing and balancing chemical equations is essential for representing chemical changes accurately.

• Chemical Equation: A symbolic representation of a chemical reaction showing reactants and products.

• Balancing: Ensures the same number of each atom on both sides of the equation, reflecting the Law of Conservation of Mass.

• Example:

  • Unbalanced: $\mathrm{H_2 + O_2 \rightarrow H_2O}$

  • Balanced: $2\mathrm{H_2} + \mathrm{O_2} \rightarrow 2\mathrm{H_2O}$

Types of Chemical Reactions

Chemical reactions are classified based on the changes occurring among reactants and products.

• Synthesis (Combination): Two or more substances combine to form one product.

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

• Single Replacement: One element replaces another in a compound.

• Double Replacement: Exchange of ions between two compounds.

• Combustion: A substance reacts with oxygen, releasing energy.

• Example: $\mathrm{2Na + Cl_2 \rightarrow 2NaCl}$ (Synthesis)

Oxidation and Reduction (Redox Reactions)

Redox reactions involve the transfer of electrons between substances.

• Oxidation: Loss of electrons by a substance.

• Reduction: Gain of electrons by a substance.

• Oxidizing Agent: Causes oxidation by accepting electrons.

• Reducing Agent: Causes reduction by donating electrons.

• Example: $\mathrm{Zn + Cu^{2+} \rightarrow Zn^{2+} + Cu}$

Avogadro's Number and the Mole Concept

The mole is a fundamental unit for counting particles in chemistry.

• Avogadro's Number: $6.022 \times 10^{23}$ particles per mole.

• Mole: The amount of substance containing Avogadro's number of entities (atoms, molecules, ions).

• Example: 1 mole of $\mathrm{H_2O}$ contains $6.022 \times 10^{23}$ molecules.

Molar Mass and Chemical Formulas

Molar mass is used to convert between grams and moles of a substance.

• Molar Mass ($M$): The mass of one mole of a substance, expressed in g/mol.

• Calculation: Add the atomic masses of all atoms in the formula.

• Example: Molar mass of $\mathrm{H_2O}$: $2 \times 1.01 + 16.00 = 18.02$ g/mol

Stoichiometry: Mole-Mass Relationships

Stoichiometry involves quantitative relationships in chemical reactions.

• Mole Ratio: Ratio of moles of reactants and products from the balanced equation.

• Conversions: Use mole ratios to convert between grams, moles, and number of particles.

• Example: For $2\mathrm{H_2} + \mathrm{O_2} \rightarrow 2\mathrm{H_2O}$, the mole ratio of $\mathrm{H_2}$ to $\mathrm{H_2O}$ is 1:1.

Limiting Reactant and Theoretical Yield

The limiting reactant determines the maximum amount of product formed in a reaction.

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

• Theoretical Yield: The maximum amount of product possible, calculated from the limiting reactant.

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

• Example: If 5.0 g of $\mathrm{A}$ and 10.0 g of $\mathrm{B}$ react, and $\mathrm{A}$ is limiting, calculate product based on $\mathrm{A}$.

Energy Changes in Chemical Reactions

Chemical reactions can absorb or release energy, classified as endothermic or exothermic.

• Exothermic Reaction: Releases energy to the surroundings (e.g., combustion).

• Endothermic Reaction: Absorbs energy from the surroundings (e.g., photosynthesis).

• Example: $\mathrm{CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O}$ (exothermic)

Summary Table: Key Concepts in Chemical Reactions and Quantities

Concept	Definition	Example
Chemical Equation	Symbolic representation of a chemical reaction	$2\mathrm{H_2} + \mathrm{O_2} \rightarrow 2\mathrm{H_2O}$
Mole	Amount containing $6.022 \times 10^{23}$ entities	1 mole $\mathrm{Na}$ atoms
Molar Mass	Mass of one mole of substance (g/mol)	$\mathrm{H_2O}$: 18.02 g/mol
Limiting Reactant	Reactant consumed first, limits product	5.0 g $\mathrm{A}$, 10.0 g $\mathrm{B}$; $\mathrm{A}$ is limiting
Percent Yield	Actual yield divided by theoretical yield, times 100%	$\frac{8.0\,\text{g}}{10.0\,\text{g}} \times 100\% = 80\%$