Unit 1: Chemical Reactions and Stoichiometry

Chemical reactions are the fundamental processes by which substances undergo transformation to create new materials with distinct chemical identities. This unit establishes the essential skills for writing, balancing, and interpreting chemical equations as symbolic representations of these changes. By mastering stoichiometry, students can quantitatively predict the amounts of reactants required and products generated in both laboratory and industrial settings.

1.1 Introduction to Chemical Reactions

• Physical Change: A transformation that alters the physical state or appearance of a substance without changing its chemical composition (e.g., melting ice or boiling water).
• Chemical Change: A process where substances are transformed into new substances with different chemical properties and compositions (e.g., rusting of iron or burning wood).
• Chemical Reaction: The process by which one or more substances, known as reactants, are converted into one or more different substances, known as products.

1.2 Chemical Equations

• Chemical Equation: A shorthand notation that uses chemical symbols and formulas to represent a chemical reaction.
• Law of Conservation of Mass: A fundamental principle stating that mass is neither created nor destroyed in a chemical reaction; the total mass of reactants must equal the total mass of products.
• Balanced Equation: An equation in which the number of atoms of each element is the same on both the reactant and product sides.
• Coefficients: Whole numbers placed in front of chemical formulas in an equation to balance the number of atoms.

Steps for Balancing Chemical Equations (Inspection Method)

1. Write the correct formulas for reactants on the left and products on the right.
2. Count the number of atoms of each element on both sides of the equation.
3. Balance the elements one at a time using coefficients. (Start with elements that appear in only one reactant and one product).
4. Check the final equation to ensure all atoms and charges are balanced.

1.3 Types of Chemical Reactions

• Combination Reaction: Two or more substances react to form a single new substance ($A + B \rightarrow AB$).
• Decomposition Reaction: A single compound breaks down into two or more simpler substances ($AB \rightarrow A + B$). This usually requires energy in the form of heat, light, or electricity.
• Single Displacement Reaction: An element reacts with a compound and takes the place of another element in that compound ($A + BC \rightarrow AC + B$). This depends on the activity series.
• Double Displacement Reaction: Two compounds exchange positive ions (cations) to form two new compounds ($AB + CD \rightarrow AD + CB$).
• Precipitation Reaction: A type of double displacement where an insoluble solid, called a precipitate, forms and drops out of the solution.

1.4 Oxidation and Reduction (Redox) Reactions

• Oxidation: The process of losing electrons or an increase in the oxidation number of an atom.
• Reduction: The process of gaining electrons or a decrease in the oxidation number of an atom.
• Redox Reaction: A reaction where oxidation and reduction occur simultaneously through electron transfer.
• Oxidizing Agent: The substance that gains electrons (is reduced) and causes another substance to be oxidized.
• Reducing Agent: The substance that loses electrons (is oxidized) and causes another substance to be reduced.
• Oxidation Number: A positive or negative number assigned to an atom to indicate its degree of oxidation or reduction.

1.5 Molecular Masses and the Mole Concept

• Molecular Mass: The sum of the atomic masses of all atoms in a molecule, measured in atomic mass units (amu).
• Mole (mol): The SI unit for the amount of substance; one mole contains exactly $6.022 \times 10^{23}$ elementary entities.
• Avogadro’s Number: $6.022 \times 10^{23}$, the number of particles in one mole of any substance.
• Molar Mass: The mass of one mole of a substance, numerically equal to its molecular or formula mass but expressed in grams per mole (g/mol).

1.6 Stoichiometry

• Stoichiometry: The study of quantitative relationships between reactants and products in a balanced chemical reaction.
• $22.4 \text{ liters}$Molar Volume: The volume occupied by one mole of any gas at Standard Temperature and Pressure (STP), which is .
• Limiting Reactant: The reactant that is completely consumed in a reaction and determines the maximum amount of product that can be formed.
• Excess Reactant: The reactant that is not completely consumed and remains after the reaction stops.
• Theoretical Yield: The maximum amount of product that can be produced from a given amount of reactant, according to stoichiometric calculations.
• Actual Yield: The amount of product actually obtained from a reaction when performed in a laboratory.
• Percentage Yield: The ratio of the actual yield to the theoretical yield, expressed as a percentage:$\text{Percentage Yield} = \left( \frac{\text{Actual Yield}}{\text{Theoretical Yield}} \right) \times 100\%$

Key Terminology

• Actual Yield: The amount of product actually produced in a chemical reaction.
• Avogadro’s Number: The number of atoms or molecules in one mole of a substance ($6.022 \times 10^{23}$).
• Chemical Equation: A representation of a chemical reaction using symbols and formulas.
• Combination Reaction: A reaction in which two or more substances combine to form a single product.
• Decomposition Reaction: A reaction in which a single compound breaks down into simpler substances.
• Double Displacement: A reaction involving the exchange of ions between two compounds.
• Excess Reactant: The reactant that remains after a chemical reaction is complete.
• Law of Conservation of Mass: Mass is neither created nor destroyed during a chemical reaction.
• Limiting Reactant: The reactant that limits the amount of product formed.
• Molar Mass: The mass in grams of one mole of a substance.
• Mole: The amount of a substance that contains Avogadro’s number of particles.
• Oxidation: The loss of electrons or an increase in oxidation state.
• Oxidizing Agent: A substance that causes oxidation by accepting electrons.
• Percentage Yield: The efficiency of a reaction, calculated as (Actual Yield / Theoretical Yield) × 100.
• Precipitate: An insoluble solid that emerges from a liquid solution.
• Redox Reaction: A reaction involving the transfer of electrons between species.
• Reducing Agent: A substance that causes reduction by donating electrons.
• Reduction: The gain of electrons or a decrease in oxidation state.
• Single Displacement: A reaction where one element replaces another in a compound.
• Stoichiometry: The calculation of quantities of substances involved in chemical reactions.
• Theoretical Yield: The predicted amount of product based on the balanced equation.