Chapter 3: Chemical Reactions

Overview of Chemical Reactions

  • Chemical Reaction (or Chemical Change):

    • Defined as a change in chemical composition, including the change in molecular structure.

    • Involves Reactants turning into Products, which are structurally different from the reactants.

Chemical Equations

  • Description:

    • Represents chemical reactions symbolically.

    • Example: Methane reacts with oxygen to yield carbon dioxide and water.

  • Components of Chemical Equations:

    • Identity of reactants and products

    • Molar Ratios:

    • Indicates the relative amounts of substances involved.

Physical States in Chemical Equations

  • Notation for physical states within equations:

    • $s$ - solid

    • $l$ - liquid

    • $g$ - gas

    • $aq$ - aqueous (dissolved in water)

Importance of Balancing Chemical Equations

  • Law of Conservation of Matter: (formulated by Lavoisier)

    • States that matter cannot be created or destroyed.

    • For Chemists: The mass of the reactants equals the mass of the products.

    • Reflection of the quantities of each element is unchanged throughout the reaction.

  • Balancing Chemical Equations:

    • Chemical equations must be balanced for mass conservation.

    • There should be the same number of atoms of each element on both sides of the equation.

Steps in Balancing Chemical Reactions

  1. Write the equation with the formulas/symbols of reactants and products, including physical states if necessary.

  2. Start balancing the compound with the largest number of different elements using integers as coefficients.

  3. Balance individual elements last.

  4. Verify that the number of atoms of each element is equal on both sides of the equation.

  5. Simplify coefficients if possible by dividing by a common whole number.

Example of Balancing:

  • Reaction: $2 ext{HgO}(s) ightarrow 2 ext{Hg}(l) + ext{O}_2(g)$

    • Confirmed balanced with respect to each element involved.

Types of Chemical Reactions

  • Chemical Equilibrium:

    • Can include reactions that run in reverse, indicated with double arrows (⇄), suggesting the ability to reach an equilibrium state.

Reversibility of Reactions

  • Example:

    • $Ca^{2+}(aq) + 2 HCO3^-(aq) ightleftharpoons CaCO3(s) + CO2(g) + H2O(l)$

Ionic Compounds

  • Definition and Components:

    • Ionic compounds are generally formed from metals and non-metals.

    • Composed of ions that carry positive or negative charges, e.g., sodium chloride ($ ext{NaCl}$).

  • Dissolution of Ionic Compounds in Water:

    • Ions become mobile in solution, which is a critical factor for conductivity.

Conductivity

  • Electrolytes vs. Nonelectrolytes:

    • Electrolyte:

    • Is a substance that, when dissolved in water, conducts electricity.

    • Strong Electrolyte: Fully dissociates into ions (e.g., NaCl).

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

    • Nonelectrolyte:

    • Does not conduct electricity when dissolved (e.g., sugar in water).

Recognizing Reactions in Solution

  • Indicators of reactions occurring in solutions include:

    • Formation of a precipitate (an insoluble solid).

    • Formation of a gas.

    • Formation of a weak electrolyte or nonelectrolyte.

Solubility Rules

  • Guiding principles to predict the solubility of compounds in water:

    • For molecular compounds, they must be polar for solubility.

    • For ionic compounds, presence of an ion from the soluble compounds list indicates solubility.

Examples to Determine Solubility

  • $NH4NO3$, $NaOH$, $PbI2$, $K2CO_3$, and $LiCl$ provide insights on soluble versus insoluble salts.

    • Example of an insoluble salt: $PbI_2$.

Types of Chemical Reactions

  • Exchange Reactions:

    • Involves an exchange of ions between two compounds.

  • Precipitation Reactions:

    • Chemical reactions that result in the formation of an insoluble solid (precipitate).

Definitions Related to Reactions

  • Complete (“molecular”) equation:

    • An equation detailing all substances as they exist in molecular form, despite some being ionic.

  • Ionic equation:

    • Represents reactions involving ions in solution where soluble substances are shown as predominant species.

  • Spectator ions:

    • Ions that remain unchanged during a reaction.

  • Net ionic equation:

    • An equation that excludes the spectator ions, showing only the species that participate in the reaction.

Final Note on Balancing and Classification

  • Ensuring the mathematical and chemical accuracy of equations through consistent practices in balancing, alongside recognizing types of reactions will solidify foundational chemical knowledge.