CH4-1: Chemical Reactions and Stoichiometry: Solutions, Electrolytes, and Precipitation

Chapter 4: Chemical Reactions and Stoichiometry

Balancing Chemical Equations

Balancing chemical equations is essential to represent chemical reactions accurately, ensuring the conservation of mass. This means the number of atoms for each element is the same on both sides of the equation.

• Reactants are written on the left, products on the right.

• Coefficients are used to balance the number of atoms; subscripts are never changed.

• Coefficients should be the smallest set of whole numbers possible.

• Physical states are indicated: (s) for solid, (l) for liquid, (g) for gas, (aq) for aqueous.

• Reaction conditions (e.g., heat, catalyst) are shown above or below the reaction arrow.

Example:

Combustion of propane:

$\mathrm{C_3H_8 (g) + 5\ O_2 (g) \rightarrow 3\ CO_2 (g) + 4\ H_2O (l)}$

Solutions and Solubility

A solution is a homogeneous mixture of two or more substances. The component present in the greatest amount is the solvent, and the other components are solutes. If water is the solvent, the solution is called aqueous.

• Solvent: Substance present in the largest quantity.

• Solute: Substance(s) dissolved in the solvent.

Example: In a solution of salt in water, water is the solvent and salt is the solute.

Ionic Solutions

Solutions can be classified by their ability to conduct electricity, which depends on the presence of ions.

• Electrolyte: A substance that forms ions in solution and conducts electricity. Most ionic compounds are electrolytes.

• Nonelectrolyte: A substance that does not form ions in solution and does not conduct electricity. Most molecular compounds are nonelectrolytes.

Example: NaCl is an electrolyte; sugar (C12H22O11) is a nonelectrolyte.

Classification of Electrolytes

Electrolytes are further classified based on the extent to which they dissociate into ions in solution.

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

• Weak Electrolyte: Partially dissociates into ions (e.g., CH3COOH, NH3).

• Nonelectrolyte: Does not dissociate into ions (e.g., glucose, ethanol).

Strong Electrolytes	Weak Electrolytes	Nonelectrolytes
HCl, HBr, HI, NaCl, KBr, NaOH, KOH, other soluble ionic compounds	CH3COOH, HF, H2O (very weak), NH3	H2O, C2H5OH, C6H12O6, most organic compounds

Solubility of Ionic Compounds

Solubility is the ability of a substance to dissolve in a solvent. Substances that dissolve readily are called soluble; those with limited solubility are slightly soluble or insoluble.

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

Soluble Compounds	Exceptions
All nitrates (NO3-), acetates (CH3COO-), and most alkali metal salts	None
Most chlorides, bromides, iodides	Ag+, Pb2+, Hg22+
Most sulfates (SO42-)	Ba2+, Pb2+, Ca2+, Sr2+
Most carbonates, phosphates, sulfides, and hydroxides	Insoluble except with alkali metals and NH4+

Example: BaSO4 is insoluble; KNO3 is soluble.

Net Ionic Equations

Chemical equations can be written in different forms to show varying levels of detail about the species involved in a reaction.

• Molecular (Formula) Equation: Shows all reactants and products as compounds.

• Complete Ionic Equation: Shows all strong electrolytes as ions.

• Net Ionic Equation: Shows only the species that actually change during the reaction (omits spectator ions).

Example:

• Molecular: $\mathrm{HCl\ (aq) + NaOH\ (aq) \rightarrow H_2O\ (l) + NaCl\ (aq)}$

• Complete Ionic: $\mathrm{H^+\ (aq) + Cl^-\ (aq) + Na^+\ (aq) + OH^-\ (aq) \rightarrow H_2O\ (l) + Na^+\ (aq) + Cl^-\ (aq)}$

• Net Ionic: $\mathrm{H^+\ (aq) + OH^-\ (aq) \rightarrow H_2O\ (l)}$

Notes:

• Write a compound as ions only if it is both soluble and a strong electrolyte.

• Polyatomic ions remain intact unless otherwise specified.

• Only aqueous species are split into ions; gases, solids, and liquids are not.

Types of Chemical Reactions

Chemical reactions are classified into categories based on their characteristics. The main types discussed are:

• Precipitation reactions

• Acid-base reactions

• Oxidation-reduction (redox) reactions

Precipitation Reactions

A precipitation reaction occurs when two solutions are mixed and an insoluble solid (precipitate) forms. This is a type of metathesis (double displacement) reaction, where ions are exchanged between reactants.

• General form: $\mathrm{AX\ (aq) + BY\ (aq) \rightarrow AY\ (s) + BX\ (aq)}$

• The precipitate is the insoluble product (AY in the example above).

Example:

• $\mathrm{2AgNO_3\ (aq) + MgCl_2\ (aq) \rightarrow 2AgCl\ (s) + Mg(NO_3)_2\ (aq)}$

Additional info: Precipitation reactions are important in analytical chemistry for identifying ions in solution and in industrial processes for removing unwanted ions.