What is a spontaneous process
the formation of a solution without the requirement of energy from an external source
Two criteria that favor, but do not guarantee, the spontaneous formation of a solution:
-a decrease in the internal energy of the system (an exothermic change, as discussed in the previous chapter on thermochemistry)
-The system becomes more disordered (entropy increases) when matter spreads out in a system
What is an ideal solution?
the intermolecular forces (IMFs) between:
solute–solute,
solvent–solvent, and
solute–solvent
They are essentially the same strength. That’s why no extra energy is absorbed or released when the solution forms.
What are the energy changes during solution formation?
Breaking solute-solute and solvent-solvent attractions → requires energy (endothermic)
Forming solute-solvent attractions → releases energy (exothermic, called solvation)
What energy changes occur when an ionic compound dissolves in water?
Breaking ion–ion attractions (solute–solute) → requires energy (endothermic)
Forming ion–water attractions (solute–solvent) → releases energy (exothermic, solvation)
When is an ionic compound poorly soluble in water? Give an example.
If solute–solute forces > solvation forces, dissolution is highly endothermic → low solubility
Example: Calcium carbonate (CaCO₃)
Can endothermic dissolutions still happen spontaneously? Give an example.
Yes, because entropy (disorder) increases during solution formation
Example: Ammonium nitrate (NH₄NO₃) in cold packs absorbs heat but dissolves spontaneously
Q: When is an ionic compound highly soluble in water? Give an example.
If solute–solvent (ion–water) attractions > solute–solute (ion–ion) attractions, dissolution is exothermic → high solubility
Example: Sodium hydroxide (NaOH)
Q: What are electrolytes and nonelectrolytes?
Electrolytes: Substances that produce ions in solution
Nonelectrolytes: Substances that do not produce ions in solution
Q: What distinguishes strong and weak electrolytes?
Strong electrolytes: Nearly 100% of the dissolved substance produces ions
Weak electrolytes: Only a small fraction of the substance produces ions
How can you tell if a substance is a strong, weak, or non-electrolyte?
By measuring electrical conductance in solution
More ions → higher conductivity
Qualitative test: brighter light in a circuit indicates more ions
Q: How do ionic compounds dissolve in water?
-Water molecules surround and solvate ions, reducing strong electrostatic forces.
-This process, called dissociation, is a physical change.
-Ionic compounds typically dissociate nearly completely → strong electrolytes.
What are ion-dipole attractions, and how do they help dissolve ionic compounds?
Ion-dipole attraction: electrostatic attraction between an ion and the dipole of a polar molecule.
Water’s positive H atoms attract anions (Cl⁻) and its negative O atoms attract cations (K⁺), helping ions separate and disperse.
How can covalent compounds conduct electricity? Give an example.
Some covalent compounds react with water to produce ions.
Example: HCl gas → HCl(aq) reacts with water to form H₃O⁺ and Cl⁻.
Strong acids like HCl → strong electrolytes; weak acids/bases → weak electrolytes.
What is a solution?
A homogeneous mixture of two or more substances in the same phase. (liquid, solid or gas)
What is a solvent?
The substance is present in larger amounts
What is a solute?
The substance is present in smaller amounts
What is Concentration?
the amount of a solute that is
present in a certain amount of solution, or
mixed with a certain amount of solvent
Hydration
water molecules clustering around solute particles, stabilizing them in solution through ion–dipole or hydrogen bonding interactions.
prevents them from turning solid
supersaturated
above the limit of solubility!
saturated
concentration of solute is at the (max) limit of solubility
unsaturated
concentration of solute is below the limit of solubility
Supersaturation
the solution has dissolved more solute than its equilibrium limit, making it unstable and prone to rapid crystallization.
Is the dissolving of gas an exothermic or endothermic?
exothermic because energy is released as the gas molecules become hydrated as they enter the liquid phase.
