Introduction and Definitions

• Chemical reactions only occur when molecules or atoms collide/interact with each other

• Solutions are a uniform mixture of one or more solutes dissolved in a solvent.

  • Alloys are solid solutions of two or more metals

• Solutions can be described quantitatively by figuring out how much solute is dissolved in a specific amount of solvent, aka concentration

  • Solubility is the maximum amount of solute that can dissolve in a solvent in grams per liter

  • Molar solubility is the max amount of solute that can dissolve in a solvent in moles per liter

• Qualitative terms can also describe a solution

  • A concentrated solution is a large amount of solute dissolved in the solvent

  • A dilute solution has a small amount of dissolved solute

  • Saturated solutions have the maximum amount of solute dissolved in solvent

    • Sometimes in equilibrium if there is solid solute in the solution as well

  • Unsaturated solutions have less than the max amount of solute dissolved in the solvent.

    • Never any undissolved solute in solution

  • Supersaturated solution has more than the max amount dissolved

    • Obtained at a high temperature and carefully cooling to avoid crystallizing

    • Solution is metastable: can be crystallized if shaked or disturbed

Solubility and the Solution Process

• Solutions can be made with a solvent and solute with similar polarities but not with very different polarities

• Solution process

  • Solute molecules separate to allow solute to fit between them. Energy is used to break attraction between solute molecules

  • Solvent molecules separate to allow solute between them. Energy used to break attraction between solvent molecules

  • Separated molecules are combined. Energy released when new attractions form between solvent and solute

  • If more energy is released than used, solvent will dissolve in the solute. Excess energy will release as heat and temperature will increase.

• If the solution can have more arrangements or increases in states, it will have high entropy. High entropy helps the solution process

Examples of the Solution Process

Dissolving Ionic Compounds

• Ionic compounds dissolve into aqueous solutions

  • The high lattice energy makes it near impossible to dissolve but the highly polar water molecule is attracted to the ions and enough energy is released.

  • The water molecule attraction releases enough energy so dissolution is favored.

  • Entropy increases because of the crystal structure breaking up, which helps the solution process

Gas Mixtures

• Gasses mix freely with other gasses because of entropy only.

  • Gasses fill up vacuum containers because increases the amount of possible arrangements it could have, which increases entropy

Rates of Dissolution

• Heating the solution, grinding up chemicals, and stirring vigorously can increase how fast a solution process occurs

  • All three of these increases surface area of the solid

  • Heat increases solvent molecule motion to increase collision speed

  • Grinding increases solute exposure to solvent

  • Stirring moves solute away from other solute molecules

Aqueous Solutions

Classification of Solutes

• Aqueous solutions have water as the solvent

  • Ionic substances dissociate completely in water

  • Some ions dissolve through ionization where their solutions can conduct electricity. These are called electrolytes

    • Weak electrolytes dissociate only slightly and conduct electricity poorly

    • If the solution does not conduct electricity, these are nonelectrolytes

Strong Electrolytes

• Strong electrolytes are ionic compounds that are dissolvable in water, such as NaCl, KBr, and Mg(NO3)2

• Covalent compounds in the gas state can ionize completely when dissolved in water

  • Only HCl, HBr, and HI

Weak Electrolytes

• Molecular compounds that are soluble but only partially ionize

  • Most ionize less than 10 percent

• Weak electrolyte reactions with water results in dynamic equilibrium

Nonelectrolytes

• Dissolve but don't form ions

  • sugars like glucose, sucrose, and alcohols such as methanol, ethanol, and propanol

Concentration of Solutions

Definitions and Units

Molarity (M)

• Molarity is the most common unit.

  • Molarity is number of moles of solute divided by the liters of solvent dissolved in

Effect of Temperature on Solubility

• Most solids are more soluble in hot solvents vs cold solvents

• Increasing temperature increases disorder. If a molecule could move from a lower entropy state to a higher entropy state, it would

Effects of Pressure on Solubility

• External pressure has no significant effect on liquid or solid solubility because they are not compressed when pressure is increased

• Gasses, when compressed, increase solubility because there is less space for the molecules to be, increasing the frequency of gas molecules converting to liquid

• Henry’s law is the solubility of gasses

  • solubility of a gas = kP

    • k, Henry’s proportionality constant

    • P is gas partial pressure