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Chapter 8: Solutions

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