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

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