SCH3U - Unit 4 - Solutions and Solubility

Lesson 1 - Solutions and Solubility

• Solubility - The amount of solute that can dissolve in a given amount of solvent

• Unsaturated Solution - A solution that has less solute dissolved than the greatest soluble amount

• Saturated Solution - A solution in which no more solute can be dissolved

• Supersaturated Solution - A solution in which there is more solute than the greatest soluble amount, and the excess does not dissolve

• Intermolecular Forces and Solubility

  • London Dispersion Forces - Weak intermolecular forces that form a temporary dipole

  • Dipole-Dipole Forces - A strong intermolecular force, where the atoms have a large difference in electronegativity, that forms a dipole

  • Hydrogen Bonds - The strongest intermolecular force that forms a permanent dipole between hydrogen and fluorine, oxygen, and nitrogen

  • Molecules need similar intermolecular forces for a solute to be dissolved in a solvent

  • The size of the molecules and “accessibility” of the dipoles can affect solubility

    • Larger molecules tend to be less soluble

  • In an ion-dipole interaction, water will dissolve the ions in a compound, where the anion is attracted to the positive hydrogen, and the cation is attracted to the negative oxygen

    • Electrolyte - A compound which forms ions in a solution and can conduct electricity

  • In a dipole-dipole interaction, water molecules are attracted to the dipole in the solute molecule and allow the solute to dissolve

• Ionic solids that are insoluble in water have a greater attraction to themselves, so water cannot break it apart

  • Coulomb’s Law - F = (Q₁Q₂)/r², where Q₁Q₂ represents the amount of electrons transferred

    • The force of attraction increases when more electrons are transferred, and decreases as the radius increases

• Solubility graphs and temperature

  • Temperature affects the speed at which particles move

  • When solutes dissolve in water, you have to break the force of attraction between the molecules of the solute and the solvent

  • When the temperature is higher, there is more energy

  • The increased energy can break bonds apart easier, which helps the solute dissolve in the solvent

  • With gases, the increased kinetic energy causes the gases to move more, which overcomes the forces of attraction to the solvent, allowing the gases to leave the solution

• Pressure

  • The more pressure is exerted above a solution of a liquid and a gas, the more gas remains dissolved (ex. soda)

Lesson 2 - Concentration and Dilution

• Concentration - How much of a solute is dissolved in a solvent

  • Molarity - The concentration of a solution in moles/litre, or M

    • c = n/v

      • n is the amount of solute, in moles

      • v is the volume of the solution, in litres

  • Mass/Volume Percent

    • c = m/v (100)

      • m is the mass of the solute, in grams

      • v is the volume of the solution, in millilitres

  • Mass/Mass Percent

    • c = m/m_s (100)

      • m_s is the mass of the solution, in grams

  • Volume/Volume Percent

    • c = v/v_s (100)

      • v_s is the volume, in millilitres, of the solution

  • Parts Per Million (ppm)

    • m/m_s (10⁶)

  • Parts Per Billion (ppb)

    • m/m_s (10⁹)

• if the unit is not specified, assume it is Molarity (mol/L)

• Dilution - Reducing the concentration of a solution

  • c₁v₁ = c₂v₂

    • C₁ - concentration of the original solution in mol/L

    • V₁ - volume of the original solution in L

    • C₂ - concentration of the new solution in mol/L

    • V₂ - volume of the new solution in L

Lesson 3 - Beer’s Law

• The colour of a solution comes from a chemical which is emitting light in the visible spectrum

• The wavelength of light can be measured

• When light passes through a solution, it loses its intensity, and is proportional to the concentration

• A colorimeter shines light through a sample on a cuvette and detects how much light is transmitted

  • The more light is absorbed, the less light is transmitted

  • The colour of light used is different to the solution so it is not absorbed

• Beer’s Law - How much light a sample absorbs is proportional to the concentration of the sample

  • A = εBC

    • A - absorbance (no units)

    • ε - molar absorption coefficient (a constant that is unique to each solution, measured in 1/(mol/L * cm)

    • B - length of path or cuvette in cm

    • C - concentration of the solution in mol/L

Lesson 4 - Acids and Bases

• Dissociation - Molecules break apart into their various components in a solution

• Ionization - Ions are formed as the result of a compound being added to a solvent

  • Ex. HCl is a molecular compound, but will become ions in water

• Arrhenius’ Theory

  • Acid - A hydrogen containing electrolyte compound that ionizes to produce H⁺ ions and neutralizes bases

    • Acid ionizes in water to increase the hydrogen ion concentration

    • Turns blue litmus paper red

  • Base - Ionic hydroxide electrolytes that dissociate to produce OH^-ions and neutralize acids

    • Bases dissociate in water to increase the hydroxide ion concentration

    • Turns red litmus paper blue

  • Neutral - A substance that can be electrolytes or nonelectrolytes, ions or molecules, and does not affect litmus paper

    • Neutral substances have a pH of 7

  • pH - Power of Hydrogen that is based on a log scale from 1 to 14

    • 1 is very acidic, and 14 is very basic

    • pH = -log[H⁺]

      • [H⁺] = 10^-pH

    • pOH = -log[OH^-]

    • pH + pOH = 14

• Bronsted-Lowry Theory - Acids are proton donors, and bases are proton acceptors

  • Water can be an acid or base, depending on what it reacts with

    • Amphoteric Molecule - A molecule that can act as an acid or a base

    • Autoionization of Water - Two water molecules can ionize to become a hydroxide and a hydronium ion

  • Conjugate Acid - A substance formed by adding a proton to the base

  • Conjugate Base - A substance formed by removing a proton from the acid

  • Conjugate Acid-Base Pair - An acid and base such as HA and A^- that differ only in the presence or absence of a proton

  • Hydrogen ions (H⁺) and hydronium (H₃O⁺) are interchangeable when calculated pH

Lesson 5 - Titration

• Neutralization Reactions

  • Acids and bases neutralize each other

  • Acid + Base → Salt + Water

    • ex. HCl + NaOH → NaCl + H₂O

  • Equivalence Point - The moles of acid equal the moles of base, and results in pH 7

    • Having excess of one will not result in the equivalence point

• Titration - A technique that quantitatively determines the concentration of an unknown base using the known mole ratio between an acid and base

  • Four regions in a titration curve

    • Initial pH region - pH of solution before the addition of any acid or base

    • Before equivalence point - shows the pH steadily rising or falling, depending on the pH of the acid or base

    • Equivalence point - pH is equivalent to 7

    • After equivalence point - pH is greater or less than 7 (under/overshoot)

  • Can determine the concentration of an unknown substance

  • Titration is spontaneous and fast, where the reaction is 99% completed and there is a whole number mole ratio of reactants and products

  • Titration terms:

    • Buret - Glassware that delivers a specific volume of a standard titrant solution

    • Titrant - Solution in buret during titration that is being added

    • Standard Solution - Solution with an accurately known concentration

    • Analyte - The initial compound being studied

    • Indicator - A compound which changes colour based on pH

    • Endpoint - A point at which a sharp change in property occurs

• Steps of a titration experiment

  • Add a known amount of analyte to a flask with an indicator

  • Add a titrant until the indicator changes color, showing neutralization

  • Record the volume of titrant that it took to neutralize the analyte