1062: CHEM Ch 6

Solutions

Homogeneous mixture of two or more components

• Solute: minor component

• Solvent: major component

Solutions vs. Mixtures

• Solution: Homogeneous (same composition throughout) and typically won’t unmix once mixed

  • Subtype of a mixture

• Mixture: Heterogenous (different throughout) and easy to unmix

Solution Volume

Isn’t always equivalent to the sum of the volume components but somewhere between

• Affected by the density of the solution versus the components density

Aqueous (aq)

When a solution is dissolved in water (solvent)

• solubility rules and ions diffuse

Immiscible

Solute and solvent don’t mix and they form two separate layers instead

Miscible

The solute and solvent mix into one solution and don’t unmix

Saturated Solution

No more solute will dissolve

Unsaturated Solution

Added solute will dissolve

Solution Notes (Mass and Volume)

• Mass_solution = Mass_solute + Mass_solvent

  • Mass = A + B

• Volume_solution ≠ Volume_solute + Volume_solvent

  • Volume ≤ A + B

Molality

Molarity

Moles of solute per liter of solution

• Affected by the temperature

Making a Solution

Add enough solvent to the solute sitting in the flask to make the correct concentration of solution.

• Same process for mass/L solution and dilution

Mass to Mass Concentrations

Use %concentration, ppm, or ppb concentrations

• 10% solution has 10g solute for 90g solvent ( = solute/solution)(same as pph)

• 10ppm solution has 10 micrograms (10^-6) solute per gram of solution (mg solute/mg solution x 10⁶)

• 10ppb solution has 10 nanograms (10^-9) solute per gram solution

Mass % Given Density

Use the volume multiplied by the density to find the solution’s mass and divide the solute by the calculated solution’s mass

Mass/Mass Conversion Example - A beverage contains caffeine at a concentration of 35 ppm, interpreted as mass per mass.

Correct Conversions:

• The same as 0.0035% by mass

• The same as 35 mg of caffeine per kg solution

• The same as 3.5 × 10⁴ ppb

• The same as 35 micrograms of caffeine per g solution

Concentrated vs. Diluted

• Concentrated: more moles per volume

• Diluted: fewer moles per volume

• The moles of solute between these two doe’t change but the volume of the solution does

Dilution Formula

Latent Heat

Additional heat stored during a phase change that is released when the substance reverts phases.

Furthering Reactions

Cooling it, lowers the temperature, causing ∆G to become more negative

• -T∆S: decreases in magnitude

Factor Affecting Solubility

Molecule structure, molecule size, polarity, temperature, pressure, enthalpy change, entropy change, strength of solute-solute interactions, strength of solvent-solvent interactions, strength of solute-solvent interactions.

Solubility + Temperature

Solubility is always specified by the temperature at which it is measured

• Temperature affects solubility

Solubility Trends (Water)

• Hydrocarbons (CH₃): not very soluble in water

• Alcohols (OH): very soluble

Solubility + Entropy

It is favorable for molecules to mix into a solution

• S_mixed > S_unmixed

Determining A Molecules Solubility

Assess the molecule’s structure

• Is it polar, an ion, covalently bonded, and what IMFs are present?

• Compare the strength of the solvent-solvent and solute-solute IMFs to determine if the solute-solvent interactions are stronger

• “Like dissolves like”: molecules with similar structures and IMFs tend to be soluble in each other

Changes During Dissolving

1. Endothermic as interactions between particles in the solute are overcome

2. Endothermic as some interactions between particles in the solvent are overcome

3. Exothermic as new interactions are formed between the solute and solvent (the stronger the more soluble)

IMFs

• London Dispersion Forces: happen in all molecules (especially non-polar) when there are momentary charge distributions

• Hydrogen Bonding: the strongest and happens in polar molecules, especially alcohol groups (OH)

  • Can go both ways and are stronger if they do

• Dipole-Dipole Interactions: attractions between partial charges of polar molecules; they are the second strongest

Mass → Volume

Moles → Mass

IMFs For Molecular Compounds

• Draw out the lewis structure

• Find polar bonds (highly electronegative bond)

• Determine molecular geometry (electron geometry)

VESPER Theory

Hybridization → # of electron groups → groups

• Combine for overall geometry and use with bond polarity to determine overall polarity

Ionic Bond

Electrostatic attraction between ions

• Stronger than other IMFs

• Largely synonymous with ion-ion interactions

• ion-ion > ion-dipole > other IMFs

Ionic Solid

A 3D ionic lattice, ordered every other, and held together by ionic bonds

• High melting, brittle, non-conducting solid, conducting liquid, and soluble in water

• Conducts electricity when dissolved in water (aq)

Carbon Tail

The longer the carbon chain, the lower the solubility

Non-Polar Molecules + Water

Solubility is influenced by entropy so water molecules form a cage-like structure, which decreases entropy, so oil molecules clump instead.

Salts - Solubility

Ionic compounds whose electrostatic interactions within the crystal must be disrupted

• When molecular compounds dissolve IMFs between separate molecules are disrupted

Salt Dissolution (Water)

Water interacts with ions on the surface, forming a dynamic cluster around the ion with the oppositely charged end orientated towards an ion. This collision drives the ion and its water shell to separate.

• Strongly stabilizing (-∆H) and dynamic, as the water molecules of the shell are interchangeable

Solvation

When solvent molecules interact with and stabilize solute molecules

• Hydration: water is the solvent

Dissolution - System vs. Surroundings

• System: solute and solvent molecules that interact with it

• Surroundings: water molecules not interacting

• Temperature change measures surroundings, not the system specifically

Dissolution - Exothermic

As the measured temperature rises, the formed bonds are stronger and more stable than the reactants’ bonds.

• Enthalpy-driven

Dissolution - Entropy Driven

If a reaction is endothermic, entropy must increase, and -T∆S must overcome ∆H, such that it is entropy-driven

• Needs -∆G to occur

• ∆G more negative = more soluble

Amphipathic Molecules

Molecules large enough to have different regions that can be classified as polar or non-polar

• Ex: biomolecules

• Non-polar regions case a decrease in entropy (∆S) when interacting with water

• Polar regions interact with water and little to no increase in entropy

Amphipathic Molecules in Water

Molecules form structures, such as spherical micelles, which orient polar regions in contact with water and non-polar regions inside

• Other ordered structures

• Larger molecular aggregates

Colloid

When the particles in a solution maintain the structure of a solid

• Stable, as thermal motion is suspended

• Particles are suspended before they settle

• Larger molecules are unstable and settle due to gravity

Reaction Rate

As the reaction proceeds the probability of collisions between the reactants decreases

• Reaction forward = probability of collision of reactions

• Reaction begins occurring backward as the concentration of products reaches capacity, until it reaches equilibrium

Emulsions

When the particles of a solution occupy a separate, suspended, appearance

• Unstable, and the two liquid phases often separate

Solubility of Gas + Temperature Increase

Not highly soluble and vaporizes as heat is added because gases’ IMFs (especially with liquids are weak)

Gas Dissolution - Dipole Induced Dipole

Most gases have a slightly favorable -∆H and a slightly unfavorable -∆S of solution

• Less disordered in liquid and escape when the temperature rises/thermal energy is added

• Increasing temperature doesn’t always increase solubility

Alloys

Solutions of solids in solids, as atoms mix and are resolidified with emergent (new) properties

New Interactions

Ion-dipole interactions are weaker than ion-ion interactions and stronger than hydrogen bonding

• If a process occurs, incorporate the increased number of ion-dipole interactions in aqueous solutions when determining new bond strength

Exothermic Dissolution

Some thermal energy is required to break bonds, but even more energy is released when forming new bonds

• Products are more stable with stronger bonds (solute-solvent interactions are stronger)

Entropy of Dissolution

Mixing doesn’t always increase entropy so use context to determine its sign

• Doesn’t determine solubility alone

• ∆G must always be negative

Polyatomic Ions

Don’t dissociate when dissolved in water

• Represented in a similar ionic structure

• Water molecules surround dissolved ions

Endothermic Dissolution

More thermal energy is required to break bonds than is released when new bonds form.

• Products are less stable, and the solute-solvent interactions are weaker

• +∆S is necessary

Ionic Concentration

Multiply by the number of ions or total ions to determine the molarity of an ion or total ion molarity of a solution

Temperature Effects on Solubility

Affects the entropy (∆S) and thus determines the sign of ∆G when ∆H is positive

• -∆S = low T

• +∆S = high T

Lattice Energy

Energy is released when an ionic lattice forms from ions in the gas phase

• More charge = stronger attractions

• Larger ions = smaller attractions

• The stronger the interactions, the more energy required to overcome, which decreased solubility and increased the melting point

Solubility Trends

Ionic compounds with a ± 1 charge are soluble and ions with higher charges tend to be insoluble

• Insoluble = +∆G (-∆S)

• Lower charges means the water molecules in the hydration shell can exchange (≠ -∆S)