Chemistry 9.1: The nature of solutions

general solubility rules for aqueous solutions

*The MCAT will not expect you to memorize all the solubility rules, but its worth knowing two absolutes: - all group 1 metals, and all nitrate salts are soluble. please know this.

Solution components

A solution consist of a solute which is dissolved in a solvent. - solutes: (NaCl, NH3, CO2, etc) - solvents: (H2O, benzene or ethanol) Solvent - component that remains in the same phase after mixing it. ---- if two components are already in same phase (liquid - liquid) the one present in greater quantity is solvent. --- if two components are in equal quantities, the one most commonly used as a solvent is the solvent

ideal solution

A solution with an enthalpy of dissolution that is equal to zero.

When NaCl dissolves in water, aqueous Na+ and Cl- ions result. The force of attraction that exists between Na+ and H2O is called a(n) __________ interaction. A) dipole-dipole B) ion-ion C) hydrogen bonding D) ion-dipole E) London dispersion force

D) Ion-dipole This also exists between the H+ OF Water and Cl- O- of H20 and Na+

dilute vs concentrated solutions

Dilute - when the proportion of solute to solvent is small. - Adding 1 teaspoon of sugar in 1 jug of water. Concentrated - when the proportion of solute to solvent is large. -- dumping container of sugar in 1 jug of water.

Solutions

Homogenous (same throughout) mixtures of two or more substances that combine to form a single-phase, usually the liquid phase. - MCAT focuses mostly on solids dissolved into aq solutions. - Solutions can be formed from different combinations of the phases of matter. -- gasses can be dissolved In liquids (carbonating soda) --- liquids in other liquids (ethanol in water) --- solids in solids (metal alloys) A mixture where substances are distributed evenly. Has a solvent & solute.

Gibbs free energy

When Gibbs free energy is negative, the dissolution occurs spontaneously, and solute is said to be soluble. *you can infer what happens in the opposite.

Solvation

Solvation - the electrostatic interaction between solute and solvent molecules. -- aka dissolution (or hydration when H20 is solvent). - involves breaking intermolecular interactions between solute - solute and solvent - solvent in order to form new interactions between solute - solvent.

Solutions vs Mixtures

It is important to know that every solution is a mixture, but not every mixture is a solution. - solutions are a type of mixture that form homogenous products. - mixtures that aren't solutions are heterogenous. There properties do not interact all that much chemically like when gases are dissolved in gasses. mixture - a system containing multiple substances that have been physically combined but are not chemically combined.

if more solute is added to a saturated solution

It will not dissolve, because it has already reached its maximum dissolution ability. -- most often, the solute will remain in solid form, and precipitate (insoluble solid) to the bottom of the container.

Aqueous solutions (aq)

Solution in which the solvent is water. -- in some solutions, such as acids the formation of H3O+ can occur. -- caused by transfer of H+ to water. ex: CH3COOH (aq) + H2O (l) ⇌ CH3COO- (aq) + H3O+ (aq) *MOST TESTED solution type on mcat.

When new interactions are weaker

Salvation is an endothermic process and favored at high temps. -- most dissolutions are of this type -- examples: ammonium nitrate in water or sugar in water. -- because the new solute-solvent interactions are weaker than the solute-solute interactions, energy must be supplied to the system to facilitate these less stable interactions. Sometimes overall strength does reach strength of original interactions. - in this case overall enthalpy change for dissolution is close to zero = ideal solution.

When new interactions are stronger

Salvation is exothermic and the process is favored at low temps. --- based on Lechat's principle. Ex: Dissolution of gases such as CO2 in water is exothermic, because only interactions that must be broken are those of the water molecules.

How do solutes interact with solvents?

Solutes interact with solvent by the way of intermolecular forces such as, ion-dipole, dipole-dipole, or hydrogen bonding. - dissolved solutes are also relatively free to interact w/other dissolved solutes. CHEMICAL REACTION thus occur EASILY in solution.

Solubility Guidelines

Solutes with molar solubility greater than 0.1M = considered soluble -- less than 0.1M = not soluble. = Sparingly soluble.

Solubility

The maximum amount of of substance that can be dissolved in a particular solvent at a given temperature. - lets us know how much solute will dissolve in a given solvent. - when the maximum amount of solute has been added the dissolved solute is in equilibrium with its undissolved state. = we say the solution is saturated

When does a dissolution occur spontaneously?

When change in entropy is higher than the change in (Enthalpy) - considers energy to break bonds and form new ones - entropy when solutes are dissolved ∆G =∆H-T∆S