MCAT KAPLAN Solutions chapter 9

What does it mean when a solutions is Saturated?

- Its when a maximum amount of solute has been added. -When the dissolved solute is in equilibrium with its undissolved state.

Why do we salt icy roads in winter?

- Salt mixes with snow and ice and initially dissolves into a small amount of liquid water 2) this disturbs the equilibrium such that the rate of melting is unchanged (salt cannot interact with the solid water stabilized in lattice structure), but the rate of freezing is decreased (solute displaces some water molecules from -solid liquid interference) 3) the imbalance causes more ice to melt than water to freeze. 4) the melting of ice(an endothermic process) causes solution temperature to fall below ambient temperature. This creates a temperature gradient that will drive warm air into the aqueous solution.

When is solvation endothermic?

- The solvation is endothermic when the new interactions are weaker than the original ones -favored at high temperatures. -most dissolutions are of this type. EX: mixing sugar with water. the new interactions are weaker than the original interactions. This means that energy( in this case heat) must be provided to facilitate the formation of these weaker, less stable interactions.

What is a concentrated solution?

- When the proportion of solute to solvent is large.

ideal solution

- enthalpy of dissolution is equal to zero

Can solutions be formed from different combinations of the three phases of matter? What type of solution combination will be the focus of the MCAT?

- yes, Solutions can made from the combinations of solids and liquids, Gases and liquids (soda), solids and solids (melting metals together), and liquids and liquids (ethanol in water) . -The MCAT will focus on solids dissolved into aqueous solutions.

What is a dilute solution?

-A solution in which the proportion of solute to solvent is small.

When is the vant hoff factor (i) used?

-For solutes that dissociate, (i) is used in freezing point depression, boiling point elevation, osmotic pressure calculations.

What is the solubility of substances in term of Gibbs free energy.

-It is a function of thermodynamics. -When the ∆G<0 for the dissolution reaction at a given temp, the process will be spontaneous, and the solute is said to be soluble. -When the ∆G>0 at a given temp, the process will be non-sponteneous and the solute is said to be insoluble.

What occurs once a solute has been introduced into a solvent?

-Most of the change taking place is dissociation of solute. -however, once solute is dissolved, the reverse process- precipitation of the solute- will begin to occur.

What is the saturation point?

-The equilibrium of a solution. -where the solute concentration is at its maximum value for the given temperature and pressure.

How does vapor pressure depression go hand in hand with boiling point elevation?

-The lowering of a solutions vapor pressure would mean that a higher temperature is needed to match atmospheric pressure, thereby raising the boiling point.

normality (N)

-The molarity of the stuff of interest in the reaction. -the number of equivalents of interest per liter of solution

What would be considered the solute if the 2 substances being mixed are in the same phase?

-The solvent would be considered the substance with the greater quantity. - the solute is the lesser amount - the solvent is usually aqueous solution

What is thermodynamically favored when a solution is dilute (unsaturated)?

-The thermocynamically favored process is dissolution. that means initially the rate of dissolution >(greater than) rate of precipitation. -as the solution continues to be saturated, the rate of dissolution lessens until the saturation point of the solution is reached. The solution is now in equilibrium, for which the rates of dissolution and precipitation are equal and the concentration of dissolved solute is constant. ∆G=0 at this point.

What is important to note about freezing point depression and boiling point elevation?

-The values that are given by these 2 concepts only calculate the amount the normal freezing/boiling point lowered. -They do not tell us the actual boiling or freezing point.

Are gases mixed with gases thought of as solutions? -Why is that?

-They can be but gas mixtures are more properly defined as mixtures. -this is because gas molecules do not interact all that much chemically, as described by the kinetic molecular theory of gases.

Why does the formation of a complex increase the solubility of other salts containing the same ions?

-This is because it uses up the products of those dissolution reactions, shifting the equilibrium to the right (the opposite of the common ion effect)

Is the condensation rate affected by the presence of solute particles?

-Unlike the the evaporation rate which is reduced by the presence of solute particles, The condensation rate is unaffected by the presence of solute particles. -since the condensation rate stays the same and the evaporation rate reduces with added solute, there is an overall, net reduction in vapor pressure as a result (less vapor particles above solution due to decreased evaporation and constant condensation).

What direction does water move, higher or lower solute concentration?

-Water towards higher solute concentration.

When are solutes considered soluble?

-When they have a molar solubility above 0.1 M in solution. - When the change in Gibbs free energy is negative at a given temperature.

Is the solubility product constant (Ksp) temperature dependent?

-Yes, like all equilibrium constants(Keq, Ka, Kb, and Kw), the solubility product constant is temperature dependent. -the solubility product constant increases with increasing temperature for non-gas solutes and decreases for gas solutes.

complex ion

-also called coordinated ion. -refers to a molecule that in which a cation is bonded to at least 1 electron pair donor aka a ligand or (which could include water).

coordinate covalent bond

-bonds that hold together complex ion complexes. forms when one atom donates a pair of electrons to be shared with an atom or ion that needs two electrons to become stable

ligands

-electron pair donors that combine with a cation to form a complex ion - its a Nucleophile ; lewis Acid -is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex.

When will molarity (M) be used?

-molarity will be used for: 1) Rate laws 2) law of mass action 3) Osmotic pressure 4) Ph 5) pOH 6) Nernst equation

Does the common ion effect have any effect on the Ksp (solubility product constant) why?

-no -The common ion effect only results in a reduction in molar solubility of common salt. -This means that the Ksp will remain constant even if the molar solubility changed. 2) since the solution already contains 1 of the constituent ions from the products side of the dissociation equilibrium, the system will shift to the left side, reforming the solid salt due to an increase in salt concentration. as a result the solubility for the solid is reduced, and less solid dissolves in the solution.

Should the law of mass action of solutions(Ksp) have a denominator?

-no -this is because pure solids and liquids do not appear in the equilibrium constant. -Because pure solutes and pure solvents are added to create a solution, Ksp expressions should never have denominators.

Is the solubility product constant (Ksp) pressure dependent?

-only when the solution consists of a gas dissolved into a liquid. -Ksp will be larger for gases at higher pressure than lower ones.

what happens, in terms of entropy, during the formation of a solution?

-since entropy is a measure of molecular disorder, we can define our change in that way; increased disorder will increase entropy. -EX: dissolving NaCl into water: - the solvation of NaCl crystals into water will allow the NaCl molecules restricted into their crystal structure to move freely in solution. This means that there will be an increase in (S) due to the increased disorder of the Na+ Cl- ions. -The water on the other hand, due to the addition of the ions, will see a decrease in entropy. this is because water molecules are more restricted in their ability to move around due to their neighbors (Na+ Cl-). - In the end, the dissolution of NaCl will see a greater increase in entropy than the decrease in entropy experienced by water. -This means that the overall entropy change will be positive. and the dissolution will be spontaneous.

What ions are usually given in the mcat that are often used as counterions to what is actually important?

-sodium and nitrate ions are usually given.

When is solvation exothermic?

-solvation is exothermic when the new interactions are stronger than the original ones -favored at low temperatures -EX: The dissolution of gases into liquids is an exothermic process because the only significant interactions that must be broken are those between water molecules- CO2, as a gas demonstrates minimal intermolecular interactions.

IP values to see if solution has reached equilibrium?

1) IP<Ksp: unsaturated, solute will continue to dissolve. 2) IP=Ksp: saturated, solution is at equilibrium. 3) IP>Ksp: supersaturated, precipitation will occur.

colligative properties

1) Vapor pressure depression 2) Boiling point elevation 3) freezing point depression 4) osmotic pressure.

What are the rules of aqueous solutions needed to know for the MCAT?

1) all salts containing (NH4+) and alkali metal cations (Na+) are water soluble. 2) all salts containing nitrate (NO3-) and acetate (CH3COO-) anions are water soluble. 3) halides(Cl-, ,Br-, I- ) exception of florides . halides with Pb2+ and Ag+ and HG22+ are insoluble. - all salts of group 1 metals, and all nitrate salts are soluble. Example of insoluble compounds: all carbonate, phosphate, sulfides, sulfites - all hydroxides except when combine to GROUP 1 metal

What does the spontaneity of solvation depend on?

1) enthalpy -solutions may form spontaneously for both endothermic and exothermic solvations. 2) entropy

molecular level of Raoul's law

1. condensation rate is unaffected by the presence of solute particles 2. the evaporation rate is reduced by the presence of solute particles 3. there is an overall, net reduction vapor pressure as a result

solubility

A measure of the maximum amount of solute that will dissolve in a particular solvent at a given temperature. - solubility is directly proportional to temperature - solubility is not affected by pressure

common ion effect

a decrease in the solubility of an ionic compound caused by the addition of a common ion. EX: If a salt like CaF2 is dissolved into water already containing Ca++ ions(from some other salt, perhaps CaCl2), the solution will dissolve less CaF2 than would an equal amount of pure water.

parts per million (ppm)

a method of expressing a concentration of a dissolved substance in a solution. - 1 ppm is equivalent to 1 milligram per liter.

Le Chatelier's Principle

a principle stating that if a constraint (such as a change in pressure, temperature, or concentration of a reactant) is applied to a system in equilibrium, the equilibrium will shift so as to tend to counteract the effect of the constraint.

solutions

are homogeneous (same throughout) mixtures of 2 or more substances that combine to form a single phase, usually the liquid phase.

Are all mixtures considered solutions?

no, not all mixtures are considered solutions. but all solutions are considered mixtures.

Microstate Entropy

number of energy

Hydration

solution process with water as the solvent

boiling point elevation

the difference in temperature between the boiling point of a solution and the boiling point of the pure solvent. ∆Tb=iKbm ∆Tb= the increase in boiling point. i= van't hoff factor Kb= proportionality constant characteristic of a particular solvent( provided on test day) m= molality of solution.

freezing point depression

the difference in temperature between the freezing point of a solution and the freezing point of the pure solvent. -the presence of solute particles in a solution interferes with the formation of the lattice arrangement of solvent molecules in a solid state. thus a greater amount of energy must be removed from the solution. (resulting in a lower temperature)

solubility product constant (Ksp)

the equilibrium constant expression for the dissolving of a sparingly soluble compound. - temperature dependent Ksp=[A^n+]^m[B^m-]^n -EX:Ksp of silver cloride ksp=[Ag+][Cl-]

Molarity

the number of moles of solute per liter of solution -The volume refers to the volume of the solution not the solvent used to prepare the solution. - the most common on the MCAT

van't Hoff factor

the number of particles into which a compound dissociates in solution. Ex: i=2 for NaCl because each formula unit of Nacl dissociates into 2 particles- a sodium ion (Na+) and a chloride ion(Cl-)- when it dissolves. -covalent molecules like glucose do not readily dissolve and therefore have an i value of 1 (i=1)

osmotic pressure

the pressure that would have to be applied to a pure solvent to prevent it from passing into a given solution by osmosis, often used to express the concentration of the solution.

solvent

the substance in which a solute dissolves -component of the solution that remains in the same phase after mixing.

saturated

when a solution is at equilibrium.

Precipitation

when you add more solute to an already saturated solution, rather than dissolve, the solute will precipitate and remain in solid form the the bottom of the container.

freezing point depression equation

ΔTf = iKfm water Kf:1.86 Tf:Freezing point of solution i: vant hoff factor KF:cryoscopic constant m:molarity

percent composition by mass

mass of solute/mass of solution x 100%

Molar solubility

molarity of a solute in a saturated solution/solution that has reached equilibrium.

Molality (m)

moles of solute/kg of solvent -used in boiling point elevation and freezing point depression.

solute

The substance that is dissolved in a solution. Ex. NaCl, NH3,C6H12O6,or CO2

Raoult's Law

As solute is added to a solvent, the vapor pressure of the solvent decreases proportionately

Raoult's Law Equation

PA= XAPA° PA= vapor pressure of solvent A when solutes are present. XA= is the mole fraction of solvent A in solution. PA°= is the vapor pressure of solvent A in its pure state.

sparingly soluble salts

Solutes that dissolve minimally in the solvent, molar solubility under 0.1 M.

Solvation/Dissolution

The electrostatic interaction between solute and solvent molecules.

ion product (IP)

The general term for the reaction quotient of a dissolving ionic compound; compared to Ksp to determine the saturation status of a solution. IP=[A^n+]^m[B^m-]^n

dilution

The process of adding a solvent to a solution of high concentration to produce a solution of lower concentration. -The concentration of a solution after dilution can be determined using the following equation. MiVi=MfVf M=molarity V= volime i=initial values f=final values

chelation

The process of binding metal ions to the same ligand at multiple points.

law of mass action

The rate of a chemical reaction is proportional to the product of the concentrations of the reactants

mole fraction (X)

The ratio of the moles of solute in solution to the total number of moles of both solvent and solute - the sum of mole fraction in a system is equal to 1

What occurs in the process of solvation?

involves breaking intermolecular interactions bonds between solute molecules and between solvent molecules - and forming new intermolecular interactions between solute and solvent molecules together.

aqueous solution

a solution in which water is the solvent. -denoted by symbol (aq) -

formation constant (Kf)

an equilibrium constant for the formation of a complex ion from the hydrated metal ion and ligands. Kf=[M^xL^y]/[M]^x[L]^y

unit of concentration

concentration is denote by the amount of solute in a solvent - on the MCAT concentration are commonly expressed as precent composition by mass, mole of fraction, molarity, molality, normality

supersaturated solution

contains more dissolved solute than a saturated solution at the same temperature

Is freezing point depression a colligative property?

freezing point depression is a colligative property.

hydronium ion

hydrogen ion combines with a water molecule to form a hydronium ion, H3O(+) -in some solutions, such as acids, the formation of this complex can occur.