PHT121 Lecture 1

Solvent

A liquid substance capable of dissolving other substances. the component that determined the phase of the solution

Why are most real solutions non-ideal?

Because solute-solute, solute-solvent and solvent-solvent forces of interaction are unequal.

Why is the the vapor pressures exerted by solids are usually much lower than those exerted by liquids?

Because the intermolecular forces in solids are stronger than those in liquids so that the escaping tendency for surface molecules is higher in liquids

How can gas molecules overcome the attractive forces?

If the kinetic energy is less than the attractive forces, a liquid or solid will form. The average kinetic energy of the particles in a gas is great enough to overcome the forces of attraction between them. The molecules of a gas move apart when they collide. ... The molecules in a liquid (or solid) do not move apart. The thermal motions of molecules of substance can overcome the attractive force that exist between the molecules.

Solubility

The amount of solute that can be dissolved by the solvent Is the amount of solute that passes into solution when an equilibrium is established between the solution and excess (undissolved substance).

Solutions

homogeneous mixtures

Freezing

liquid to solid

What is the unit of molarity?

m/liter

Melting

solid to liquid

Thermal motion

the chaotic, random motion of atoms and molecules

Molality method of expressing a concentration

the concentration of a solution expressed in moles of solute per kilogram of solvent The number of moles of solute divided by the mass of the solvent, i.e. mol/kg.

Partial pressure

the contribution each gas in a mixture of gases makes to the total pressure

Supersaturated solution

a solution that holds more dissolved solute than is required to reach equilibrium at a given temperature A solution containing a solute(s) more than its solubility limit.

In a liquid solvent containing a dissolved solute (solution): molecules of both the solvent and solute may show a tendency to escape from the surface and so contribute to the vapor pressure. The relative tendencies to escape will depend on:

1. The numbers of the different molecules in the surface of the solution 2. The strengths of the attractive forces between adjacent solvent molecules 3. The strengths of the attractive forces between solute and solvent molecules

Sublimation

A change directly from the solid to the gaseous state without becoming liquid

Non-ideal solution

A non-ideal solution is a solution whose properties are generally not very predictable on account of the intermolecular forces between the molecules. None. Non-ideal solutions by definition cannot be dealt with through Raoult's Law. Raoult's Law is strictly for ideal solutions only. Example of non-ideal solutions: 1- solution of Ethanol and Acetone. 2- solution of Ethanol and water.

Normal Solution

A solution containing one gram equivalent of solute per liter of solution. contains the equivalent weight of the solute (gm) found in one liter of solution.

Solute

A substance that is dissolved in a solution. it is usually dispersed as molecules(sugar) or ions (NaCl)throughout the solvent.

Quantity per quantity method of expressing a concentration

Concentrations are often expressed simply as the weight or volume of solute that is found in a given weight or volume of the solution. w/v (majority) w/w, v/v

Unsaturated solution

GIMME MORE! A solution that contains less solute than a saturated solution does and that is able to dissolve additional solute A solution having a solute (s) less than its solubility limit

Normality

Gram equivalent weight of solute per liter of solution, often denoted by N. the number of equivalents of a substance dissolved in a liter of solution Normal solution is one that contains the equivalent weight of the solute (gm) found in one liter of solution.

Are solutions a single or double phase?

Single

Raoult's law is met example

Solution 1: Energetic molecules evaporate at a certain rate Solution 2: Energetic molecules evaporate at a certain rate Solution 1 mixed with Solution 2: In an ideal mixture of these two liquids, the tendency of the two different sorts of molecules to escape is unchanged. If the red molecules still have the same tendency to escape as before, that must mean that the intermolecular forces between two red molecules must be exactly the same as the intermolecular forces between a red and a blue molecule. • If the forces were any different, the tendency to escape would change

Which is greater in quantity a solute or a solvent?

Solvent

Condensed system

Systems where in only the solid and liquid states are considered and the vapor is ignored All condensed systems have the inherent ability to give rise to a vapor pressure. •Surface loss of vapor from liquids by the process of evaporation is more common than surface loss of vapor from solids via sublimation.

What causes the attraction between liquid molecules?

The average kinetic energy of the particles in a liquid (or solid) is small enough that the forces of attraction between them is sufficient to hold the particles close together. The molecules in a liquid (or solid) do not move apart Van der Waals forces of attraction leads to some degree of coherence between the molecules of liquid.

Evaporation

The change of a substance from a liquid to a gas

Condensation

The change of state from a gas to a liquid

Parts method of expressing a concentration

The number of 'parts' of solute dissolved in a stated number of 'parts' of solution. Parts per million: the number of molecules (or atoms) of a substance in a mixture for every 1 million molecules (or atoms) in that mixture 1. solid in a liquid: parts by weight (g) of solid in parts by volume (ml) of solution 2. liquids in liquids: parts by volume of solute in parts by volume of solution 3. gases in liquids: parts by weight of gas in parts by weight of solution.

Molarity method of expressing a concentration

The number of moles of solute in 1 liter of solution Molar concentration and its symbol M describe the molarity of a solution • 1 M = mol wt / liter

Mole fraction

The ratio of the moles of solute in solution to the total number of moles of both solvent and solute

Mole Fraction method of expressing a concentration

The ratio of the moles of solute in solution to the total number of moles of both solvent and solute Remember back in high school when you were memorizing ''part over whole'', yeah yeah that shit is exactly what this is. The number of moles of solute divided by the total number of moles of solute and solvent, i.e. mole fraction of solute= n1/n1+n2 • Where n1 and n2: numbers of moles of solute and solvent, respectively.

Why does a molecule evaporates?

Think of it like that, the molecule that evaporates is more energetic and doesn't have a strong enough attraction to keep it from evaporating. In a pure liquid, some of the more energetic molecules have enough energy to overcome the intermolecular attractions and escape from the surface to form a vapour. • The smaller the intermolecular forces, the more molecules will be able to escape at any particular temperature.

Total vapor pressure equation of a binary solution

X1 & X2 : mole fractions of the solute and solvent P1 & P2 : partial vapor pressures exerted above the solution by solute and solvent P1o & P2o : vapor pressures exerted by pure solute and pure solvent respectively.

Does the vapor pressure increases with temperature?

Yeah yup.

Raoult's Law

a law stating that the vapour pressure of an ideal solution is proportional to the mole fraction of solvent. In the model it is assumed that the strengths of all intermolecular forces are identical, i.e. solvent-solvent, solutesolvent and solute-solute interactions are the same and are equal to the strength of the intermolecular interactions in the pure solvent or pure solute. • Because of this equality the tendencies of solute and solvent molecules to escape from the surface of the solution will be determined only by their relative numbers in the surface. • Since a solution is homogeneous by definition then the relative numbers of these surface molecules will be reflected by the relative numbers in the whole of the solution which can be expressed by the mole fractions of the components

Thermodynamic activity

a measure of the "effective concentration" of a species in a mixture, in the sense that the species' chemical potential depends on the activity of a real solution in the same way that it would depend on concentration for an ideal solution.

Binary solution

a mixture of only two components (the solute and the solvent)

Van Der Waals forces

a slight attraction that develops between the oppositely charged regions of nearby molecules

Non-volatile solute

a solute that does not exert a vapor pressure

Saturated solution

a solution that cannot dissolve any more solute under the given conditions The solution obtained when maximum amount of solute is dissolved in a solvent under given conditions.

Deposition

gas to solid

What is raoult's law

law that deals with vapor pressure in solutions. Adding non-volatile solvents will decrease the vapor pressure of a solution compared to its vapor pressure in pure solvent form. Raoult's law allows you to calculate the new vapor pressure of the solution by multiplying the mole fraction of the solvent (liquid) with the vapor pressure of the pure solvent Pv= XaPa Think of it this way: if 97% of the solution is solvent, then the new vapor pressure will be 97% of the vapor pressure of the pure solvent

Equivalent weight

the mass in grams of 1 equivalent of that acid or base

Vapor pressure

the pressure exerted by a vapor over a liquid a measure of the force exerted by a gas above a liquid The pressure exerted by the vapor at equilibrium is referred to as the vapor pressure of the substance. In a closed system evaporation of liquids takes place till an equilibrium is set up between the molecules leaving and molecules re-entering the liquid (condensate). The vapor above the liquid is then saturated and exerts a maximum vapor pressure; after that equilibrium was attained between evaporation and condensation.

Ideal gas law

the relationship PV=nRT, which describes the behavior of an ideal gas

Positive deviation from Raoult's Law

➢If the attractive forces between solute-solvent molecules are weaker than solute-solute or solvent-solvent then the components will have little affinity for each other. The escaping tendency of the surface molecules in such a system is increased when compared with an ideal solution. ➢ Therefore vapour pressure of solutions is higher in systems showing positive deviation. • i.e. P1, P2 and P are greater than expected from Raoult's law • The thermodynamic activities of the components are greater than their mole fractions, i.e. a1 > X1 and a2 > X2. e.g. 1-alcohol + benzene (small deviation) 2- water + diethyl ether - less miscible (greater + D) 3- benzene + water - immiscible (very large +D) When the attraction between the solvent molecules and solute molecules is weaker than the attractions of the molecules of themselves.

Negative deviation from Raoult's Law

➢If the solute and solvent have a strong affinity for each other that results in the formation of a complex or compound. So, negative deviation from Raoult's law occurs. and the vapor pressure of the solution is low. ➢ P1, P2 and P are lower than expected and a1< X1 and a2< X2. e.g. 1-chloroform + acetone, 2- pyridine + acetic acid and 3-water + nitric acid