Solution Chemistry

Solutions• Solution chemistry- where water is the

solvent (called aqueous solutions).

• Solutions—homogeneous mixture of two or more substances in which dissolved substance is ionized, its composition is the same throughout its volume.

• Usually a solid is dissolved in a liquid, but liquids can be dissolved in other liquids and gases can be dissolved in a liquid as well.

Solutions

• Solutions are homogeneous mixtures of two or more pure substances.

• In a solution, the solute is dispersed uniformly throughout the solvent.

Solubility?The amount of a substance that dissolves in a given volume of solvent at a given temperature.

Common units: grams solute/ 100 grams of solvent or

grams solute/ 100mL of water

Solutions

How does a solid dissolve into a liquid?

What ‘drives’ the dissolution process?

What are the energetics of dissolution?

How Does a Solution Form?1. Solvent molecules attracted to surface ions.2. Each ion is surrounded by solvent molecules.3. Enthalpy (DH) changes with each interaction broken or

formed.

Ionic solid dissolving in water

How Does a Solution Form?1. Solvent molecules attracted to surface ions.2. Each ion is surrounded by solvent molecules.3. Enthalpy (DH) changes with each interaction broken or

formed.

How Does a Solution Form

The ions are solvated (surrounded by solvent).

If the solvent is water, the ions are hydrated.

The intermolecular force here is ion-dipole.

Factors Affecting Solubility

•Solute-solvent interaction - Like dissolves like•Temperature Factor -

i) Solids/Liquids- Solubility increases with Temperature. Increase K.E. increases motion and collision between solute / solvent.

ii) gas - Solubility decreases with TemperatureIncrease K.E. result in gas escaping to atmosphere.•Pressure Factor -

i) Solids/Liquids - Very little effect. Solids and Liquids are already lose together, extra pressure will not increase solubility.

ii) gas - Solubility increases with Pressure.Increase pressure squeezes gas solute into solvent.

Properties of Water

2-

+

+

Water is polar

Water forms hydrogen bonds

When water freezes it expands

•hydrogen bonding is the weak intermolecular bond between the H end of one molecule and the O, N, or F end of another molecule•

2- +

+

2-

+

+

• Polar liquids tend to dissolve in polar solvents. “Likes dissolve likes”• Miscible liquids: mix in any proportions.

• water and ethanol are miscible- broken hydrogen bonds in both pure liquids are re-established in the mixture.

• Immiscible liquids: do not mix. • The number of carbon atoms in a chain affect solubility: the

more C atoms the less soluble in water.

Solute-Solvent Interactions

• The number of -OH groups within a molecule increases solubility in water.

• The more polar bonds in the molecule, the better it dissolves in a polar solvent.

• The less polar the molecule the less it dissolves in a polar solvent and the better is dissolves in a non-polar solvent.

• The greater the contact between the solute and solvent, the faster the substance will dissolve. Therefore, stirring and crushing, which increases the surface area, increase the rate of solution

Solute-Solvent Interactions Cont.

Ionic Solutes

Polar water molecules interacting with positive and negative ions of a salt.

+

+

+

+

+

+

++

++

++ +

++++

+

++

+

+

+

+++

++

--

- ------

-- -

-

-

-

---

-

--

- -

--

-

---

-- - -

- -

+

-

-

+ -

Positive end of the water molecule attracts to the anion (negative ion) Negative end of the water molecule attracts to the cation (positive ion).Making it a strong electrolyte..

Electrolytes Compounds- dissolved in water conduct an electric current.

Strength correlates with number of ions in solution.

Strong electrolytes conduct a much stronger current because have more ions

Nonelectrolytes Do not conduct an electric current because do not contain ions.

Electrolytes

Strong Weak NonElectrolyte Electrolyte Electrolyte

Polar Molecules

Sugar is a molecular solid with weak bonds. It does not form ions and therefore does not ionize in solution. Meaning sugar cannot act as an electrolyte.

In order for a substance to dissolve in water, the water molecules must be more attracted to the new substance added.

Remember water molecules are weakly bonded to each other by hydrogen bonds.

Will this substance dissolve in water?

Yes, because water will be attracted to it.

Substances that don’t dissolve are called insolubleE.g. Petroleum (crude oil), which are non-polar

So if you want to dissolve grease which is non-polar, you need to use a non-polar solvent.

Petroleum in a non-polar organic molecule

If solution process absorbs energy- solubility will be INCREASED as the temperature is increased.

If the solution process releases energy- solubility will DECREASE with increasing temperature.

Temperature-

• Absorption of energy and increase in solubility: • sugar dissolves better in warm water than cold.• As temperature increases, solubility of solids

generally increases.• Exception: Ce2(SO4)3

Temperature Cont.

Gases and liquids: • gases get less soluble as temperature increases.

• Example: carbonated beverages• Thermal pollution: lakes get too warm, CO2 and

O2 become less soluble and are not available for plants or animals.

Temperature Effects:

Pressure:

• Higher the pressure, the more molecules of gas are close to the solvent and the greater the chance of a gas molecule striking the surface and entering the solution.

– higher the pressure, the greater the solubility.

Pressure Effects:

• Carbonated beverages are bottled with a partial pressure of CO2 > 1 atm.

• Opening bottle, the partial pressure of CO2 decreases and the solubility of CO2 decreases.

• bubbles of CO2 escape from solution.

Pressure Effects:

Degree of saturation

• Saturated solution Solvent holds as much

solute as is possible at that temperature.

Undissolved solid remains in flask.

Dissolved solute is in dynamic equilibrium with solid solute particles.

Degree of saturation

• Unsaturated Solution Less than the maximum

amount of solute for that temperature is dissolved in the solvent.

No solid remains in flask.

Degree of saturation

• Supersaturated Solvent holds more solute than is normally possible at

that temperature.These solutions are unstable; crystallization can often

be stimulated by adding a “seed crystal” or scratching the side of the flask.

Degree of saturation

Unsaturated, Saturated or Supersaturated?

How much solute can be dissolved in a solution?

Saturated Solutions

Saturated/unsaturated solutions

To dissolve 120 g the temp must be raised

to 80oC

at 50o 88.0 g of KNO3 will dissolve

1. Which substance’s solubility increases the most with temperature?2. Which substance’s solubility changes the least with temperature?3. What is the solubility of KI at 8oC?4. What temperature is needed to dissolve 160 g of potassium iodide, KI, in 100 g of water?

Colloids:

Suspensions of particles larger than individual ions or molecules, but too small to be settled out by gravity.

Tyndall Effect

• Colloidal suspensions can scatter rays of light.

• This phenomenon is known as the Tyndall effect.

Colloids in Biological Systems

Some molecules have a polar, hydrophilic (water-loving) end and a nonpolar, hydrophobic (water-hating) end.

Colloids in Biological Systems

Sodium stearate is one example of such a molecule.

Colloids in Biological Systems

These molecules can aid in the emulsification of fats and oils in aqueous solutions.

Ways of Expressing Concentrations of Solutions

Concentration

• Mass Percent or Volume Percent• Mole Fraction• Molarity (M)• Molality (M)

Mass Percentage

Mass % of A =mass of A in solutiontotal mass of solution 100

Volume of A in solutionTotal volume of solutionVolume % of A = 100

Parts per Million andParts per Billion (for mass or volume)

ppm =mass of A in solutiontotal mass of solution 106

Parts per Million (ppm)

Parts per Billion (ppb)

ppb =mass of A in solutiontotal mass of solution 109

moles of Atotal moles in solutionXA =

Mole Fraction (X)

• A could be either solvent or solute– make sure you find the quantity you need!

mol of soluteL of solutionM =

Molarity (M)

• Concentration of solution; or its strength

• Can change with temperature.

mol of solutekg of solventm =

Molality (m)

Not temperature dependent- Mass and moles are not dependent on temperature

Changing Molarity to Molality

Problem

• Give the concentration (molarity of 0.0012 grams of NaCl in 545 mL of water)

• Molar mass of NaCl = 58.44 g/mol

Honors: Problem

• Give the concentration (in molality) of 0.0012 grams of NaCl in 545 mL of water

• Density of water @ 25°C = 0.9970 g/mL• MW of NaCl = 58.442 g/mol

Honors Problem

• A 10.7 molal solution of NaOH has a density of 1.33 g/cm3 at 20°C. molar mass of NaOH = 39.996 g/mol and molar mass of H2O = 18.0153 g/mol.

– Calculate the mole fraction of NaOH, the weight percentage of NaOH and the molarity of the solution.

Honors practice

• An aqueous solution of NaCl is created using 133 g of NaCl diluted to a total solution volume of 1.00 L.– Calculate the molarity, molality, and mass

percent of the solution– Density of 1.08 g/mL– Molar mass of NaCl = 58.442 g/mol.

Acid/Base Chemistry

Acids• from the Latin word acere “sour”• taste sour (but you wouldn’t taste an acid to

see)• change litmus paper red• corrosive to some metals (reacts to create

hydrogen gas – H2)• Donates a hydrogen ion (H+) to another

substance• Create a hydrogen ion (H+) or hydronium ion

(H3O+) when dissolved in waterHCl H+ + Cl-

HydrochloricAcid

Hydrogenion

Chlorideion

Examples: hydrochloric acid, vinegar, lemon juice, rainwater

H2O Notice howthe hydrogenion is releasedwhen the acid

is in water

Bases (Alkalis)• taste bitter • feel slippery or soapy• change litmus paper blue• react with oils and grease- soaps• Accept a hydrogen ion (H+) • create a hydroxide ion (OH-) when dissolved

in water

Examples: sodium hydroxide, Drano, Tums, baking soda

NaOH Na+ + OH-

SodiumHydroxide

Sodiumion

Hydroxideion

H2O

Notice howthe hydroxideion is releasedwhen the baseis in water; this

ion can accept a hydrogen ion

(H+)

Neutralization Reaction

• occurs when acids and bases react with each other to produce water and salt– acids release a hydrogen ion (H+) and bases

release a hydroxide ion (OH-) water (H2O)– the negative ion from the acid joins with the

positive ion of a base salt

HCl + NaOH H2O + NaClHydrochloric

Acid(acid)

Sodium Hydroxide

(base)

WaterSodium Chloride(salt)

Both the salt and water are neutral substances; therefore, that is why this is referred to as a neutralization reaction.

Acid, Base, or Neutralization?

Zn + 2H+ Zn2+ + H2

NH3 + H2O NH4+ + OH-

HClO + LiOH LiClO + H2O

HCl + H2O H3O+ + Cl-

Acid – because H2 gas was given off

Acid – because H3O+ is present in the products

Base – because OH- is present in the products

Neutralization – because of the salt and water in the products

WhoTheory:Acid=

When

Arrhenius increases H+ 1880’s

Brønsted proton donor 1923

Lowry ditto 1923

Lewis Electron-pair acceptor 1923

Three definitions of acid

Some Definitions• Arrhenius acids and bases

– Acid: Substance that, when dissolved in water, increases the concentration of hydrogen ions (protons, H+).

– Base: Substance that, when dissolved in water, increases the concentration of hydroxide ions.

Some Definitions• Brønsted–Lowry: must have both

1. an Acid: Proton donor

and2. a Base: Proton acceptor

The Brønsted-Lowry acid donates a proton (H+ ion),

while the Brønsted-Lowry base (H+ ion) accepts it.

Brønsted-Lowry Acids and Bases:

Which is the acid and which is the base in each of these rxns?

A Brønsted–Lowry acid……must have a removable (acidic) proton.

HCl, H2O, H2SO4

A Brønsted–Lowry base……must have a pair of nonbonding electrons.

NH3, H2O

Types of Proton acceptors and donators:

• Monoprotic acid- Donates 1 proton (H+)– HCl, HF, HI, HClO3

• Diprotic acid- Donates 2 protons (2H+)– H2S, H2SO4

• Triprotic acid- Donates 3 protons (3 H+)– H3PO4

If it can be either…

...it is amphiprotic.

HCO3–

HSO4 –

H2O

What Happens When an Acid Dissolves in Water?

• Water acts as a Brønsted–Lowry base and abstracts a proton (H+) from the acid.

• As a result, the conjugate base of the acid and a hydronium ion are formed.

Conjugate Acids and Bases:• From the Latin word conjugare, meaning “to join

together.”• Reactions between acids and bases always yield

their conjugate bases and acids.

Acid and Base Strength

• Strong acids are completely dissociated in water.– Their conjugate bases are

quite weak.• Weak acids only dissociate

partially in water.– Their conjugate bases are

weak bases.

Acid and Base Strength

• Substances with negligible acidity do not dissociate in water.– Their conjugate bases are

exceedingly strong.

Autoionization of WaterAs we have seen, water is amphoteric.• In pure water, a few molecules act as bases and a few

act as acids.

This process is called autoionization.

Ion-Product Constant

• The equilibrium expression for this process isKc = [H3O+] [OH–]

• This special equilibrium constant is referred to as the ion-product constant for water, Kw.

• At 25°C, Kw = 1.0 10-14

pH

pH is defined as the negative base-10 logarithm of the hydronium ion concentration.

pH = –log [H3O+]

pH

• In pure water,

Kw = [H3O+] [OH–] = 1.0 10-14

• Because in pure water [H3O+] = [OH-],

[H3O+] = (1.0 10-14)1/2 = 1.0 10-7

pH• Therefore, in pure water,

pH = –log [H3O+] = –log (1.0 10-7) = 7.00

• An acid has a higher [H3O+] than pure water, so its pH is <7

• A base has a lower [H3O+] than pure water, so its pH is >7.

pH

These are the pH values for several common substances.

Other “p” Scales

• The “p” in pH tells us to take the negative log of the quantity (in this case, hydronium ions).

• Some similar examples are– pOH –log [OH-]– pKw –log Kw

Watch This!

Because[H3O+] [OH−] = Kw = 1.0 10-14,

we know that

–log [H3O+] + – log [OH−] = – log Kw = 14.00

or, in other words,pH + pOH = pKw = 14.00

How Do We Measure pH?

– Litmus paper• “Red” paper turns

blue above ~pH = 8• “Blue” paper turns

red below ~pH = 5– An indicator

• Compound that changes color in solution.

How Do We Measure pH?

pH metersmeasure the voltage in

the solution