Reactions

in Solution

AP Chemistry

S

o

l

u

t

i

o

n

s

solvent

solution: a homogeneous mixture of

two or more substances

-- The ______ is present in greatest

quantity.

-- Any other substance present is

called a ______.

aqueous solutions: solutions in which water is the

dissolving medium (i.e., the solvent)

electrolyte: any substance whose aqueous solution

will conduct electricity

e.g.,

-- as opposed to a nonelectrolyte,

e.g.,

solute

HCl, NaCl, KOH

any sugar (C6H12O6, C12H22O11)

or any alcohol (CH3OH, CH3CH2OH)

OH H

+O

H HO

HH

OH H

d+d+

d–

+ – + – + – + – – + –+ – + – +– + – + – +–

+

-- then solvation: solvent particles surround solute

particles (if solvent is H2O, we say “hydration”)

-- solvent particles

“pull” solute ions

from the crystal

Intermolecular forces (IMFs) operate between

solvent and solute particles. Solutions form

when solute-solvent IMFs are comparable

to solute-solute IMFs.

+

As a general rule, ionic solids dissociate into ions in

aqueous solution. The partial (–) charge on the O and

the partial (+) charge on the H atoms allow H2O to

interact strongly with, and “pull out,” ions in the crystal

lattice. Thus, ionic compounds are often strong

electrolytes.

OH H

– + – + –+ – + – +–

+– – + –+ – + – +–

+O

H HO

HH

OH H

d+d+

d–

+

For molecular compounds, structural integrity of

molecules is maintained. Substance may dissolve,

but generally won’t split into

ions. Thus, mol. comps.

tend to be nonelectrolytes.

-- major exceptions: acids and NH3 (ammonia)

** When molecular compounds DO split into ions,

it is called ionization, not dissociation.

Strong electrolytes exist almost completely as ions

in aqueous solution.

e.g.,

Weak electrolytes produce only a small concentration

of ions in reaching equilibrium.

e.g.,

HCl(aq) H+(aq) + Cl–(aq)

KCl(aq) K+(aq) + Cl–(aq)

lots of

product

CH3COOH(aq) CH3COO–(aq) + H+(aq)

HF(aq) H+(aq) + F–(aq)

lots of

reactant

(note the one-sided arrow)

(note the double arrow)

Some of the strong electrolytes are the

strong acids and strong bases.

STRONG ACIDS STRONG BASES

the hydroxides of...

“strong base

cations”

Li, Na, K, Rb,

Cs, Ca, Sr, Ba

hydrochloric, HCl

hydrobromic, HBr

hydroiodic, HI

chloric, HClO3

perchloric, HClO4

nitric, HNO3

sulfuric, H2SO4

If… “most”

Be careful to distinguish between

dissolution and dissociation/ionization

in regard to strongs or weaks. For

example, CH3COOH dissolves completely, but ionizes

only slightly; it is therefore a weak electrolyte. On the

other hand, Ba(OH)2 dissolves very little, but the

amount that does dissolve dissociates almost

completely. Ba(OH)2 is a strong electrolyte.

The question is: Of the amount that dissolves, what

fraction dissociates/ionizes?

If… “not much”

STRONG

WEAK

Sometimes it is not readily apparent whether a solution

has been formed or whether a chemical reaction took

place. To help you decide, consider the following:

If the product is evaporated to dryness, a solution

would give you what you started with.

e.g., NaCl(s) + H2O(l) NaCl(aq)

Ni(s) + 2 HCl(aq) NiCl2(aq) + H2(g)

(SOLN)

(RXN)

Precipitation reactions are reactions in solution that

form an insoluble product.

The insoluble product is called a... precipitate.

Solubility

Crystallization is the opposite of the solution process.

-- When the rates of solution and crystallization

are equal, __________ is established.

SOLUTE + SOLVENT SOLUTIONsolution

crystallization

A saturated solution of NaCl,

in which the rates of solution

and crystallization are equal.

equilibrium

(aq)

Suppose you mix solutions of

lead(II) nitrate and sodium iodide.

The ions present are... Pb2+, NO3–, Na+, I–

Pb2+ + 2 NO3– + 2 Na+ + 2 I–

Pb(NO3)2 NaI

Write the overall ionic equation…

PbI2 + 2 NO3– + 2 Na+

(aq) (aq) (aq) (aq) (aq)(ppt)

Cancel the spectator ions to

get the net ionic equation…

Pb2+ + 2 I– PbI2(aq) (aq) (ppt)

Solubility Trends

The solubility of MOST solids increases with

temperature.

The rate at which solids dissolve increases with

increasing surface area of the solid.

The solubility of gases decreases with

increases in temperature.

The solubility of gases increases with the

pressure above the solution.

Therefore…Solids tend to dissolve best when:

o Heatedo Stirredo Ground into small particles

Gases tend to dissolve best when:

o The solution is coldo Pressure is high

Solubility Chart

ConcentrationCalculation Concentration

• In a solution, the solute is distributed evenly

throughout the solvent. This means that any part of a

solution has the same ratio of solute to solvent as any

other part of the solution.

• This ratio is the concentration of the solution.

• The concentration is the amount of a particular

substance in a given quantity of a solution

Concentration, continuedCalculating Concentration, continued

• Concentrations can be expressed in many forms.

• % by mass (g of solute/100 g solution)

• M = Molarity (mol of solute/L of solution)

• m = Molality (mol of solute/kg of solvent)

• Ppm = Parts per million (g solute/1,000,000 g solution)

Molarity

A solution’s concentration tells us

the amount of solute per solvent.

A common unit of concentration

is molarity. -- equation:

L

mol M

mol

LM

What mass of magnesium nitrite is needed to make

3.25 L of a 0.35 M solution?

mol = M L = 0.35 M (3.25 L ) = 1.1375 mol

mol 1

g 116.31.1375 mol = 130 g Mg(NO2)2

Mg2+ NO2– Mg(NO2)2

Steps for Properly Mixing an Aqueous Solution

1. Fill an appropriate container (e.g., graduated

cylinder or volumetric flask) mostly full of water

(~80% full). This is an approximate technique

and should take very little time.

2. Weigh out the proper amount

of solute and mix it into the

water from Step 1.

3. “Top off” the solution to the

proper volume and mix.

DONE.

• Preparing 1.000 L of a 0.5000 M Solution

What is the conc. of sodium ions in a 0.025 M

solution of sodium phosphate?

strong electrolyte

Na3PO4(aq)

0.025 M 0.075 M

3 Na+(aq) + PO43–(aq)

Na+ PO43–

Na3PO4

Solution

Stoichiometry

What volume of 0.150 M sulfuric acid is needed to neutralize

26 g sodium hydroxide? 22

NaOH H2SO4 V of gases

at STPV of sol’ns

H2SO4 NaOH+ Na2SO4 + H2O

NaOH g 40

NaOH mol 1

NaOH mol 2

SOH mol 1 4226 g NaOH = 0.325 mol H2SO4

M

mol L

42

42

SOH M 0.150

SOH mol 0.325

= 2.2 L of 0.150 M H2SO4

LMpart.

vol.

mass mass

vol.

part.

mol

LM

mol

Dilutions

Aqueous acids (and sometimes bases) can be

purchased in concentrated form and diluted to any

lower concentration. A purchased bottle of acid is

called a concentrate or a stock solution.

-- **Safety Tip: When diluting, add acid or base

to water, not the other way around.

Dilution Equation: MCVC = MDVDC = conc.

D = dilute

14.8

Conc. phosphoric acid is 14.8 M. What volume of

concentrate is req’d to make 25.0 L of 0.500 M acid?

(VC) = 0.500 (25.0) VC = 0.845 L

What mass of lead(II) nitrate will consume

85.0 mL of 0.45 M sodium iodide?

22Pb(NO3)2 NaI+ PbI2 + NaNO3

NaI Pb(NO3)2

mol NaI = M L = 0.45 M (0.085 L )

= 0.03825 mol NaI

NaI mol 2

)Pb(NO mol 1 23

23

23

)Pb(NO mol 1

)Pb(NO g 331.20.03825 mol

NaI

= 6.3 g Pb(NO3)2