Solutions, Solubility, and Reaction Types

Solutions, Solubility,and Reaction Types

Brown, LeMay Ch 4

AP Chemistry

4.1: Solutions & Expressing Concentration

Solution: homogeneous mixture of 2 or more substances

–solute: component present in lesser quantity

–solvent: component present in greater quantity

•Molarity (M) =moles solute=molvolume of solution L

Since volume of a solution changes with temperature, M of a solution changeswith temp. too. M is a good unit in measuring the conc. of a solution underconstant temp conditions like during an experiment in the lab. Ex. Titration

•Molality (m) =moles solute=molkg of solvent kg

Since mass of a solution is temp. independent, m does not change withchange in temp, hence m is a good unit to use for solution conc. whentemp is changing. Ex. B.P. Elevation, F.P. Depression

Which is more concentrated- 1Maq NaCl or I m aq NaCl ?

4.1: Solutions & Expressing Concentration

•*Normality (N): associated with acid & basestrength. Normality = molarity x n (where n = thenumber of protons exchanged in a reaction). = (moles solute)(# of acid/base equivalents) =volume of solutionmol/L

•Ex. N of 1M H2SO4 will be 2N.

•N was used earlier to express concentration ofacids/bases but is obsolete now.

Mole Fraction

•XA= nA/ nA + nB …..

•Practice Problem: What is the mole fraction ofeach component in a solution in which 3.57 g ofsodium chloride, NaCl, is dissolved in 25.0 g ofwater? Ans: X H2O 0.0421, X NaCl 0.958

•Note: Sum total of mole fractions of all componentsis always 1

•Mole fraction is a good unit but not so easy tocompute, hence has limited use.

Mass Percent

Mass percent is an easy way to express the concentration ofsolution and is most commonly used. Mass percent can bewritten in as mass/mass (m/m) or mass/volume (m/v) orvolume/volume (v/v).

Let us look at each of these carefully.

Mass % = mass of solute X100

mass of solution

Mass % = mass of solute X100

volume of solution

Mass %= volume of solute X 100

volume of solution

Practice Problem on Mass %

•What does 2% (m/m) aq NaCl mean? 2% m/V?2% v/v

•What mass % unit are you most likely to findvinegar’s conc. expressed in? Salt solution’s?

•Calculate the mole fraction of HCl in a solutionof hydrochloric acid containing 36% HCl byweight. 0.22

•What is the molality of a solution made bydissolving 5.0g of toluene (C7H8) in 225g ofbenzene (C6H6)? 0.24m

Titration movie on Titration

-It is a method to determine the molarity of unknown acid or base

-In titration, an acid or base of unknown molarity is titratedagainst a standard solution (whose M is known) of acid or base.

-The end point in a titration is indicated by a color change by theindicator. Indicators are weak acids or bases and are added insmall quantity (1-3 drops) to indicate the end point.

-At equivalence point (which should be close to end point),

moles of H+ = moles of OH-

M1V1= M2 V2 (sometimes used to get moles , M= moles/L , somoles= M XV)

-What other ways can you get the moles- for a solid acid or base?For a gas?

-color change by indicator indicates end point

-end point and equivalence points should be close. Equivalencepoint is defined when moles of acid and base become equal in atitration and end point is where the indicator changes color.

•Dilution: (Minitial)(Vinitial) = (Mfinal)(Vfinal)

Ex: How much water must be added to a solution thatcontains 100.0 g NaOH in 500. mL of solution to make thefinal concentration 2.00 M?

Minitial = (100.0 g NaOH)/(40.00 g/mol NaOH) = 5.00 M0.500 L

(Mi)(Vi) = (Mf)(Vf) = (5.00 M)(0.500 L) = (2.00 M)(Vf)

Vf = 1.25 L, so 0.75 L water must be added.

1.What is the molality of lead nitrate in 0.726 MPb(NO3)2 is: (The density of the solution is 1.202g/ml) Ans. 0.755m

2.A solution is prepared by dissolving 15.0g of NH3 in250g of water. The density of the resulting solution is0.974 g/ml. What is the mole fraction of NH3 in thesolution. Ans. 0.060

3.How many mL of a 3M NaOH solution are required tocompletely neutralize 20.0 mL of 1.5M H2SO4? (Startby writing a balanced equation!) Ans. 20.0 mL

4.How many g of NaOH is required to completely reactwith 100. mL of 1M HCl?

Practice Problems: M, m and Titrations

4.2: Properties of Solutes

•Ionic compounds (salts): dissociate in water by solvation,in which polar H2O molecules surround and separate ionsfrom ionic crystal lattice

•Molecular compounds: do not ionize becauseintramolecular bonds are stronger than bonds that formbetween H2O and molecule

•Acids may ionize or not, depending on their strength.Bases dissociate based upon their strength. Refer toNaming Notes for further details.

Electrolyte: substance which, in aqueous solution, ionizesand thus conducts electricity. Ex: salt in water. (salt dissolving in water)

Non-electrolyte: substance which, in aqueous solution,does not dissociate and thus does not conduct electricity

Strong & weak electrolytes: conductivity depends ondegree of dissociation and equilibrium position:

HA (aq) ↔ H+ (aq) + A- (aq)Strong = nearly completely dissociatedWeak = partially dissociated

•( electrolytes animation)

•(Conductivity of different substances)

4.3: Acids & Bases

–Acids: H+ donor

–Bases: H+ acceptor

–Be (OH)2 and Mg(OH)2 are weak bases. Why?

Figure 1: Strong & weak: depends on degree ofdissociation (Ka value, Table 4.1, p. 117)

Strong Acids

Strong Bases

HBr, HCl, HI

HNO3, H2SO4

HClO4, HClO3

Group 1A’s: LiOH,NaOH, KOH, RbOH,CsOH

Heavy 2A’s: Ca(OH)2,Sr(OH)2, Ba(OH)2

4.4: Ionic Equations

1.Molecular equation: shows complete chemical equation withstates of matter, undissociated

BaCl2 (aq) + Na2SO4 (aq) → 2 NaCl (aq) + BaSO4 (s)

2.Complete ionic equation: shows complete chemical equation withstates of matter, dissociated if appropriate

Ba2+(aq) + 2 Cl-(aq) + 2 Na+(aq) + SO42-(aq) →

2 Cl-(aq) + 2 Na+(aq) + BaSO4 (s)

•Spectator ions: present in reaction but do not “participate”;depend on solubility rulesCl- (aq) and Na+ (aq)

( Precipitation Animation)

3. Net ionic equation: shows chemical equation without spectatorionsBa2+ (aq) + SO42- (aq) → BaSO4 (s)

Good Practice for writing equations

Solubility Rules (memorize them!)

Figure 2: Compounds that are always soluble.

Compoundscontaining…

Important Exceptions

NO3-

CH3COO-

Cl-, Br-, I-

SO42-

None

Ag+, Hg22+, Pb2+

Hg22+, Pb2+,Sr2+, Ba2+, Ca2+

•Also, compounds with Group 1A and NH4+ cations

Figure 3: Compounds that always insoluble.

Compoundscontaining…

Important Exceptions

•Other anions (e.g., BrO3- or Cr2O72-) are generally insoluble except with thesoluble cations (Group 1A and NH4+)

•Precipitation Reaction Animation

CO32-, CrO42-, C2O42-

PO43-

S2-, O2-, OH-

Alkali metals, NH4+

Alkali metals, NH4+,Sr2+, Ba2+, Ca2+

AP Exam: Predicting Reaction Products

Reactants:

1.Write all soluble ionic compounds, strong acids, andstrong bases in aqueous solution as their constituentions. Never write NaCl or Ca(NO3)2, HCl or HNO3, NaOHor Ba(OH)2, etc.

Products:

1.Assume a reaction occurs.

2.Inspect all of the ions to see if any precipitates will form.

3.Cross out any spectator ions.

4.Balance the reaction (and states of matter are notneeded.)

5.For reaction prediction, please go to podcasts and WS onthe website.