Chapter 4

Reactions in Aqueous Solution

AqueousReactions

Solutions

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

• The solvent is present in greatest abundance.

• All other substances are solutes.

4.1 General Properties of Aqueous Solutions

AqueousReactions

Solutions

AqueousReactions

Dissociation• When an ionic

substance dissolves in water, the solvent pulls the individual ions from the crystal and solvates them.

• This process is called dissociation.

• An electrolyte is a substances that dissociates into ions when dissolved in water.

AqueousReactions

Hydration Process• Water molecules surround

the ions.• Note the positions of the

positive and negative charges.

AqueousReactions

Solutions• An electrolyte is a

substance that dissociates into ions when dissolved in water.

• A nonelectrolyte may dissolve in water, but it does not dissociate into ions when it does so.

AqueousReactions

Electrolytes and Nonelectrolytes

• Soluble ionic compounds tend to be electrolytes.

• Molecular compounds tend to be nonelectrolytes, except for acids and bases.

AqueousReactions

Electrolytes

• A strong electrolyte dissociates completelywhen dissolved in water.

• A weak electrolyte only dissociates partiallywhen dissolved in water.

AqueousReactions

Electrolytes

AqueousReactions

Strong Electrolytes Are…

• Strong acids

• Strong bases

AqueousReactions

Strong Electrolytes Are…

• Strong acids

• Strong bases

• Soluble ionic salts

AqueousReactions

• A strong electrolyte is completely dissociated in water.KCl(s) KCl(aq) K+(aq) + Cl(aq)

• A weak electrolyte is partiallydissociated in water.

NH3(aq) + H2O(l) NH4+(aq) + OH(aq)

• Non-electrolytes do not form ions.

C12H22O11(s) C12H22O11(aq)

MSOffice1

슬라이드 12

MSOffice1 I had to use the line option to recreate the arrows so that the weak electrolyte could have the proper equilibrium arows. , 2006-07-29

AqueousReactions

4.2 Precipitation Reactions

When one mixes ions that form compounds that are insoluble (as could be predicted by the solubility guidelines), a precipitate is formed.

AqueousReactions

Precipitation Reactions

AqueousReactions

How can a precipitation reaction be predicted ?

Solubility Rules

AqueousReactions

Metathesis (Exchange) Reactions

• Metathesis comes from a Greek word that means “to transpose.”

• It appears as though the ions in the reactant compounds exchange, or transpose, ions:

AgNO3(aq) + KCl(aq) AgCl(s) + KNO3(aq)

AqueousReactions

Solution Chemistry

• It is helpful to pay attention to exactlywhat species are present in a reaction mixture (i.e., solid, liquid, gas, aqueous solution).

• If we are to understand reactivity, we must be aware of just what is changing during the course of a reaction.

AqueousReactions

Types of Chemical Reactions

• Molecular equation (components written as compounds):

AgNO3(aq) + NaCl(aq) AgCl(s) + NaNO3(aq)

• Complete Ionic Equation (electrolytes written as ions):all strong electrolytes (strong acids, strong bases, and soluble ionic salts) are dissociated into their ions.

Ag+(aq) + NO3(aq) + Na+(aq) + Cl(aq)

AgCl(s) + Na+(aq) + NO3(aq)

• Net ionic equation (ions participating in the reactionAg+(aq) + Cl(aq) AgCl(s)

(Na+ and NO3 are spectator ions.)

AqueousReactions

Writing Net Ionic Equations

1. Write a balanced molecular equation.

2. Dissociate all strong electrolytes.

3. Cross out anything that remains unchanged from the left side to the right side of the equation.

4. Write the net ionic equation with the species that remain.

AqueousReactions

4.3 Acids, Bases, & Neutralization Reactions

• The Swedish physicist and chemist S. A. Arrhenius defined acids as substances that increase the concentration of H+

when dissolved in water.

• Both the Danish chemist J. N. Brønstedand the British chemist T. M. Lowry defined them as proton donors.

Acids

There are only seven strong acids:• Hydrochloric (HCl)

• Hydrobromic (HBr)

• Hydroiodic (HI)

• Nitric (HNO3)

• Sulfuric (H2SO4)

• Chloric (HClO3)

• Perchloric (HClO4)

AqueousReactions

Bases

• Brønsted and Lowry defined them as proton acceptors.

The strong bases are the soluble metal salts of hydroxide ion:• Alkali metals

• Calcium

• Strontium

• Barium

• Arrhenius defined bases as substances that increase the concentration of OH− when dissolved in water.

AqueousReactions

Acid-Base Reactions: the acid donates a proton (H+) to the base.

AqueousReactions

• When a strong acid reacts with a strong base, the net ionic equation is

HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)

H+(aq) + Cl−(aq) + Na+(aq) + OH−(aq) Na+(aq) + Cl−(aq) + H2O(l)

H+(aq) + OH−(aq) H2O(l)

Neutralization Reactions• Generally, when solutions of an acid and a base are combined,

the products are a salt and water:

CH3COOH(aq) + NaOH(aq) CH3COONa(aq) + H2O(l)

AqueousReactions

• Some metathesis reactions do not give the product expected.

• In this reaction, the expected product (H2CO3) decomposes to give a gaseous product (CO2):

Neutralization Reactions with Gas-Forming Reactions

CaCO3(s) + HCl(aq) CaCl2(aq) + CO2(g) + H2O(l)

NaHCO3(aq) + HBr(aq) NaBr(aq) + CO2(g) + H2O(l)

AqueousReactions

• Similarly, when a sulfite reacts with an acid, the products are a salt, sulfur dioxide, and water:

SrSO3(s) + 2HI(aq) SrI2(aq) + SO2(g) + H2O(l)

Na2S(aq) + H2SO4(aq) Na2SO4(aq) + H2S(g)

• This reaction gives the predicted product, but you had better carry it out in the hood, or you will be veryunpopular!

• But just as in the previous examples, a gas is formed as a product of this reaction:

AqueousReactions

4.4 Oxidation-Reduction Reactions

• An oxidation occurs when an atom or ion loses electrons.

• A reduction occurs when an atom or ion gains electrons.

• One cannot occur without the other.

AqueousReactions

Oxidation Numbers: To determine if an oxidation–reduction reaction has occurred, we assign an oxidation number to each element in a neutral compound or charged entity.

1. Elements in their elemental form have an oxidation number of 0.

2. The oxidation number of a monatomic ion is the same as its charge.

3. Nonmetals tend to have negative oxidation numbers, although some are positive in certain compounds or ions.

AqueousReactions

• Oxygen has an oxidation number of −2, except in the peroxide ion, in which it has an oxidation number of −1.

• Hydrogen is −1 when bonded to a metal, +1 when bonded to a nonmetal.

• Fluorine always has an oxidation number of −1.

• The other halogens have an oxidation number of −1 when they are negative; they can have positive oxidation numbers, however, most notably in oxyanions.

4. The sum of the oxidation numbers in a

neutral compound is 0.

5. The sum of the oxidation numbers in a polyatomic ion is the charge on the ion.

AqueousReactions

Sample Problem

Assign oxidation numbers to all atoms in the following:

CO2, K2Cr2O7, PCl5

CO2 – oxygen is -2×2 = -4, making C +4.

K2Cr2O7 – oxygen -2×7 = -14.

K (group 1A) is +1×2 = +2

2Cr must equal +12 = -14+2

each Cr = +6

PCl5 – Cl (group VIIA) = -1×5 = -5

P = +5

AqueousReactions

Oxidation of Metals by Acids & Bases

• In displacement reactions, ions oxidize an element.

• The ions, then, are reduced.

Displacement Reactions

AqueousReactions

Displacement Reactions

In this reaction,

silver ions oxidize

copper metal:

Cu(s) + 2Ag+(aq) Cu2+(aq) + 2Ag(s)

AqueousReactions

Displacement Reactions

The reverse reaction,

however, does not

occur:

Cu2+(aq) + 2Ag(s) Cu(s) + 2Ag+(aq)x

AqueousReactions

Activity Series

AqueousReactions

Molarity

• Two solutions can contain the same compounds but be quite different because the proportions of those compounds are different.

• Molarity is one way to measure the concentration of a solution:

moles of solute

volume of solution in litersMolarity (M) =

AqueousReactions

Mixing a Solution

• To create a solution of a known molarity, one weighs out a known mass (and, therefore, number of moles) of the solute.

• The solute is added to a volumetric flask, and solvent is added to the line on the neck of the flask.

AqueousReactions

Sample Problem

What is the molarity of 100 mL solution containing 15.6g of NaOH?

1 mol NaOH15.6g NaOH 0.390 mol NaOH

40.0g NaOH

0.390 mol NaOH3.90 NaOH

1 L100 mL

1000 mL

M = M

AqueousReactions

Sample Problem

How many moles of HCl are contained in 250 mL of a 0.600 molar solution?

1 L 0.600 mol HCl250. mL 0.150 mol HCl

1000 mL 1 L

AqueousReactions

Other Concentration Units

AqueousReactions

Other Concentration Units

ppm ppb ppt

AqueousReactions

Sample Problem

What is the molarity of a solution that is 4.0 ppb Cr6+?

6+ 6+8 6+

6 6+

4.0 g4.0 ppb

L soln.

4.0 g 1 g Cr 1mol Cr7.7 10 Cr

L soln. 10 g 52.0 gCr

M

AqueousReactions

© 2012 Pearson Education, Inc.

Dilution• One can also dilute a more concentrated solution by

– Using a pipet to deliver a volume of the solution to a new volumetric flask, and

– Adding solvent to the line on the neck of the new flask.

AqueousReactions

DilutionThe molarity of the new solution can be determined from the equation

Mc Vc = Md Vd,Moles solute before dilution = moles solute after dilution

where Mc and Md are the molarity of the concentrated and dilute solutions, respectively, and Vc and Vd are the volumes of the two solutions.

How many mL of 15.0 Mnitric acid solution should be dilute to make 250. mL of a 0.50 M solution?

final finalinitial

initial

C VV

C

0.50 250. mL

15.0

8.3 mL

M

M

AqueousReactions

4.6 Using Molarities inStoichiometric Calculations

AqueousReactions

Titration

Titration is an analytical technique in which one can calculate the concentration of a solute in a solution.

Equivalence pointThe point when all reactants are completely consumed.

End pointClose to the equivalence point marked by an indicator.

AqueousReactions

Titration

AqueousReactions

Sample Problem

What volume of 0.200 M HCl solution is need to neutralize 25.0 mL of a 0.450 M KOH solution?

The balanced molecular equation is:

HCl(aq) + KOH(aq) KCl(aq) + H2O(l)

-

-

- +

++

0.450 mol KOH 1mol OH 1 L25.0 mL

1 L 1 mol KOH 1000 mL

0.01125 mol OH

at neutralization, mol OH mol H

1mol HCl 1 L 1000 mL0.01125 mol H

1 mol H 0.200 mol HCL 1 L

56.3 mL HCl

AqueousReactions

Homeworks

필수 숙제: 18, 24, 36, 46, 68, 86, 115추가 연습20, 22, 32, 40, 48, 50, 58, 62, 74