Reactions in Aqueous Solution

Chapter 4 (semester 2/2011)

4.1 General Properties of Aqueous Solutions4.2 Precipitation Reactions4.3 Acid- Base Reactions4.4 Oxidation – Reduction Reactions4.5 Concentration of Solutions

4.1

4.1 General Properties of Aqueous Solutions

Solution = [ solute + solvent]

[smaller amount + larger amount]

[CLEAR, HOMOGENEOUS MIXTURE]

Aqueous solution = solute (liquid “or” solid) + solvent (water)

Solution Solvent Solute

Sea water

Air (g)

Alloy

H2O

N2

Cu

Salt (NaCl)

O2, Ar, CH4

Ni

Two types of Solutes

nonelectrolyte weak electrolyte strong electrolyte

4.2

Non-electrolyte

When dissolved in water does not conduct electricity

Electrolyte

When dissolved in water can conduct electricity

Incomplete dissociation/reversible

100%dissociation/

One sided reaction

Ref: Raymond Chang Chemistry, Tenth Edition

Figure 4.1, Page 122

Strong Electrolyte – 100% dissociation

NaCl (s) Na+ (aq) + Cl- (aq)H2O

Weak Electrolyte – not completely dissociated

CH3COOH CH3COO- (aq) + H+ (aq)

Conduct electricity in solution?

Dissociation means breaking up into..Cations (+) and Anions (-)

4.3

Note: Pure water contains very few ions, cannot conduct electricity (extremely weak electrolyte)

Hydration: is the process in which an ion is surrounded by water molecules arranged in a specific manner.

Water, electrically neutral molecule has a positive poles and negative poles, it is a polar solvent.

Ex: when NaCl dissolves in water Na+ ions and Cl- ions are separated from each other and undergo

“hydration”.

Hydration helps to stabilize ions in solution and prevents cations from combining with anions.

4.4

Ionization of acetic acid

CH3COOH CH3COO- (aq) + H+ (aq)

4.5

A reversible reaction. The reaction can occur in both directions.

Acetic acid is a weak electrolyte because its ionization in water is incomplete.

Nonelectrolyte does not conduct electricity?

No cations (+) and anions (-) in solution

4.6

C6H12O6 (s) C6H12O6 (aq)H2O

4.2 Precipitation Reactions (Metathesis reaction) or (Double Displacement reaction)

One product is insoluble solid

molecular equation

ionic equation

net ionic equation

Pb2+ + 2NO3- + 2Na+ + 2I- PbI2 (s) + 2Na+ + 2NO3

-

Na+ and NO3- are spectator ions

Pb(NO3)2 (aq) + 2NaI (aq) PbI2 (s) + 2NaNO3 (aq)

precipitate

Pb2+ + 2I- PbI2 (s)

4.7

4.8

Solubility: Maximum amount of solute that will dissolve in a given quantity of solvent in a specific temperature.

Substances:

soluble

Slightly soluble

insoluble

Soluble : fair amount is visibly dissolves when added to water

-All ionic compounds are strong electrolytes, but they are not

equally soluble.

-Even insoluble compounds dissolve to a certain extent

Examples are: (NH4)2CO3, (NH4)3PO4,(NH4)2S,(NH4)2CrO4

ACID: Arrhenius acid is a substance that produces H+ (H3O+) in water

BASE: Arrhenius base is a substance that produces OH- in water

4.9

4.3 Acid-Base Reactions

Hydronium ion, hydrated proton, H3O+

4.10

A Brønsted acid is a proton donorA Brønsted base is a proton acceptor

acidbase acid base

4.11

A Brønsted acid must contain at least one ionizable proton!

Monoprotic acids

HCl H+ + Cl-

HNO3 H+ + NO3-

CH3COOH H+ + CH3COO-

Strong electrolyte, strong acid

Strong electrolyte, strong acid

Weak electrolyte, weak acid

Diprotic acidsH2SO4 H+ + HSO4

-

HSO4- H+ + SO4

2-

Strong electrolyte, strong acid

Weak electrolyte, weak acid

Triprotic acidsH3PO4 H+ + H2PO4

-

H2PO4- H+ + HPO4

2-

HPO42- H+ + PO4

3-

Weak electrolyte, weak acid

Weak electrolyte, weak acid

Weak electrolyte, weak acid

4.12

Practice question: Identify each of the following species as a Brønsted acid, base, or both. (a) HI, (b) CH3COO-, (c) H2PO4

-

HI (aq) H+ (aq) + Br- (aq) Brønsted acid

CH3COO- (aq) + H+ (aq) CH3COOH (aq) Brønsted base

H2PO4- (aq) H+ (aq) + HPO4

2- (aq)

H2PO4- (aq) + H+ (aq) H3PO4 (aq)

Brønsted acid

Brønsted base

4.13

Neutralization Reaction

acid + base salt + water

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

H+ + Cl- + Na+ + OH- Na+ + Cl- + H2O

H+ + OH- H2O

4.14

4.4 Oxidation-Reduction Reactions

REDOX REACTIONS(electron transfer reactions)

2Mg (s) + O2 (g) 2MgO (s)

2Mg 2Mg2+ + 4e-

O2 + 4e- 2O2-

Oxidation half-reaction (loss of e-)

Reduction half-reaction (gain e-)

2Mg + O2 2MgO

4.15

Oxidized

Reducing Agent (donates electrons

to oxygen and causes oxygen to

be reduced)

Reduced

Oxidizing Agent (accepts electrons from Magnesium

and causes Magnesium to be

oxidized)

OILRIG

Oxidation Is Loss

Reduction Is Gain

Zn (s) + CuSO4 (aq) ZnSO4 (aq) + Cu (s)

Zn is oxidizedZn Zn2+ + 2e-

Cu2+ is reducedCu2+ + 2e- Cu

Zn is the reducing agent

Cu2+ is the oxidizing agent

4.16

Copper wire reacts with silver nitrate to form silver metal.What is the oxidizing agent in the reaction?

Cu (s) + 2AgNO3 (aq) Cu(NO3)2 (aq) + 2Ag (s)

Cu Cu2+ + 2e-

Ag+ + 1e- Ag Ag+ is reduced Ag+ is the oxidizing agent

NaIO3

Na = +1 O = -2

3x(-2) + 1 + ? = 0

I = +5

IF7

F = -1

7x(-1) + ? = 0

I = +7

K2Cr2O7

O = -2 K = +1

7x(-2) + 2x(+1) + 2x(?) = 0

Cr = +6

Oxidation numbers of all the elements in the following ?

4.17

Types of Redox Reactions

(i) Combination Reaction

Two or more substances combine to form a single product.

0 0 +4 -2

S(s) + O2 (g) SO2(g)

(ii) Decomposition Reaction

Breakdown of a compound into two or more components.

+2 -2 0 0

2 HgO(s) 2Hg(l) + O2(g)

4.18

(iii) Displacement Reaction

Halogen displacement

According to Activity Series

F2 > Cl2 > Br2 > I2

i.e Molecular fluorine can replace chloride, bromide and iodide ions in solution. On the other hand, Molecular chlorine can replace bromide and iodide ions in solution

0 -1 -1 0

Cl2 (g) + 2 KBr(aq) 2KCl(aq) + Br2(l)

0 -1 -1 0 Cl2 (g) + 2 NaI(aq) 2NaCl(aq) + I2(l)

4.19

Ca2+ + CO32- CaCO3

NH3 + H+ NH4+

Zn + 2HCl ZnCl2 + H2

Ca + F2 CaF2

Precipitation

Acid-Base

Redox (H2 Displacement)

Redox (Combination)

Classify the following reactions.

4.20

4.5 Concentration of Solutions

concentration : amount of solute present in a given quantity of solvent or solution.

M = molarity =moles of solute

liters of solution

4.21

Most commonly used unit is “Molarity”

V

nM

Dilution is the procedure for preparing a less concentrated solution from a more concentrated solution.

Dilution

Add Solvent

Moles of solutebefore dilution (i)

Moles of soluteafter dilution (f)=

MiVi MfVf=4.22

How would you prepare 60.0 mL of 0.2 MHNO3 from a stock solution of 4.00 M HNO3?

MiVi = MfVf

Mi = 4.00 Mf = 0.200 Vf = 0.06 L Vi = ? L

4.23

Vi =MfVf

Mi

=0.200 x 0.06

4.00= 0.003 L = 3 mL

3 mL of acid + 57 mL of water = 60 mL of solution

Ref: power point presentation for instructors

(Mc- Graw hill series,www.mhhe.com/chemistry)

25

Chemistry in Action: Breath Analyzer

3CH3COOH + 2Cr2(SO4)3 + 2K2SO4 + 11H2O

3CH3CH2OH + 2K2Cr2O7 + 8H2SO4 +6

+3

26

Preparing a Solution of Known Concentration