copyright©2000 by houghton mifflin company. all rights reserved. 1 reactions chapter 4

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1

Reactions

Chapter 4

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2

Aqueous Solutions

Water is the dissolving medium, or solvent.

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3

Figure 4.1 The Water Molecule is Polar

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4

Some Properties of Water

Water is “bent” or V-shaped. The O-H bonds are covalent. Water is a polar molecule. Hydration occurs when salts dissolve in

water.

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• http://mathinscience.info/teach/k5_science/chemistry/mastering_matter/matter_animations/salt_cystl_watr_mol_web.swf

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Figure 4.2Polar Water Molecules Interact with the Positive and Negative Ions of a

Salt

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A Solute

dissolves in water (or other “solvent”)

changes phase (if different from the solvent)

is present in lesser amount (if the same phase as the solvent)

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A Solvent

retains its phase (if different from the solute)

is present in greater amount (if the same phase as the solute)

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Electrolytes

Strong - conduct current efficiently

NaCl, HNO3

Weak - conduct only a small current

vinegar, tap water

Non - no current flows

pure water, sugar solution

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12

Figure 4.5BaCI2 Dissolving

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Acids

Strong acids - dissociate completely to produce H+ in solution

hydrochloric and sulfuric acid

Weak acids - dissociate to a slight extent to give H+ in solution

acetic and formic acid

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Figure 4.6HCI (aq) is Completely Ionized

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15

Figure 4.8Acetic Acid in Water

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Bases

Strong bases - react completely with water to give OH ions.

sodium hydroxide

Weak bases - react only slightly with water to give OH ions.

ammonia

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20

Types of Double Displacement Reactions

Precipitation reactionsAgNO3(aq) + NaCl(aq) AgCl(s) + NaNO3(aq)

Acid-base reactionsNaOH(aq) + HCl(aq) NaCl(aq) + H2O(l)

Oxidation-reduction reactionsFe2O3(s) + Al(s) Fe(l) + Al2O3(s)

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21

Simple Rules for Solubility

1. Most nitrate (NO3) salts are soluble.

2. Most alkali (group 1A) salts and NH4+ are soluble.

3. Most Cl, Br, and I salts are soluble (NOT Ag+, Pb2+, Hg22+)

4. Most sulfate salts are soluble (NOT BaSO4, PbSO4, HgSO4, CaSO4)

5. Most OH salts are only slightly soluble (NaOH, KOH are soluble, Ba(OH)2, Ca(OH)2 are marginally soluble)

6. Most S2, CO32, CrO4

2, PO43 salts are only slightly soluble.

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Describing Reactions in Solution

1. Molecular equation (reactants and products as compounds)

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

2. Complete ionic equation (all strong electrolytes shown as ions)

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

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

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Describing Reactions in Solution (continued)

3. Net ionic equation (show only components that actually react)

Ag+(aq) + Cl(aq) AgCl(s)

Na+ and NO3 are spectator ions.

Quick Review – Are you Ready?

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47%

53% 1. Yes

2. No

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

When Fe(NO3)2 dissolves in water, what particles are present in the solution?

0%

6%

88%

6% 1. Fe+ and (NO3)2–

2. Fe2+ and 2 NO3–

3. Fe and 2 NO3

4. Fe and N2 and 3 O2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Pb(NO3)2 + 2 KI PbI2 + 2 KNO3

What is the observable evidence that the above reaction occurs?

88%

0%

0%

12% 1. An explosion.

2. A gas forms.

3. The solution boils.

4. A solid forms.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

When an acid and a base react to form a salt and water, the reaction is called a(n):

0%

100%

0%

0% 1. cancellation.

2. elimination.

3. neutralization.

4. adduct formation.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Which mixture will produce a precipitate?

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1. NH3 + HF

2. HClO4 + KOH

3. Na2SO4 + PbCl24. Ca(NO3)2 + KC2H3O2

5. (NH4)2CO3 + Na3PO4

12%

24%

47%

18% 0%

1. 2 3 4 5

1. 1.

2. 2

3. 3

4. 4

5. 5

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

What is(are) the spectator ion(s) in the following reaction?

KOH (aq) + HF (aq) KF (aq) + H2O (l)

0%

6%

82%

6%

6% 1. K+

2. F–

3. K+ and F–

4. OH– and H+

5. OH–

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Which of the following reactions has the net ionic equation:

H+ (aq) + OH– (aq) H2O (l)

1) HClO4 + KOH

2) HF + NaOH

3) H3PO4 + KOH

4) H2SO4 + Ca(OH)2

53%

0%

0%

41%

6% 1. Reaction 1

2. Reactions 1, 4

3. Reactions 2, 3

4. Reactions 2, 4

5. Reactions 3, 4

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

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32

Other Reactions

• Single displacement

• Combustion

• Synthesis

• Decomposition

All of these involve a transfer of electrons and are oxidation-reduction (Redox) reactions

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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.

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34

Rules for Assigning Oxidation States

1. Oxidation state of an atom in an element = 0

2. Oxidation state of monatomic ion = charge

3. Oxygen = 2 in covalent compounds (except in peroxides where it = 1)

4. H = +1 when bonded to nonmetal; -1 when bonded to metal

5. Fluorine = 1 in compoundsThe other halogens have an oxidation number of −1

when they are negative; they can have positive oxidation numbers, however, most notably in oxyanions (polyatomic ions with oxygen)

6. Sum of oxidation states = 0 in compounds Sum of oxidation states = charge of the ion

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Redox Reactions - Terms

• Oxidation – loss of electrons (increase in oxidation number)

• Reduction – gain of electrons (decrease in oxidation number)

• Oxidizing agent – the compound that is the electron acceptor

• Reducing agent – the compound that is the electron donor

When an atom undergoes reduction, it _______ electrons.

0%

0%

6%

94% 1. gains

2. loses

3. retains

4. balances

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

What is the oxidation number of sulfur in the following compound?

SO2

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6%

0%

94%

0% 1. -4

2. +4

3. -2

4. +2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

What is the oxidation number of oxygen in the following compound?

MgO

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12%

88%

0%

0% 1. -4

2. +4

3. -2

4. +2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

What is the oxidation number of Manganese in the following compound?

MnO2

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6%

0%

94%

0% 1. -4

2. +4

3. -2

4. +2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

What is the oxidation number of Fluorine in the following compound?

F2

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18%

41%

6%

35% 1. -1

2. +1

3. 0

4. -2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

What is the oxidation number of Sulfur in the following compound?

SO42-

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6%

6%

82%

6% 1. +8

2. +6

3. -2

4. +4

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

What is the oxidation number of manganese in the permanganate ion?

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59%

0%

29%

12% 1. +8

2. +6

3. -6

4. +7

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

What is the oxidation number of nitrogen in the nitrite ion?

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44

24%

0%

12%

65% 1. +3

2. +5

3. -5

4. +2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

What is reduced in the following equation?

Al + Cu(NO3)2 Cu + Al(NO3)3

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0%

0%

89%

11% 1. Al

2. Cu

3. Cu(NO3)2

4. Al(NO3)3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

What is oxidized in the following equation?

Al + Cu(NO3)2 Cu + Al(NO3)3

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25%

25%

25%

25% 1. Al

2. Cu

3. Cu(NO3)2

4. Al(NO3)3

What is reducing agent in the following equation?

Al + Cu(NO3)2 Cu + Al(NO3)3

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25%

25%

25%

25% 1. Al

2. Cu

3. Cu(NO3)2

4. Al(NO3)3

What is oxidizing agent in the following equation?

Al + Cu(NO3)2 Cu + Al(NO3)3

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48

25%

25%

25%

25% 1. Al

2. Cu

3. Cu(NO3)2

4. Al(NO3)3

Which substance is serving as the reducing agent in the following reaction?

14H+ + Cr2O72- + 3Ni 3Ni2+ + 2Cr3+ + 7H2O

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20%

20%

20%

20%

20% A. H2O

B. H+

C. Ni2+

D. Ni

E. Cr2O72-

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50

Balancing by Half-Reaction Method

1. Write separate reduction, oxidation reactions.

2. For each half-reaction:

Balance elements (except H, O)

Balance O using H2O

Balance H using H+

Balance charge using electrons

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Balancing by Half-Reaction Method (continued)

3. If necessary, multiply by integer to equalize electron count.

4. Add half-reactions.

5. Check that elements and charges are balanced.

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52

Half-Reaction Method - Balancing in Base

1. Balance as in acid.

2. Add OH that equals H+ ions (both sides!)

3. Form water by combining H+, OH.

4. Check elements and charges for balance.