Lecture 5. Chemical Reaction (Part 3. Redox)

CHEMICAL REACTIONS:

Reduction-Oxidation Reaction

GENERAL CHEMISTRY

LECTURE 5

cont.

5.1 Redox Reaction

5.2 Oxidation Number of Atoms

5.3 Reducing and Oxidizing agent in a Redox Reaction

Scope of the Study

5.4 Activity series of metals

Figure 5.1 The redox process in compound formation.

*key chemical event is the net movement of electrons from one reactant to another

Oxidation

Redox Terminology

Reduction

Oxidation (electron loss of Mg): Mg Mg2+ + 2e-

Reduction (electron gain by O2): O2 + 2e- O2-

Consider: Mg + O2 MgO

O2 oxidizes Mg, O2 is the Oxidizing agent

Mg reduces O2, Mg is the Reducing agent

LEORA

GEROA

is the loss of electrons (increased in the oxidation state)

is the gain of electrons (decreased in the oxidation state)

Reaction of Sodium and Chlorine

Oxidation number (O.N) or oxidation state is the charge of an atom in a molecule when electrons are transferred completely.

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

2. Oxidation state of monatomic ion = charge of the ion

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

4. Hydrogen = +1 in covalent compounds

5. Fluorine = 1 in compounds

6. Sum of oxidation states = 0 in compounds

7. Sum of oxidation states = charge of the ion in ions

General Rules for Assigning Oxidation States

Exercise No. 1 Determining the Oxidation Number of an Element

1. Determine the oxidation number (O.N.) of each element in these compounds:

(a) Zinc chloride (b) Sulfur trioxide

(c) Nitrate ion

SOLUTION:

(a) ZnCl2

(b) SO3

(c) NO3-

The O.N. for zinc is +2 and that for chloride is -1.

Oxygen has an O.N. of -2

S + 3(-2) = 0; Therefore, S = +6

Oxygen has an O.N of -2

N + 3(-2) = -1; Therefore, N =+5

(d) Hydrogen phosphate ion

(d) HPO42- Oxygen has an O.N of -2.

Hydrogen has an O.N of +1

1 + P + 4(-2) = -2; Therefore, P =+5

2. Find the oxidation states for each of the elements in each of the following compounds:

K2Cr2O7 CO3

2-

MnO2 PCl5 SF4

K = +1; Cr = +6; O = 2

C = +4; O = 2

Mn = +4; O = 2

P = +5; Cl = 1

S = +4; F = 1

Exercise No. 2 Determining the Oxidation Number of an Element

Figure 5.2 A summary of terminology for oxidation-reduction (redox) reactions.

LEORA

GEROA

3. Which of the following are oxidation-reduction reactions? Identify the oxidizing agent and the reducing agent.

Zn(s) + 2HCl(aq) ZnCl2(aq) + H2(g)

Cr2O72-(aq) + 2OH-(aq) 2CrO4

2-(aq) + H2O(l)

2CuCl(aq) CuCl2(aq) + Cu(s)

Recognizing Oxidizing and Reducing Agents Exercise No. 3

Zn(s) + 2H+ (aq) + 2Cl-(aq) Zn2+(aq) + 2Cl(aq) + H2(g)

0 +1 1 +2 1 0

1. Identify the oxidation states of each atom in the equation.

Determining Oxidizing and Reducing Agent


H+ gained e Zn(s) + 2H+ (aq) + 2Cl-(aq) Zn2+(aq) + 2Cl(aq) + H2(g)

0 +1 1 +2 1 0

Zn lost e-

The oxidation state of chlorine remains unchanged.


2. Which gained or lose electrons?

gained of e- : reduction Zn(s) + 2H+ (aq) + 2Cl-(aq) Zn2+(aq) + 2Cl(aq) + H2(g)

0 +1 1 +2 1 0

lost of e- : oxidation


2. Which gained or lose electrons?

gained of e- : reduction Zn(s) + 2H+ (aq) + 2Cl-(aq) Zn2+(aq) + 2Cl(aq) + H2(g)

0 +1 1 +2 1 0

lost of e- : oxidation

Zn is the reducing agent HCl is the oxdizing agent


3. Identify OA and RA

LEORA

GEROA

3. Which of the following are oxidation-reduction reactions? Identify the oxidizing agent and the reducing agent.


Cr2O72-(aq) + 2OH-(aq) 2CrO4

2-(aq) + H2O(l)


Recognizing Oxidizing and Reducing Agents Exercise No. 3

Disproportionation reaction

The same substance is both oxidized and reduced.

Examples:


2H2O2(l) 2H2O(l) + O2(g)

Recognizing Oxidizing and Reducing Agents

4. Identify the oxidizing agent and reducing agent in each of the following:

(a) 2Al(s) + 3H2SO4(aq) Al2(SO4)3(aq) + 3H2(g)

(b) PbO(s) + CO(g) Pb(s) + CO2(g)

(c) 2H2(g) + O2(g) 2H2O(g)

Assign an O.N. for each atom and see which atom gained and which atom lost electrons in going from reactants to products.

An increase in O.N. means the species was oxidized (and is the reducing agent) and a decrease in O.N. means the species was reduced (is the oxidizing agent).

(a) 2Al(s) + 3H2SO4(aq) Al2(SO4)3(aq) + 3H2(g)

0 0 +6 +1 -2 +3 +6 -2 SOLUTION:

The O.N. of Al increases; Al is oxidized; it is the reducing agent.

The O.N. of H decreases; H is reduced; H2SO4 is the oxidizing agent.

Exercise No. 4

Recognizing Oxidizing and Reducing Agents

continued

(c) 2H2(g) + O2(g) 2H2O(g)

(b) PbO(s) + CO(g) Pb(s) + CO2(g)

+2 -2 +2 -2 0 +4 -2

0 0 +1 -2

The O.N. of C increases; C is oxidized; CO is the reducing agent.

The O.N. of Pb decreases; Pb is reduced; PbO is the oxidizing agent.

The O.N. of H increases; it is oxidized; H2 is the reducing agent.

The O.N. of O decreases; it is reduced; O2 is the oxidizing agent.

Exercise No. 4

Displacing One Element by Another:

Activity Series

The activity series of metals

-elements are rank in order of their ability to displace one another.

Figure 5.4 An active metal displacing hydrogen from water.

The activity series of the metals.

(a) 2Al(s) + 6H2O(g) 2Al(OH)3(s) + 3H2(g)

(b) Ni(s) + 2H+ (g) Ni+ (g) + H2(g)

Figure 5.5 Displacing one metal by another.

The TOP METAL will reduce the CATION of the METAL BELOW!

Ba(s) + Ca2+

(aq) Ca(s) + Ba2+

(aq)

Mg(s) + Zn2+

(aq) Zn(s) + Mg2+

(aq)

How about: Al(s) + Na

+(aq) ?

Sr(s) + Al3+

(aq) ? No reaction

Al(s) + Sr2+

(aq)

Additional Exercises Identifying the Type of Redox Reaction

PROBLEM: Classify each of the following redox reactions as a combination, decomposition, or displacement reaction, write a balanced molecular equation for each, as well as total and net ionic equations for part (c), and identify the oxidizing and reducing agents:

(a) Magnesium(s) + nitrogen(g) magnesium nitride (aq)

(b) Hydrogen peroxide(l) water(l) + oxygen gas

(c) Aluminum(s) + lead(II) nitrate(aq) aluminum nitrate(aq) + lead(s)

(a) Magnesium(s) + nitrogen(g) magnesium nitride (aq)

(a) Combination

Mg is the reducing agent; N2 is the oxidizing agent.

3

0 0 +2 -3

Mg(s) + N2(g) Mg3N2(aq)

(b) Decomposition

H2O2 is the oxidizing and reducing agent.

H2O2(l) H2O(l) + O2(g)

+1 -1 +1 -2 0

or

2H2O2(l) 2H2O(l) + O2(g)

1 2

(b) Hydrogen peroxide(l) water(l) + oxygen gas


(c) Displacement

Al(s) + Pb(NO3)2(aq) Al(NO3)3(aq) + Pb(s)

0 +2 +5 -2 +3 +5 -2 0

2Al(s) + 3Pb(NO3)2(aq) 2Al(NO3)3(aq) + 3Pb(s)

Pb(NO3)2 is the oxidizing agent; Al is the reducing agent.

(c) Aluminum(s) + lead(II) nitrate(aq) aluminum nitrate(aq) + lead(s)


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Lecture 5. Chemical Reaction (Part 3. Redox)