Electron Transfer Reactions
Textbook pp. 600-407
Vocabulary
oxidation
half-reaction equation
oxidizing agent
reduction
oxidation number
reducing agent
oxidation—reduction (redox) reaction
MAIN IDEA: Many reactions involve the transfer of electrons. These reactions are
called oxidation—reduction reactions or redox reactions. In a redox reaction, elec
trons transfer from one entity to another. Neither oxidation nor reduction can occur
without the other Oxidation numbers are assigned to the entities in order to deter
mine which is being oxidized and which is being reduced.
LEARNING TIP
Electron Transfer
Remember that in a redox reaction,
the number of electrons lost by
oxidation must always be equal to
the number of electrons gained by
reduction.
LEARNING TIP
1. Explain in your own words, the process that occurs when an entity undergoes
each reaction. C fl
(a) oxidation:
(b) reduction:
2. Write the oxidation and reduction half-reaction equations for the reactions
represented by the following equations. C
2 Fe(s) + ALO3(aq)
(a) FeO(aq) -r 2 Al(s)
—,
A Redox Mnemonic
Use the mnemonic “LEO says GER’
to help remember the difference
between reduction and oxidation.
LEO: Losing Electrons is Oxidation
GER: Gaining Electrons is Reduction
(b) Cu(s) + 2W (aq)
—÷
Cu2(aq) ± H,L
(c) SnCI,(aq) + 2 l-IgCI,(aq)
(d) 4 NaCIO(aq) + HS(g)
—
—*
SnCl4(aq) + 2 HgCI(aq)
4 NaCI(aq) + H2504(aq)
3. Identify the entity that is oxidized and the entity that is reduced in the
chemical reactions represented by the following equations. C
(a) Cr9072jaq) + 8 Ht + 3 H,C,04(s) —÷ 2Cr3 (aq) + 7 H10(l) + 6 C02(g)
(b) 2 Cr3 (aq) + H,O(l) * 6 ClOj(aq)
(c) Cu(s)
AgNO3(aq)
—
Cu(N03),(aq)+ Ag(s)
(d) MnO,,(aq) + 5 Fe2(aq) + SHt(aq)
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Cr,07(aq) + 6 ClO,(aq) + 2 Ht(aq)
—*
Mn(aq) + 5 Fe3(aq) + 4 1-LOll)
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4. Determine the oxidation states of each element in the following chemical
compounds. 0
(a) N,
(e) GO,
(b) NH3
(f) NaNO3
(c) ZnO
(g) Na,S04
(d) CaRlO)2
(h) Al(OH),
5. Jermaine says All combustion reactions are redox reactions.” is he correct?
0
Explain.
6. Is it possible to have a reaction in which oxidation occurs and reduction does
not? Explain.U3
7. Give oxidation numbers for the underlined atoms in the following molecules
and ions. C
(a) MgN
(e) C204
NLL
(b) GaO
(1) ZnO,2
(c) CsQ
(g) NaBH4
(d) C012
(h) WOI2
9.1 Electron Transfer Reactions
179
8. For
(I)
(ii)
(iii)
(a)
each of the complete redox reactions given below, do the following:
Break down each reaction into its half-reactions.
Identify the oxidizing agent.
Identify the reducing agent. 0
2Sr+02—.2SrO
(b) 2 Li + H,
—*
2 NH
(c) 2Cs + Br2—*2CsBr
(d) 3 Mg + N2
-÷
Mg3N,
9. The compound iron(lll) oxide, Fe203 (more commonly known as rust)
is a major contributor to the degradation of vehicles and structures. The
oxidation of iron metal occurs when it comes into contact with oxygen in the
atmosphere and rust slowly begins to form as the reaction proceeds. o flfl
(a) Write the half-reactions for the formation of this compound.
(b) Write the balanced overall reaction that results in the formation of Fe,O
(c) Galvanization is a process by which a coating of zinc is applied to the
iron surface to protect against rust. Describe how this zinc layer protects
against rust.
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Balancing Redox Reaction
Equations
Textbook pp. 608—617
MAIN IDEA: In a balanced redox equation, the total numbers of each type of atom or
ion on either side of the equation are equal and the numbers of electrons transferred are
equal. Redox equations may be balanced by using the oxidation numbers method or the
half-reactions method. These reactions can occur in either acidic or basic conditions. In
these cases it may be necessary to add water molecules, hydrogen ions and/or hydroxide
ions to balance the equation.
1. ‘Write the balanced half-reactions for each of the following.
(a) 2 Fe(s) + 3 C12(g) —> 2 FeCI3(s)
Reduction:
Oxidation:
(b) Mg(s) + 2 AgNO,( q)
Oxidation:
—>
STUDY TIP
Different Methods of Solution
It is helpful to know multiple methods
of solution. You can use one method
to solve a problem and another to
verify your work.
Mg(NO,),(aq) + 2 Ag(s)
Red u c t ion:
(c) 2 MnO4jaq) + 16 H(aq) + 5 C2042jaq) —.
2 Mn2
+ 10 C02(g) + 8 H,O(l)
Reduction:
Oxidation:
(d) 2 Al(s) + 3 1(s) —*2 AP(aq) + 6 1(aq)
Reduction:
Oxidation:
2. Balance the following equations for reactions occurring in acidic conditions:
(a) Cr20,2(aq) + I (aq) —> Cr’(aq) + 12(g)
—*
(c) Fe2(aq) + H,02(1)
Fe3 (aq) + 1-1,00)
—,
(d) Cr0-2 (aq) + SO,2Qq)
NEL
Cl,(g)
(b) MnOj(aq) + Cr(aq)
Mn2(aq)
Cr (aq) + S042(aq)
9.2 Balancing Redox Reaction Equations
181
3. Balance the following equations for reactions occurring in basic conditions. El
(a) CN(aq) + Mn04(aq) —* CNOiaq) + Mn02(s)
(b) Br2(g)
—>
Br01(aq) + Br(aq)
(c) Mn2(aq) + H202(l)
—>
Mn02(s) + F110(I)
(d) Bi(OH)3(aq) + SnO,2(aq)
—.
Sn032(aq) + Bi(s)
4. Consider the following reaction:
Pb(OH)42(aq) + C10(aq)
,
PbO(s) + C1(aq)
(a) How many electrons are transferred for each Pb(OH)42 ion?
(b) How many electrons are transferred for each Cl0
ion?
(c) Write the balanced chemical equation using the oxidation numbers
El
method assuming the reaction occurs in basic conditions.
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5. Potassium permanganate is a deep purple compound that can be used to
determine the concentration of sulfur dioxide in the atmosphere. Sulfur
dioxide is one of the compounds responsible for acid precipitation. The
reaction between sulfur dioxide and potassium permanganate is shown below:
SO(g) + Mn04(aq)
—*
SO4(aq) + Mn24(aq)
Balance the above equation in acidic conditions, using either the oxidation
fl
numbers method or the halt’- reactions method.
6. When a current is passed through copper(l) ions in solution, the reaction
S
produces copper solid and copper(II) ions.
reaction using the halfthis
(a) Write the balanced chemical equation for
reaction method.
(b) What is unusual about the role of the copper(I) ion in this reaction?
NEL
9.2 Balancing Redox Reaction Equations
183
Predicting Redox Reactions
Textbook pp. 618423
Vocabulary
table
Main Idea: A relative ranking of the strength of oxidizing and reducing agents has been
developed from empirical evidence. Redox tables list the strongest oxidizing agent in the top
left corner of the table and the strongest reducing agent in the boffom right corner of the
table, Redox tables can be used to predict whether a redox reaction occurs spontaneously.
1. Rank the strength of the following as oxidizing agents, from strongest to
weakest.
Cu2 (aq), Co2 (aq), Au3 (aq). Ag (aq)
2. Rank the strength of the following as reducing agents, from strongest to
weakest.
Zn(s), Ba(s), Mg(s), Ni(s)
3. Which of the following equations represent(s) spontaneous reactions? 0
(a) Sn(s) + 2 Cu2(aq) —* Sn2’ (aq) + 2 Cu’(aq)
(h) Mg2 (aq) + 2 Ag(s)
—*
(c) Ca(s) + Cd2(aq)
(d) 2 B((aq) + Sn2(aq)
Mg(s) + 2 Agt(aq)
J
Ca2(aq) + Cd(s)
—,
Br2(g) + Sn(s)
4. A piece of solid silver is placed into a solution containing nickel(II) ions.
(a) Write the balanced chemical equation for the reaction that produces solid
nickel and silver ions.
(b) Predict whether the reaction is spontaneous. Explain your prediction.
5. Which of the following reagents can oxidize H20(l) to
H(aq), CL(aq), CL(g). Cu2’ (aq), PbNaq), Mn04(aq) (in acid)
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6. Consider the following half-reactions:
Mn04jaq) + 8 H4(aq) + 5 e
STUDY TIP
—*
Mn2(aq) + 4 H20(l)
(a) Predict whether NO ions will oxidize Mn2 to MoO1. Explain.
Using Tables
Copy tables of information that you
will use over and over again onto
index cards. Give each table an
appropriate title and keep in a handy
location.
(b) Rearrange and/or balance the equation that shows the spontaneous
reaction for N03 and Mn2
0
LEARNING TIP
NO3 (aq) + 4 H4(aq) + 3 e
—>
NO(g) + 2 H20(l)
7. Use the Standard Reduction Potentials table in your textbook (Table 1 in
Appendix 87) to answer the following questions.
(a) Predict which species in each pair is a better oxidizing agent:
(i) Br,(l) or Au3’ (aq) (ii) H2(g) or Ag (aq) (iii) Cd2 (aq) or Cr3 (aq)
(b) Predict which species in each pair is a better reducing agent.
(i) Na(s) or Li(s) (ii) Fe2(aq) or Ag(s) (iii) H(g) or h(s)
Halt-Reactions Involving Water
There are several half-reactions that
include water listed in Table 1 in
Appendix B7.
For example,
8042(aq) + H20(l) -I- 2 e
S0321aq) - 2 0H(aq)
ED,= —0.93V
The only equation for which water is
reduced is:
2 FIO(1) + 2 e
H2(g) + 2 OFh(aq)
F°,= —O.83V
The only equation in which only
water is oxidized is:
O1(g) + 2 H(aq) + 4 e
2 H20(l)
Et= -l-0.70\’
(c) Identify the strongest reducing agent and the strongest oxidizing agent
from parts (a) and (h). Justify your answer.
LEARNING TIP
8. (a) Which of the following pairs of substances, when combined, would result
in a spontaneous chemical reaction? 0
(i) Au1 — (aq) and Al(s)
(iii) AgCl(s) and Br (aq)
k
(ii) Kt(aq) and Cr(s)
(iv) Fe2(aq) and Na(s)
Metals and Metal Ions
Elemental metals always lose
electrons in redox reactions to form
cations (postively charged ions).
As a result, metals are reducing
agents. Conversely, metal cations
usually gain electrons to form
metal atoms. Hence, metal ions are
usually oxidizing agents. There are
some exceptions. For example, the
iron(ll) ion can be both oxidized and
reduced:
Oxidation: Fe2(aq) — Fe3(aq) ± e
Reduction: Fe2(aq) * 2 e —* Fe(s)
(b) Write a balanced chemical equation for each reaction in part (a) that you
predict will occur.
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9.3 Predicting Redox Reactions
185
9. A student chemist prepared several solutions to use in a chemical analysis,
as outlined below. Each of the solutions was stored in a different type of
container. 0 Sn
(1) irontlI) nitrate in an aluminum flask
(ii) nickel(ll) sulfate in a copper flask
(Hi) magnesium nitrate in a copper flask
(iv) copper(ll) sulfate in an iron drum
(a) Predict whether the student will notice any changes a day later. Explain
your reasoning.
(b) For each solution that you predict will react, what evidence of change do
you think the student will observe?
(c) Write a balanced equation for each predicted reaction.
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