Honors Chemistry Unit 13 Electrochemistry

What are oxidation numbers?

• assigned charge for the atoms/ions in the formula

• they keep track of electrons in a chemical reaction

• using the term charge is not accurate for molecular compounds

  • in these, imagine all bonds as ionic and give electrons to the atom with highest electronegativity

• unbounded elements will have a charge of 0(diatomics have no polar covalent bonds)

How do we assign the oxidation numbers?

• check periodic table

• if multiple numbers listed,

  • sum of numbers is neutral(adds to 0)

  • sum of oxidation numbers in a polyatomic ion must equal the total charge of the ion

What is reduction?

• decrease in charge of a species(ions or atoms)

• done by gaining electrons

How to tell if something is reduced?

• the species that gains electrons and loses charge

What is oxidation?

• increase in charge of a species

• done by losing electrons

oxidation

A reducing agent goes through

reduction

The oxidizing agent goes through

Oxidizing = Reducing Agent

Reducing = Oxidizing Agent

ORA ROA

Losing Electrons Oxidation

Gaining Electrons Reduction

LEO GER

How to tell if a reaction is a redox reaction?

• can assign ox #s to all

• can say “any rxn with single element must be redox”

half reaction

• shows exchange of electrons

• a change in oxidation numbers shows electrons are being gained/lost

How to write half reactions

1. eliminate non redox element

2. determine if ox or re(ox, e- on right)

3. balance charges

To balance with molecule number,

1. determien half reactions

2. multiply by multiple → add to rxn

3. Co is reduced and Cl- is oxidized

Co + PbCl2 → CoCl2 + Pb

1. Co is oxidized and Cl- is reduced

2. Co is oxidized and Pb+2 is reduced

3. Co is reduced and Cl- is oxidized

4. Co is reduced and Pb+2 is oxidized

1. +6

What is the oxidation number of chromium in K2Cr2O7?

1. +6

2. +3

3. +12

4. +2

3. H2

Which of the following elements is the poorest reducing agent?

1. Al

2. Zn

3. H2

4. Ba

1. decreases

As the elements in Period 3 of the Periodic Table are considered in order of increasing atomic number, the ability of each successive element to act as a reducing agent…

1. decreases

2. increases

3. remains the same

4. Ag+

According to Reference Table J, which of these ions is most easily reduced?

1. Cu+

2. Cr3+

3. Ca2+

4. Ag+

3. Cu(s) + 2HCl(aq) → CuCl2(aq) + H2(g)

Referring to Reference Table J, which reaction will not occur under standard conditions?

1. Sn(s) + 2HCl(aq) → SnCl2(aq) + H2(g)

2. Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)

3. Cu(s) + 2HCl(aq) → CuCl2(aq) + H2(g)

4. Ba(s) + 2HCl(aq) → BaCl2(aq) + H2(g)

2. F2(g)

According to Reference Table J, which species is the strongest oxidizing agent?

1. Li+

2. F2(g)

3. Li(s)

4. F-

3. 3

Given the unbalanced equation:

___ Ag2S + 8HNO3 → ___ AgNO3 + 2NO + __ S + _H2O

What is the coefficient of Ag2S when the equation is completely balanced using the smallest whole numbers?

1. 6

2. 2

3. 3

4. 4

4. form oxides that are self protective

Iron corrodes more easily than aluminum and zinc because aluminum and zinc both

1. form oxides

2. are oxidizing agents

3. are reduced

4. form oxides that are self protective

3. Al and Zn

Which two metals resist corrosion by forming self-protective coatings?

1. Al and Fe

2. Fe and Zn

3. Al and Zn

4. Fe and Na

1. zinc

Which of the following forms a self-protective coating when exposed to air and moisture?

1. zinc

2. iron

3. sodium

4. calcium

3. H2SO4

Which substance functions as the electrolyte in an automobile battery?

1. H2O

2. PbO2

3. H2SO4

4. PbSO4

3. spontaneous oxidation-reduction reaction

The type of reaction in an electrochemical cell is best described as

1. non-spontaneous oxidation-reduction reaction

2. spontaneous oxidation reaction only

3. spontaneous oxidation-reduction reaction

4. non-spontaneous oxidation reaction, only

12 moles

Given this equation:

Mg(s) + Ni2+(aq) → Mg2+(aq) + Ni(s)

What is the total number of moles of electrons needed to completely reduce 6 moles of Ni2+(aq) ions?

the electrons are not equal lost and gained

When writing half-reactions, you use coefficients when…

the element that gets oxidized and causes other elements to get reduced

What is a reduction agent?

+4

What is the oxidation number of Mn in Na2MnO3?

1. +7

What is the oxidation number of manganese in KMNO4?

1. +7

2. +2

3. +3

4. +4

electrons

All redox reactions involve the transfer of

voltaic cell

An electrochemical cell that uses a spontaneous chemical reaction to produce a flow of electrons

electrolytic cell

An electrochemical cell that uses the flow of electrons to force a no spontaneous chemical reaction to occur

anode; cathode

Electrons flow from ____ to ____

which metals are more active: towards top, oxidize more easily; toward bottom, reduce more easily; opposite of metal form for ions

What can we find out from Table J?

anode

• site of oxidation

• negative in voltaic cells

• positive in electrolytic cells

• mass decreases

• increases activity of metal

cathode

• site of reduction

• positive in voltaic cells

• negative in electrolytic cells

• mass increase

• decreases activity of metal

salt bridge

in two container system that allows spectator ions to flow

groups 1 and 2

Electrolysis occurs in

power source

electrolytic cells need a

cathode

the object plated is always a

AnOx RedCat

acronym for where each rxn occurs

Voltaic cells, Anode is Negative, rxn Spontaneous

acronym for cell type

Anode mass decreases because atoms turn into Ions and Dissolve

acronym for mass

chemical to electrical

Voltaic cells go from _____ to ______ energy

3. decreases

As the elements in Period 3 of the Periodic Table are considered in order of increasing atomic number, the ability of each successive element to act as a reducing agent

1. increases

2. remains the same

3. decreases

1. increases

In a chemical reaction, as a species is oxidized, its oxidation number

1. increases

2. decreases

3. remains the same

4. Mg to Al3+

2Al3+(aq) + 3Mg(s) → 3Mg2+(aq) + 2Al(s)

electrons are transferred from

1. Al to Mg2+

2. Mg2+ to Al

3. Al3+ to Mg

4. Mg to Al3+

A. K

Which metal can be produced only by the electrolysis of its fused salt?

A. K

B. Pb

C. Zn

D. Ag

D. Na+ + 1e- → Na

During the electrolysis of fused NaCl, which half-reaction occurs at the negative electrode?

A. Cl2+2e- → 2Cl-

B. 2Cl- →Cl2+2e-

C. Na → Na+ + 1e-

D. Na+ + 1e- → Na