Unit 3 - Electrochemistry

Go back for these: 3.11-1, 3.11-2, 3.12-2, 3.13-1, 3.13-2, 3.15-1, 3.15-2, 3.16-1, 3.16-2, 3.17-1, 3.17-2, 3.18-1, 3.18-2, 3.20-1, 3.20-2, 3.25-2

Boron Fun Facts

• usually forms 3 covalent bonds (doesn’t follow octect rule)

• boric acid used to kill ants

• borax used as detergent

Nitrogen Fun Facts

• many oxidation states (-3 to +5)

• found in:

  • explosives

  • fertilizers

    • Ammonia from Haber process from H₂ and N₂ at low temp with catalyst

  • oxide in laughing gas

  • viagra

  • smog

  • preservatives

Carbon Fun Facts

• 3 allotropes

  • C₆₀: forms interesting tubular and spherical structures

  • Graphite: sp² hybridized

  • Diamond: sp³ hybridized

  • CO₃^-: inorganic form of carbon, essential part of cycle involcing greenhouse gas

  • CO₂: diprotic acid

  • CaCO₃: limestone

Phosphorus Fun Facts

• found in soaps, toothpaste, fertilizer, pesticides

• extracted from Ca₃(PO₄)₂ rock

• backbone in nucleic acids like DNA

• cause algae bloom

• use increasingly discouraged for environmental reasons

Halides Fun Facts

• small radii

• high ionization energy

• high electronegativity

• form -1 anions

• oxides and hydrides are acidic

Fluoride Fun Facts

(F^-) inserted instead of OH^- in tooth enamel to protect from decay

Chlorine Fun Facts

• manufactured as Cl₂ (strong oxidizing agent)

• used in disingection and sanitization

• also used to make PVC tubing

Noble Gases Fun Facts

• inert with 2 or 8 e^- in filled shells

• used as cryogens (He), inert gases (Ar), and lights (Ne)

Wet method

Ca₁₀(PO₄)₆F₂ + H₂SO₄ → H₃PO₄

Claus Process

H₂S → S

Two step oxidation process that produces H₂S (contaminant in natural gas, methane)

Contact Process

S → H₂SO₄

Four step oxidation process produces H₂SO₄ from elemental sulfur, S

Sulfuric Fun Facts

• most manufactured chemical in the world

• hald of all sulfuric acid used to solubilize phosphate in rocks by wet method

  • solubilized phosphates then used in ferilizers

• not just strong acid, but also string oxidizing agent and strong dehydration agent

Oxidation Number Assignment Rules

1. Free elements have oxidation num of 0

2. Individual ions are their charge and alkali metals (Group I) have +1

3. H has +1, unless by previous rules

4. O has -2, unless by previous rules

5. Otherwise, typically assigned by group on periodic table

   1. Group I: +1

   2. Group II: +2

   3. Group 3: +3 or -5

   4. Group IV: +4 or -4

   5. Group V: -3 or +5

   6. Group VI: -2

   7. Group VII: -1

6. Sum of individual charges must equal overall charge on molecule

OIL RIG

Oxidation is Loss of electrons

Reduction is Gain of electrons

Oxidizing Agent

species causing oxidiation of other species

species being reduced

Reducing Agent

species that causes reduction of other species

species being oxidized

Ranking Strength of Agents from a Table of Reduction Potentials

• move up the table, species on left easier to reduce (stronger oxidizing agents)

  • part of the half-cell with the largest positive reduction potential

• move down the table, species on right easier to oxidize (strong reducing agents)

  • part of the half-cell with the largest negative reduction potential

less common to see oxidation potential tables with show the reverse process

Balancing Redox Reactions in Neutral Water

1. Assign oxidation numbers

2. Create brackets to identify oxidation and reduction

3. Balance atoms that are not hydrogen or oxygen (multiplies charge around bracket)

4. Find least common multiple and balance

Balancing Redox Reactions in Acids or Bases

Follow neutral water steps

For Acids: find deficient O and put in same num of H₂O on deficient side and double H⁺ on opposite side

For Bases: find deficient O and put same num of H₂O on opposite side and double OH^- on deficient side

Battery (Voltaic/Galvanic Cells)

• ΔG (-)

• K > 1

• E (+)

• Reduction at Cathode

• Oxidation at Anode

• e^- flow from anode to cathode

• Cathode (+)

• Anode (-)

Electrolytic Cell

• ΔG (+)

• K < 1

• E (-)

• Reduction at Cathode

• Oxidation at Anode

• e^- flow from anode to cathode

• Cathode (-)

• Anode (+)

Shorthand Cell Notation

A | A^± || C^± | C

• A and B are electrodes

• || represents salt bridge

• left: anode

• right: cathode

Pt or Au electrodes: inert electrodes that serve as conductors when neither species in the half-reaction is a metal that can serve as an electrode

• dont participate in the reaction - hence inert

• used when both species are aqueous or gas

• Pt and Au used bc highly resistant to oxidation (noble metals)

Table of Standard Reduction Potentials

All values for standard conditions: 1M, 1 atm, 298K

Reference electrodes (Standard hydrogen electrode at 0.0V)

E° calculation

E°_cell = E°_cat - E°_an

Battery if E°_cell (+)

Electrolytic Cell if E°_cell (-)

Nernst Equation

E_cell = E°_cell - (0.06/n)logQ

Q (reminder)

Q = [right]/[left]

(as the reaction progresses)

Q < 1 → more reactants

Q = 1 → equal ratio of products and reactants

Q > 1 → more products

Q = K → system at equilibrium

for a battery, when E = 0, Q = K

Nernst Equation Calculation

1. Determing cathode and anode then find E°_cell

2. Combine half-cells to get unbalanced reaction

3. Balance (no acids/bases) to find n and Q

4. Set up equation

5. Plug in concentrations and solve

Dead Battery Calculation

E_cell = 0 and Q = K

0 = E°_cell - (0.06/n)logK

Faraday - Plating Calculation

Given current and time (i and t) find amount of material used or formed

Farady’s constant = 9.65 × 10⁴ C/mole e^-

Aluminum Fun Facts

• lightest metal commonly used in commercial applications

• obtained from the energy intesive Bayer process (used to make Al₂O₃ from bauxite rocks) and Hall process (used to make Al from Al₂O₃)

• oxide thart forms ruby, sapphire, topaz

Alkali Metals Fun Facts

• large radii

• low ionization energy

• low electronegativity

• form 1+ cations

• oxides and hydrides are basic

• react vigorously in water

Beryllium Fun Facts

forms covalent bonds because of relatively small size and ability to attract electrons

Calcium Fun Facts

in building materials from concrete to teeth

Mg²⁺ Fun Facts

• in chlorophyll (in plants)

  • makes molecule more rigid to permit e^- transport for photosynthesis instead of energy loss through vibrations

Electrochem to Thermo

∆G° = -nFE°_cell

n = num e^- in balanced rxn

E°_cell (+) → ∆G° (-)

E°_cell (-) → ∆G° (+)

gives the answer in J, need to divide by 1000 to get kJ

Spontaneity from K

• K > 1 → product favored, ∆G (-)

• K = Q → reaction at equilibrium, ∆G = 0

• K < 1 → reactant favored, ∆G (+)

Metal Activity Series

4 buckets, on periodic table from left to right: (increasing reactivity of metals from right to left)

• Group I: React in Cold Water

• Group II: React in Hot Water

• Transition: React in Acid

• Coinage Metals: Don’t React

most reactive metals in lower right corner

least reactive metals in top right corner

Battery Design

• pushing boundaries by trying to make smallest, least expensive, highter power, longest life, reversible, environmentally safe

• Secondary batteries built with consideration given to minimizing liquid or gas production so that contents better contained in casing for recharging

• at current limits, Li-ion best, and known to catch fire

• modern batteries made with solids and pastes rather than liquids and gases to avoid losses of material

• primary batteries single use

• rechargeable = secondary batteries that are environmentally superior and convenient

• inefficient batteries produce heat instead of work; heat dissipation important practically and for safety

• lightweight batteries use less dense, higher charge density materials like lithium

Primary Batteries

disposed of after one cycle

Secondary Batteries

• reversible

• built with consideration given to minimizing liquid or gas production so contents of reaction are better contained in battery casing for recharging

Alkaline Battery

Primary battery, household usage

Zn-Hg, Zn-air Battery

Primary battery, hearing aids

Li-ion Battery

Secondary battery, electronics/electric cars

Lead-acid Battery

Secondary battery, cars (with altenator)

NiCd Battery

Secondary battery, household

NiMH

Secondary battery, household

Rust

• made by oxidation of iron in presence of water or air

• Fe + H₂O + O₂ → Fe(OH)₂ + Fe(OH)₃

• can be orevented by adding metals through specific processes:

  • Galvanization with Zn

  • sacrificial electrodes with Mg, Al

  • Stainless steel with Cr and V added to Fe

  • Waxes and coating

Green Chemistry

12 principles developed in 1998 aimed to reduce waste, increase safety, and reduce environmental impacts of chemical processes

Atom Economy

• synthetic methods designed to maximize incorporation of all materials used in process into final product

• (AE) Atom Economy = (MW of product atoms utilized)/(MW of all reactants) x 100%

• high atom economies desired (fewer atoms go to waste)