CHAPTER 8 (redox)

question

reduction
answer

forms of matter high in energy
the opposite of oxidation, the removal of oxygen from an oxide

A LOSS OF OXYGEN ATOMS or
A GAIN OF HYDROGEN ATOMS or
A GAIN OF ELECTRONS

question

oxidation
answer

forms of matter low in energy
when oxygen combines with other elements or compounds

A GAIN OF OXYGEN ATOMS or
A LOSS OF HYDROGEN ATOMS or
A LOSS OF ELECTRONS

question

oxidation number
answer

the charge on a simple ion

an increase in oxidation number (increase in positive charge) = oxidation
a decrease in oxidation number = reduction

question

in a redox reaction
answer

if one substance is oxidized, the other must cause it to be oxidized
they are equal and opposite components of a reaction
question

oxidizing agent
answer

whatever CAUSES a substance to be oxidized – whatever is BEING reduced
question

reducing agent
answer

whatever CAUSES a substance to be reduced, whatever is BEING oxidized
question

electrochemistry cells and batteries
answer

an electric current in a wire is simply a flow of electrons

oxidation-reduction reactions in which electrons are transferred from one substance to another can be used to produce electricity (ex. dry cell and storage batteries)

by placing the reactants in separate compartments and connecting them with a wire, the electrons will flow through the wire to get from one substance to another (electrochemical cell)

question

electrodes
answer

the two pieces of metal where electrons are transferred in an electrochemical cell
question

anode
answer

the electrode where OXIDATION occurs
question

cathode
answer

the electrode where REDUCTION occurs
question

dry cell
answer

used in flashlights and many other small portable devices

zinc anode (container itself), carbon cathode rod in center of cell, space between cathode and anode contains moist paste of graphite powder, manganese dioxide, ammonium chloride. anode reaction is OXIDATION of ZINC cylinder to zinc ions, cathode reaction involves REDUCTINO of manganese dioxide

question

battery
answer

a collection of electrochemical cells

lead storage batteries can be recharged – discharges as it supplies electricity when you turn on the ignition to start a car or when a motor is off and the lights are on – but it is recharged when the car is moving and an electric current is supplied to the battery by the mechanical action of the car

lithium battery – extraordinarily low density and provides fairly high voltage

question

fuel cell
answer

(a kind of battery)
fuel is oxidized at the anode and oxygen is reduced at the cathode with 70-75% efficiency (compared to fossil fuels with 35-40% efficiency)
question

corrosion
answer

20% of all the iron and steel production in the US each year goes to replace corroded items

in moist air, iron is oxidized – as it is oxidized, oxygen is reduced resulting in the formation of insoluble iron II hydroxide, further getting oxidized to iron III hydroxide

oxidation and reduction often occur at separate points on the metal’s surface – electrons are transferred through the iron metal

question

aluminum’s oxidation
answer

aluminum surface reacts with oxygen in the air to form a thin layer of oxide – however, instead of being porous and flaky like iron oxide, aluminum oxide is hard, tough, and adheres strongly, protecting the metal from further oxidation
question

silver’s tarnish
answer

tarnish on silver results from oxidation of the silver surface by hydrogen sulfide in the air from food (H2S) – producing a film of black silver sulfide (Ag2S)
question

chemical explosions
answer

usually based on the result of redox reactions – happen when a chemical reaction occurs rapidly and with a considerable increase in volume

a reaction of solid and/or liquid reactants that generates gaseous products involves a huge volume increase and a possibility of explosion

question

oxygen
answer

oxygen itself is the most common oxidizing agent

comprises almost 2/3 of our body mass

comprises about half of the accessible portion of Earth by mass

combines with many metals to form metal oxides and nonmetals to form nonmetal oxides

OZONE – is a powerful oxidizing agent and a harmful air pollutant (if low in the atmosphere)

question

common oxidants
answer

used as antiseptics (to living tissues), disinfectants (to nonliving things), bleaches, and play a role in many chemical syntheses

hydrogen peroxide (h2o2) is a common oxidizing agent that has the advantage of being turned into water in most reactions

question

chlorine dioxide
answer

substitute for elemental chlorine in the bleaching and processing of paper

reduces creation of dioxins as byproducts (carcinogens), forms fewer harmful byproducts

question

antioxidants
answer

certain reducing agents in food chemistry

ascorbic acid (vitamin C) – which prevents the browning of fruit by inhibiting air oxidation

question

hydrogen as reducer
answer

excellent reducing agent that can free many metals from their ores and reduce many chemical compounds
question

catalyst
answer

a substance that increases the rate of a chemical reaction without itself being used up – often present in redox reactions
question

activation energy
answer

for a chemical reaction – the minimum energy needed ot get the reaction started
question

cellular respiration and redox
answer

we obtain energy for all our physical and mental activities by metabolizing food through cellular respiration – where carbohydrates are OXIDIZED (in animals) and formed during photosynthesis (in plants)

in food metabolization we focus on oxidation
in photosynthesis we focus on reduction (provides food AND oxygen)

question

oxidation number rules
answer

an element in elemental form has na oxidation # of 0

a monatomic ion has an oxidation # equal to its charge ( we can predict normal oxidation numbers according to position in periodic table and the tendency to acquire or lose electrons to attain a complete octet.)

In compounds, we calculate the oxidation number on an atom as follows: start with the charge on the atom stripped of its valence electrons and then decrease this number by one for each electron assigned to the atom (e.g. in the Lewis electron dot structure) as follows: unshared electrons are assigned to the atom they surround. Shared electrons are assigned to the more electronegative element; if they are equally shared in a pure covalent bond between two atoms of the same element, one electron of the pair is assigned to each atom

the sum of all oxidation numbers of all atoms in a neutral molecule should be zero; the sum in a charged ion should be the charge on the ion

Get instant access to
all materials

Become a Member