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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 |
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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 |
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the charge on a simple ion
an increase in oxidation number (increase in positive charge) = oxidation
a decrease in oxidation number = reduction |
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if one substance is oxidized, the other must cause it to be oxidized
they are equal and opposite components of a reaction |
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| whatever CAUSES a substance to be oxidized - whatever is BEING reduced |
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| whatever CAUSES a substance to be reduced, whatever is BEING oxidized |
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| electrochemistry cells and batteries |
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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) |
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| the two pieces of metal where electrons are transferred in an electrochemical cell |
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| the electrode where OXIDATION occurs |
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| the electrode where REDUCTION occurs |
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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 |
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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 |
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(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) |
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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 |
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| 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 |
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| 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) |
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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 |
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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) |
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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 |
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substitute for elemental chlorine in the bleaching and processing of paper
reduces creation of dioxins as byproducts (carcinogens), forms fewer harmful byproducts |
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certain reducing agents in food chemistry
ascorbic acid (vitamin C) - which prevents the browning of fruit by inhibiting air oxidation |
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| excellent reducing agent that can free many metals from their ores and reduce many chemical compounds |
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| a substance that increases the rate of a chemical reaction without itself being used up - often present in redox reactions |
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| for a chemical reaction - the minimum energy needed ot get the reaction started |
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| cellular respiration and redox |
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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) |
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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 |
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