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1

Oxidation number

The oxidation number of an element is the charge an element has or appears to hav when it is ina compound when certain rules are applied

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rules of oxidation numbers

8 rules

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rule 1

Simple elements i.e. those that are not with any other element have an oxidation number of zero

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rule 2

In the combined state, group 1 elements always have an oxidation number of +! and group 2 elements lways have an oxidation number of +2

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rule 3

in simple ions, i.e. single element ions, the oxidation number of the ion is equal to the charge on the ion

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rule 4

the halogens (group 7) in compounds where the halogen is bonded to ONE other element, has an oxidation number of -1. unless the halogen is bonded to a more electronegative element, then the oxidation number will be +1

note - does not work for halogens in complex ions, the charge in a complex ion represents a missing element

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rule 5

oxygen has an oxidation number of -2. except in the compound OF₂ it is +2 as F is more electronegative element than O and in the peroxide ion [O₂²⁻] where O has an oxidation number of -1

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rule 6

Hydrogen has an oxidation number of +1 in its compounds: except when it is combined to an element less electronegative than it because then it is a hydride and has an oxidation number of -1

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hydride molecule

in a hydride molecule, there are two elements in the molecule and hydrogen is written last. Also the metal element will have a lower electronegativity value than hydrogen

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rule 7

In complex ions, i.e. where more than one element is in the ion, the oxidation numbers must add up to the charge on the ion

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rule 8

in neutral molecules, the oxidation numbers must add up to zero

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oxidation

addition of oxygen

loss of electrons

increase in oxidation number

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reduction

loss of oxygen

gain of electrons

decrease in oxidation number

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oxidising agent

causes oxidation and is itself reduced

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reducing agent

causes reduction and is itself oxidised

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oxidation + reduction

-Before discovery of electrons, reactions that involved addition of oxygen to a substance described as oxidation reactions
-Removing oxygen from substance described as reduction reaction as mass of substance got smaller due to oxygen being removed
-After discovery of electron, scientists examined reactions more closely + noticed many chem reactions involve transfer of electrons
-One substance lost electrons while other gained

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eg of oxidation reactions (before discovery of electrons, reactions that involved addition of oxygen .....) [continued point]

-Burning of coal, which is carbon to produce carbon dioxide was an oxidation
-The rusting of iron to produce iron oxide

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oxidation/reduction electron transfer

-Oxidation is when an element loses electrons

-reduction is when an element gains electrons

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electron transfer

-both oxidation + reduction occur at same time. If one element loses electrons, another gains electrons
-called REDOX reactions

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eg of electron transfer

in hback

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The Electrochemical series

list of elements in order of their standard electrode potential
-top: readily lose electrons
-bottom: unreactive

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reactivity

how easily they lose electrons

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reactivity series of metals

in hback

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electrolysis

the use of electricity to bring about a chemical reaction in an electrolyte

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electrolyte

a compound which when molten or dissolved in water will conduct an electric current

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conduction of electricity

the conduction of electricity is due to the presence of ions

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electrodes

-the two rods that clip into the electrolyte and make electrical contact with it are called the electrodes

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two types of electrodes

-Inert
-active

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inert electrode

-do not react w/ electrolye
-graphite (carbon)
-platinum

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active electrode

-react with electrode
-copper
-iron