Oxidation Numbers Flashcards
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
rules of oxidation numbers
8 rules
rule 1
Simple elements i.e. those that are not with any other element have an oxidation number of zero
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
rule 3
in simple ions, i.e. single element ions, the oxidation number of the ion is equal to the charge on the ion
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
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
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
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
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
rule 8
in neutral molecules, the oxidation numbers must add up to zero
oxidation
addition of oxygen
loss of electrons
increase in oxidation number
reduction
loss of oxygen
gain of electrons
decrease in oxidation number
oxidising agent
causes oxidation and is itself reduced
reducing agent
causes reduction and is itself oxidised
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
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
oxidation/reduction electron transfer
- Oxidation is when an element loses electrons
- reduction is when an element gains electrons
electron transfer
- both oxidation + reduction occur at same time. If one element loses electrons, another gains electrons
- called REDOX reactions
eg of electron transfer
in hback
The Electrochemical series
list of elements in order of their standard electrode potential
- top: readily lose electrons
- bottom: unreactive
reactivity
how easily they lose electrons
reactivity series of metals
in hback
electrolysis
the use of electricity to bring about a chemical reaction in an electrolyte
electrolyte
a compound which when molten or dissolved in water will conduct an electric current
conduction of electricity
the conduction of electricity is due to the presence of ions
electrodes
-the two rods that clip into the electrolyte and make electrical contact with it are called the electrodes