B2 - Redox and Periodicity
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What is an oxidation number?
a measure of the number of electrons that an atom used to bond with atoms and another element –> cannot be measured directly by experiment but are worked out
Oxidation number of an uncombined element
0
Oxidation number of a combined oxygen
-2 e.g. H2O, CaO
Oxidation number of a combined hydrogen
+1 e.g. Nh3, H2S
Oxidation number of a simple ion
charge on ion
Oxidation number of a combined flourine
-1 e.g. HF, CaF2
Oxidation number of a H in metal hydrides
-1 e.g. NaH, CaH2
Oxidation number of an O in peroxides
-1 e.g. H2O2
Oxidation number of an O bonded to F
+2 e.g. F2O
What term is sometimes used in place of oxidation number?
oxidation state
What is the overall charge of compounds?
0 —> sum of oxidation numbers is zero
What must the sum of oxidation numbers in molecular ions equal?
the overall charge
How are roman numerals used?
used in the names of compounds of elements that form ions with different charges –> the roman numeral show the oxidation state of the elements
What are polyatomic ions?
the negative ‘group ions’ that contain oxygen bonded to another element –> have ‘-ate’ ending in their name
Define redox reactions
a reaction in which both reduction and oxidation takes place is called a redox reaction
Redox reactions in terms of hydrogen
oxidation is the loss of hydrogen reduction is the gain of hydrogen
Define reducing agent
a reducing agent is a reagent that reduces (adds electrons to) another species a reducing agent is itself oxidised
Define oxidising agent
an oxidising agent is a reagent that oxidises (takes electrons from) another species an oxidising agent is itself reduced
Redox reactions using oxidation numbers
oxidation is an increase in oxidation number reduction is a decrease in oxidation number
Language pattern for redox reactions using oxidation numbers
the oxidation number of ____ has increased from _______ to ______: oxidation the oxidation number of ____ has decreased from _______ to ________: reduction
Define disproportionation
disproportionation is simultaneous oxidation and reduction of the same element
What are the vertical columns called?
groups
What are the horizontal rows called?
periods
What is meant by the term periodicity?
across each period, there is a repeating trend in properties of the elements