Topic 3 - Redox I Flashcards
Redox I
What is oxidation number(oxidation state)?
Tells you how many electrons an atom has donated or accepted to form an ion or a compound
What is the oxidation number for an uncombined element?
0 - they haven’t accepted or donated any electrons
e.g. Ag = 0
What is the oxidation number for identical atoms that are bonded?
0 - they haven’t accepted or donated any electrons
e.g. O₂ = 0
What is the oxidation number for monatomic ion?
(that’s an ion consisting of just one atom)
It is the same as its charge
e.g. Na+ = +1
What is the oxidation number for molecular ions?
(ions that are made up of a group of atoms with an overall charge
the sum of the oxidation numbers is the same as the overall charge of the ion
e.g. SO4^2- (sulfate)
charge for O= -8(4x-2)
S=+6 because -8+6=-2
Since the overall charge is -2
What is the oxidation number in a neutral compound?
0 - each atom in the compound will have its own oxidation number therefore the sum of these will be 0
e.g. MgCl₂ = 0
Mg= 2+
Cl=-1 but diatomic so 2 therefore Cl₂=-2
What is the oxidation number in hydrogen and two exceptions?
It is always +1
except
metal hydrides = -1
H₂ = 0
What is the oxidation number in Oxygen?
It is always -2
except
peroxides = -1
O₂ = 0
How do you show elements which have multiple oxidation numbers?
using Roman numerals
What two things form ions that have the ending
-ate?
Oxygen and another element
e.g. Sulfate = sulfur and oxygen
For the oxidation numbers to write chemical formulae - (not sure how to write this section)
What is oxidation?
the loss of electrons
Why is it called a redox reaction?
Reduction and oxidation happen simultaneously
What is an oxidising agent?
accepts electrons and gets reduced
What is reduction?
the gain of electrons
What is a reducing agent?
donates electrons and gets oxidised
What happens to the oxidation number when an electron is gained?
decreases
How do you figure during a reaction if an element has oxidised or reduced?
First assign the element an oxidation number before then after
If the oxidation number increases, then the element has lost electrons and has been oxidised
If the oxidation number decreases, then the element has gained electrons and has been reduced
What happens to the oxidation number when an electron is lost?
increases
What happens when metals form compounds?
they donate electrons to form positive ions therefore the oxidation number increases
What happens when
non-metals form compounds?
they gain electrons to form negative ions therefore the oxidation number decreases
What is a disproportionation reaction?
a special redox reaction which is when an element in a single species is simultaneously oxidised and reduced
This is shown by oxidation numbers
What do ionic half-equations show?
oxidation and rediction
How do you combine half equations?
e.g Magnesium burning in oxygen to form magnesium oxide
Oxygen is reduced
O₂ + 4e- –> 2O₂
Magnesium is oxidised
Mg –> Mg2+ + 2e-
HOWEVER there is 2 oxygen and only one Mg therefore you need to double the whole Mg equation
2Mg –> 2Mg2+ +4e-
Finally combine:
(no electrons should be here)
2Mg + O₂ –> 2MgO
What 3 things can you add to balance half equations if multiplying doesn’t work?
Add e- , H+ or H2O
