Topic 3 - Redox I

Question 1

What is oxidation number(oxidation state)?

Answer 1

Tells you how many electrons an atom has donated or accepted to form an ion or a compound

Question 2

What is the oxidation number for an uncombined element?

Answer 2

0 - they haven’t accepted or donated any electrons

e.g. Ag = 0

Question 3

What is the oxidation number for identical atoms that are bonded?

Answer 3

0 - they haven’t accepted or donated any electrons
e.g. O₂ = 0

Question 4

What is the oxidation number for monatomic ion?
(that’s an ion consisting of just one atom)

Answer 4

It is the same as its charge

e.g. Na+ = +1

Question 5

What is the oxidation number for molecular ions?
(ions that are made up of a group of atoms with an overall charge

Answer 5

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

Question 6

What is the oxidation number in a neutral compound?

Answer 6

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

Question 7

What is the oxidation number in hydrogen and two exceptions?

Answer 7

It is always +1
except
metal hydrides = -1
H₂ = 0

Question 8

What is the oxidation number in Oxygen?

Answer 8

It is always -2
except
peroxides = -1
O₂ = 0

Question 9

How do you show elements which have multiple oxidation numbers?

Answer 9

using Roman numerals

Question 10

What two things form ions that have the ending
-ate?

Answer 10

Oxygen and another element

e.g. Sulfate = sulfur and oxygen

Question 11

For the oxidation numbers to write chemical formulae - (not sure how to write this section)

Question 12

What is oxidation?

Answer 12

the loss of electrons

Question 13

Why is it called a redox reaction?

Answer 13

Reduction and oxidation happen simultaneously

Question 13

What is an oxidising agent?

Answer 13

accepts electrons and gets reduced

Question 14

What is reduction?

Answer 14

the gain of electrons

Question 14

What is a reducing agent?

Answer 14

donates electrons and gets oxidised

Question 14

What happens to the oxidation number when an electron is gained?

Answer 14

decreases

Question 14

How do you figure during a reaction if an element has oxidised or reduced?

Answer 14

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

Question 14

What happens to the oxidation number when an electron is lost?

Answer 14

increases

Question 15

What happens when metals form compounds?

Answer 15

they donate electrons to form positive ions therefore the oxidation number increases

Question 15

What happens when
non-metals form compounds?

Answer 15

they gain electrons to form negative ions therefore the oxidation number decreases

Question 15

What is a disproportionation reaction?

Answer 15

a special redox reaction which is when an element in a single species is simultaneously oxidised and reduced

This is shown by oxidation numbers

Question 16

What do ionic half-equations show?

Answer 16

oxidation and rediction

Question 17

How do you combine half equations?

e.g Magnesium burning in oxygen to form magnesium oxide

Answer 17

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

Question 17

What 3 things can you add to balance half equations if multiplying doesn’t work?

Answer 17

Add e- , H+ or H2O