Calculating and Recognizing Oxidation Numbers

Question 1

why are oxidation numbers used

Answer 1

• with redox reactions involving compounds that are covalent, you cant say that they lose or gain electrons as they are not like ions
• instead, you use oxidation numbers to understand the ‘charges’ of the elements forming a compound and the ‘charge’ of the compound

Question 2

what would the overall oxidation number of MgO be

Answer 2

0

Question 3

why is it 0

Answer 3

• because Mg has an oxidation number of +2
• whereas O has one og -2
• when you combine these two together you get 0

Question 4

why is the oxidation number of Mg +2

Answer 4

because the charge of a magnesium ion is 2+

Question 5

what would the oxidation numbers of S be in SO2 and why

Answer 5

• for S it would be +4
• because O has an oxidation number of -2
• but as you have two of them their total oxidation number is -4
• when combining the O2 and S you get SO4 with no oxidation number

Question 6

what is the oxidation number of an uncombined element always going to be

Answer 6

0

Question 7

what would the sum of the oxidation numbers of all the elements in a neutral compound be

Answer 7

0

Question 8

what would the sum of the oxidation numbers of all the elements in an ion be equal to

Answer 8

the charge on the ionisation

Question 9

what element would be given the negative oxidation number in a substance

Answer 9

the more electronegative element

Question 10

what is the oxidation number of fluorine always going to be

Answer 10

-1

Question 11

what are the two different oxidation numbers hydrogen can have and why

Answer 11

• the oxidation number of hydrogen is usually +1

- but if it is combined with a less electronegative element it would be -1

Question 12

what are the different kinds of oxidation numbers oxygen can have and in what conditions

Answer 12

• it is usually -2
• but in peroxides it is -1
• when combined with fluorine it is positive

Question 13

what is the definition of an oxidation number

Answer 13

• the charge that an ion has

- or the charge that it would have if its species were fully ionic

Question 14

what has happened in the half equation Zn(s) = Zn(2+) + 2e-

Answer 14

as the zinc has lost electrons, it has been oxidised (OILRIG)

Question 15

what else do you notice about how zinc changes in the half equation

Answer 15

its oxidation number increases from 0 to +2

Question 16

what has happened in the half equation Cl2(aq) + 2e- = 2Cl-(aq)

Answer 16

the chlorine has gained electrons so it has been reduced

Question 17

what do you notice about how the oxidation number of chlorine changes

Answer 17

it decreases from 0 to -1

Question 18

how can we then also define oxidation and reduction knowing how the oxidation numbers of elements change in each case

Answer 18

• oxidation is the loss of electrons which results in the increase of oxidation numbers
• whereas reduction is the gain of electrons which results in the decrease of oxidation numbers