Redox

Question 1

Define Oxidation. What happens to the oxidation number?

Answer 1

A gain of oxygen and loss of electrons when a chemical species gains oxygen or loses hydrogen. The oxidation number of the element is increased

Question 2

Define Reduction. What happens to the oxidation number?

Answer 2

Reduction is gain of electrons, loss of oxygen or gain of hydrogen. The oxidation number of the element is decreased

Question 3

Why do oxidation and reduction occur simultaneously?

Answer 3

Because any loss of electrons by one substance must be accompanied by a gain in electrons by something else, oxidation and reduction always occur together. Involves transfer of electrons

Question 4

Where does the term redox come from?

Answer 4

The term redox comes from the two reactions that occur, reduction and oxidation

Question 5

Explain the redox of potassium chloride

Answer 5

Potassium is oxidated while chloride is reduced

Question 6

Explain an ionic half equation

Answer 6

An ionic half equation shows the transfer of electrons. When oxidation occurs, the electrons are written on the right side while when reduction occurs, the electrons are written on the left side

Question 7

What is an oxidation state

Answer 7

The numbers, associated with a positive or negative sign that indicate how many electrons an atom can donate or accept

Question 8

What substances have OS of 0?

Answer 8

Any substance that is formed from one or more atoms of the same element has the OS=0 (eg. Br2, H2, O2, i2). Any molecule regardless of the number or type of atoms has the overall OS equal to 0. This is typically because the OS of each of the elements within the molecules cancel each other.

Question 9

OS of halogens

Answer 9

F, Cl, Br, i… have OS= -1

Question 10

OS of oxygen in peroxides and in F2O

Answer 10

Oxygen is –2, except in peroxides where it is in –1 and in F2O where it is +2

Question 11

OS of hydrogen and hydrites

Answer 11

Hydrogen is +1, except in hydrides like NaH where it is –1

Question 12

OS of all metals in group 1

Answer 12

+1, Na, K…

Question 13

OS of all metals in group 2

Answer 13

+2, eg. Mg

Question 14

OS of elements in group 3 (boron group)

Answer 14

+3, Al, Ga, Ti

Question 15

How do you find the OS?

Answer 15

To find the o, calculate the electron configuration

Question 16

Do metals have positive or negative OS and why?

Answer 16

Positive os because they share electrons to reach a stable configuration

Question 17

Do non metals have positive or negative OS and why?

Answer 17

Always form negative os because they gain electrons, to reach a stable configuration

Question 18

What does the roman numeral after the element’s name represent?

Answer 18

When naming, the charge on the ion is shown by using a Roman numeral after the element’s name

For example, iron can form different ions

Iron ions with a 2+ charge, Fe2+, are called iron(II) ions

Iron ions with a 3+ charge, Fe3+, are called iron(III) ions
The Roman numeral is the oxidation number of the element

Question 19

Name an example of a redox reaction

Answer 19

The reaction between zinc oxide and carbon is an example of a redox reaction

Zinc oxide is reduced because it has lost oxygen

Carbon is oxidised because it has gained oxygen

Question 20

What does +/- sign show in oxidation numbers?

Answer 20

Positive oxidation number = loss of electrons

Negative oxidation number = gain of electrons

Question 21

OS of fluorine

Answer 21

Fluorine is always –1

Question 22

What is the sum of oxidation numbers of an ion equal to?

Answer 22

The sum of the oxidation numbers in an ion is equal to the charge on the ion

SO4 2–
S = +6
Four O atoms = 4 x (–2) = –8
Sum of oxidation numbers = 6 – 8 = –2

Question 23

Which element is given a negative oxidation number in a compound or ion?

Answer 23

In either a compound or an ion, the more electronegative element is given the negative oxidation number F2O
Two F atoms = 2 x (–1) = –2
O = +2

Question 24

What do the tests for redox reactions involve? What are two common examples?

Answer 24

The tests for redox reactions involve the observation of a colour change in the solution being analysed

Two common examples are acidified potassium manganate(VII), and potassium iodide

Question 25

What is potassium manganate used to test? Explain and describe the test

Answer 25

Potassium manganate(VII), KMnO4, is an oxidising agent which is often used to test for the presence of reducing agents

When acidified potassium manganate(VII) is added to a reducing agent its colour changes from purple to colourless

Question 26

What is potassium iodide used to test? Explain and describe the test

Answer 26

Potassium iodide, KI, is a reducing agent which is often used to test for the presence of oxidising agents

When added to an acidified solution of an oxidising agent such as aqueous chlorine or hydrogen peroxide (H2O2), the solution turns a red-brown colour due to the formation of iodine, I2
The potassium iodide is oxidised as it loses electrons

The hydrogen peroxide is reduced

Therefore, potassium iodide is acting as a reducing agent

Question 27

What is an an oxidising agent?

Answer 27

An oxidising agent is a substance that oxidises another substance, and becomes reduced in the process

An oxidising agent gains electrons as another substance loses electrons

Common examples include hydrogen peroxide, fluorine and chlorine

Question 28

What is a reducing agent?

Answer 28

A reducing agent is a substance that reduces another substance, and becomes oxidised in the process

A reducing agent loses electrons as another substance gains electrons

Common examples include carbon and hydrogen

The process of reduction is very important in the chemical industry as a means of extracting metals from their ores