L1 - Intro to Redox

objectives

Understandings

• Oxidation and reduction can be considered in terms of oxygen gain/hydrogen loss, electron transfer or change in oxidation number.
• Variable oxidation numbers exist for transition metals and for most main-group non-metals.

Applications

• Deduction of the oxidation states of an atom in an ion or a compound.
• Deduction of the name of a transition metal compound from a given formula, applying oxidation numbers represented by Roman numerals.

Redox Definitions

Redox has a number of different definitions based on its’ use in Chemistry - in all chemical reactions in which this occurs elements are said to either be Oxidised or Reduced. As these processes always happen simultaneously we call these reactions REDOX REACTIONS:

Oxidation is either:

• The LOSS of Electrons
• The GAINING of OXYGEN OR LOSS of HYDROGEN
• The INCREASE in OXIDATION NUMBER

Reduction is either:

• The GAIN of Electrons
• The GAINING of HYDROGEN or LOSS of OXYGEN

The DECREASE in OXIDATION NUMBER

These definitions of Oxidation and Reduction have to go TOGETHER. I.e.

Oxidation - Application of Definitions

GAIN OF OXYGEN

2Mg + O₂ ——> 2MgO

magnesium has been oxidised as it has gained oxygen

REMOVAL (LOSS) OF HYDROGEN

C₂H₅OH ——> CH₃CHO + H₂

ethanol has been oxidised as it has ‘lost’ hydrogen

Reduction - Applications of Definitions

GAIN OF HYDROGEN

C₂H₄ + H₂ ——> C₂H₆

ethene has been reduced as it has gained hydrogen

REMOVAL (LOSS) OF OXYGEN

CuO + H₂ ——> Cu + H₂O

copper(II) oxide has been reduced as it has ‘lost’ oxygen

Redox in terms of electrons

Oxidation Numbers/States - ATOMS and SIMPLE IONS

Used to... tell if oxidation or reduction has taken place

work out what has been oxidised and/or reduced

construct half equations and balance redox equations

OXIDATION NUMBER OR STATES ARE INTERCHANGEABLE IN IBDP CHEMISTRY - BOTH CAN BE USED BUT MEAN THE SAME THING!

ATOMS AND SIMPLE IONS

The number of electrons which must be added or removed to become neutral

atoms Na in Na = 0 neutral already ... no need to add any electrons

cations Na in Na⁺ = +1 need to add 1 electron to make Na⁺ neutral

anions Cl in Cl¯ = -1 need to take 1 electron away to make Cl¯ neutral

Oxidation Numbers/States - MOLECULES

The SUM of the oxidation states adds up to ZERO

ELEMENTS H in H₂ = 0 both are the same and must add up to Zero

COMPOUNDS C in CO₂ = +4

O in CO₂ = -2 1 x +4 and 2 x -2 = Zero

Explanation

• because CO₂ is a neutral molecule, the sum of the oxidation states must be zero
• for this, one element must have a positive OS and the other must be negative

HOW DO YOU DETERMINE WHICH IS THE POSITIVE ONE?

• the more electronegative species will have the negative value
• electronegativity increases across a period and decreases down a group
• O is further to the right than C in the periodic table so it has the negative value

HOW DO YOU DETERMINE THE VALUE OF AN ELEMENT’S OXIDATION STATE?

• from its position in the periodic table and/or
• the other element(s) present in the formula

Using the Periodic Table to Predict Oxidation Number

Oxidation Numbers/States - COMPLEX IONS

The SUM of the oxidation states adds up to THE CHARGE

• e.g. NO₃^- sum of the oxidation states = - 1
• SO₄^2- sum of the oxidation states = - 2
• NH₄⁺ sum of the oxidation states = +1

Examples

• in SO₄^2- the oxidation state of S = +6 there is ONE S
• O = -2 there are FOUR O’s
• +6 + 4(-2) = -2 so the ion has a 2- charge

What is the oxidation state (OS) of Mn in MnO₄¯ ?

• the oxidation state of oxygen in most compounds is - 2
• there are 4 O’s so the sum of its oxidation states - 8
• overall charge on the ion is - 1
• therefore the sum of all the oxidation states must add up to - 1
• the oxidation states of Mn four O’s must therefore equal - 1
• therefore the oxidation state of Mn in MnO₄¯is +7

+7 + 4(-2) = - 1

Variable Oxidation States

• Many elements can exist in more than one oxidation state
• In compounds, certain elements are used as benchmarks to work out other values

HYDROGEN +1 except 0 atom (H) and molecule (H₂)

-1 hydride ion, H¯ in sodium hydride NaH

OXYGEN -2 except 0 atom (O) and molecule (O₂)

-1 in hydrogen peroxide, H₂O₂

+2 in F₂O

FLUORINE -1 except 0 atom (F) and molecule (F₂)

THE TRANSITION ELEMENTS AND MOST MAIN GROUP METALS HAVE VARIABLE OXIDATION STATES

Calculating Oxidation States

Strange Examples - Guidance

• What is the Oxidation State of O in H₂O₂?
• What is the Oxidation State of H in NaH?
• What is the Oxidation State of Cl in HClO?

THE ROLE OF OXIDATION STATE IN NAMING SPECIES

To avoid ambiguity, the oxidation state is often included in the name of a species

• manganese(IV) oxide shows that Mn is in the +4 oxidation state in MnO₂
• sulphur(VI) oxide for SO₃ S is in the +6 oxidation state
• dichromate(VI) for Cr₂O₇^2- Cr is in the +6 oxidation state
• phosphorus(V) chloride for PCl₅ P is in the +5 oxidation state
• phosphorus(III) chloride for PCl₃ P is in the +3 oxidation state