REDOX reactions Oxidation and Reduction

Redox chemistry The study of oxidation and reduction reactions Oxidation and reduction reactions involve the loss or gain of electrons. When one species is oxidized another is reduced. Neither oxidation, nor reduction, can occur alone.

Reduction Refers to reactions in which a species gains electrons.

Oxidation Refers to reactions in which a species loses electrons

Question…. When NaCl is formed from it’s elements, describe what is happening in terms of electrons. There is a transfer of an electron from Na  Cl Remember an electron has a charge of negative 1 Na + Cl  Na + + Cl -

Sodium, Na, gives up or loses an electron to become Na + Na is oxidixed Chlorine, Cl, gains an electron to become Cl - Cl is reduced

More than one electron can be lost or gained in a redox reaction In the reaction 6Li (s) + N 2(g)  2 Li 3 N (s) three electrons are gained by each nitrogen atom.

Which one was which again…? OIL RIG Oxidation is loss of electrons Reduction is gain of electrons OR LEO the lion says GER Loss of Electrons is Oxidation Gain of Electrons is Reduction

A word about oxidation Oxidation often involves a reaction with oxygen, for example when iron reacts with oxygen in the air an oxidation reaction occurs to form rust. REMEMBER, the consumption of oxygen should be treated as a clue that redox chemistry might be taking place NOT ALL OXIDATION REACTIONS INVOLVE OXYGEN.

Reducing Agent Remember, oxidation and reduction occur together When one species is oxidized, another is reduced. A reducing agent is one which causes another species to be reduced while itself is being oxidized Reducing agents lose electrons

Oxidizing Agent An oxidizing agent causes another species to be oxidized. An oxidizing agent is reduced in a redox reaction

Half reactions We can write half reactions for the oxidation or reduction that is taking place in a redox reaction

For example: 2Na (s) + Br 2(l)  2NaBr (s) Each Na gives up an electron, so the Na is oxidized and is itself a reducing agent We can write a half reaction for this part 2Na  2Na + + 2e -

2Na (s) + Br 2(l)  2NaBr (s) The reduction half reaction is: Br 2 + 2e -  2Br - Each bromine atom gains an electron so bromine is reduced, while itself being the oxidizing agent.

Summary 2Na  2Na + + 2e - oxidation Br 2 + 2e -  2Br - reduction Sodium is a reducing agent – it caused the bromine to be reduced Bromine is an oxidizing agent – it caused the sodium to be oxidized.

Redox Terminology OXIDATION—loss of electron(s) by a species; increase in oxidation number. REDUCTION—gain of electron(s); decrease in oxidation number. OXIDIZING AGENT—electron acceptor; species is reduced. REDUCING AGENT—electron donor; species is oxidized.

Oxidation numbers To identify whether atoms are oxidized or reduced Tracking how oxidation numbers change as a reaction takes place allows us to see if an atom is oxidized or reduced

Oxidation numbers Decrease in oxidation number = the atom has been reduced Increase in oxidation number = the atom has been oxidized.

Rules for Assigning Oxidation Numbers The oxidation number of any uncombined element is 0. The oxidation number of a monatomic ion equals the charge on the ion. The more-electronegative element in a binary compound is assigned the number equal to the charge it would have if it were an ion. The oxidation number of fluorine in a compound is always –1.

Rules for Assigning Oxidation Numbers Oxygen has an oxidation number of –2 unless it is combined with F, when it is +2, or it is in a peroxide, such as H 2 O 2, when it is -1 The oxidation state of hydrogen in most of its compounds is +1 unless it is combined with a metal, in which case it is -1 In compounds, the elements of groups 1 and 2 as well as aluminum have oxidation numbers of +1, +2 and +3 respectively.

Rules for Assigning Oxidation Numbers The sum of the oxidation numbers of all atoms in a neutral compound is 0. The sum of the oxidation numbers of all atoms in a polyatomic ion equals the charge of the ion.

The oxidation number of any uncombined element is 0 What is the oxidation number of H in H 2 ? What is the oxidation number of Ne in Ne? What is the oxidation number of O in O 2 ? Zero

The oxidation number of a monatomic ion equals the charge on the ion What is the oxidation number of Cl in Cl - ? What is the oxidation number of Na in Na + ? +1 What is the oxidation number of Al in Al 3+ ? +3

The more-electronegative element in a binary compound is assigned the number equal to the charge it would have if it were an ion. Electronegativity tends to d e c r e a s e down a group and i n c r e a s e across a period So the elements located in the top right hand side of the periodic table (excluding the noble gases) are the most electronegative.

The more-electronegative element in a binary compound is assigned the number equal to the charge it would have if it were an ion. What is the oxidation number of Cl in HCl? What is the oxidation number of S in H 2 S? -2

The oxidation number of fluorine in a compound is always –1. What is the oxidation number of F in CF 4 ? What is the oxidation number of F in FCl? What is the oxidation number of F in CH 2 F 2 ?

Oxygen has an oxidation number of –2 unless it is combined with F, when it is +2, or it is in a peroxide, such as H 2 O 2, when it is –1. What is the oxidation number of O in H 2 O? -2 What is the oxidation number of O in H 2 O 2 ? What is the oxidation number of O in CaO? -2 What is the oxidation number of O in Al 2 O 3 ? -2 What is the oxidation number of O in OF 2 ? +2

The oxidation state of hydrogen in most of its compounds is +1 unless it is combined with a metal, in which case it is –1 What is the oxidation number of H in H 2 O? +1 What is the oxidation number of H in HCl? +1 What is the oxidation number of H in H 3 P? +1 What is the oxidation number of H in CaH 2 ?

In compounds, the elements of groups 1 and 2 as well as aluminum have oxidation numbers of +1, +2 and +3 respectively. What is the oxidation number of Li in Li 2 O? +1 What is the oxidation number of Ca in CaO? +2 What is the oxidation number of Al in Al 2 O 3 ? +3

The sum of the oxidation numbers of all atoms in a neutral compound is 0. What is the sum of the oxidation numbers in H 2 O? What is the sum of the oxidation numbers H 2 SO 4 ? What is the sum of the oxidation numbers in CaH 2 ? ZERO!

The sum of the oxidation numbers of all atoms in a polyatomic ion equals the charge of the ion. What is the sum of the oxidation numbers in PO 4 3- ? -3 What is the sum of the oxidation numbers SO 4 2- ? -2 What is the sum of the oxidation numbers in OH - ?

Determine the oxidation numbers for each atom: Class worksheet

Balancing Redox Reactions 2 methods Half reaction method Oxidation number change method

Half reaction method for Balancing Redox Reactions 1. Assign oxidation numbers to all species 2. Write the oxidation half reaction 3. Write the reduction half-reaction 4. Balance for charge 5. Re-write equation with factors from step #4 6. Balance for mass.

Balancing Redox Reactions H 3 PO 2 + H + + Cr → Cr 3+ + P + H 2 O Step 1: Assign Oxidation numbers to everything H 3 PO 2 + H + + Cr → Cr 3+ + P + H 2 O

H 3 PO 2 + H + + Cr → Cr 3+ + P + H 2 O Steps 2 and 3: Write oxidation and reduction half reactions Cr → Cr e - Oxidation half-reaction P + + e - → P Reduction half-reaction Step 4: Balance for Charge. Cr → Cr e - Oxidation half-reaction 3(P + + e - → P) Reduction half-reaction

H 3 PO 2 + H + + Cr → Cr 3+ + P + H 2 O Step 5: Re-write the original equation with the factor from balancing for charge. Cr → Cr e - Oxidation half-reaction 3(P + + e - → P) Reduction half-reaction 3H 3 PO 2 + H + + Cr → Cr P + H 2 O

Balancing redox reactions Step 6: Balance for mass. 3H 3 PO 2 + H + + Cr → Cr P + H 2 O 3H 3 PO H + + Cr → Cr P + 6H 2 O

Oxidation number change method Step 1: Assign oxidation numbers to all species Step 2: Identify which species are oxidized and which are reduced Step 3: track the change in oxidation number for a species on either side of the equation Step 4: Use coefficients to make the increase in oxidation number equal to the decrease Step 5: balance for atoms and charge

Mg + HCl  MgCl 2 + H 2 Step 1: assign oxidation numbers to all species Mg + HCl  MgCl 2 + H 2 Step 2: Identify which species are oxidized and which are reduced H decreases in oxidation number so it is reduced Mg increases in oxidation number so it is oxidized

Mg + HCl  MgCl 2 + H 2 Step 3: track the change in oxidation number for a species on either side of the equation (+2)------>+2 (Oxidation) Mg + HCl  MgCl 2 + H (-1) >0 (reduction)

Mg + HCl  MgCI 2 + H 2 Step 4: Use coefficients to make the increase in oxidation number equal to the decrease (+2)------>+2 (Oxidation) Mg + HCl  MgCl 2 + H x(-1) >0 (reduction) Mg + 2HCl  MgCl 2 + H 2

For extra help Try the worked problems in the text book, p648, 649 and 651, 652