Oxidation and Reduction (REDOX) reactions?

1. A reaction in which electrons are transferred from

one atom to another is called an oxidation and

reduction reaction or REDOX reaction.

Rules for Assigning Oxidation Numbers

The charge the atom would have in a molecule (or an ionic compound) if electrons were completely transferred.

1. Free elements (uncombined state) have an oxidation number of zero. All Diatomics have an oxidation state of zero.

Na, Be, K, Pb, H2, O2, P4 = 0

2. In monatomic ions, the oxidation number is equal to the charge on the ion.

Li+, Li = +1; Fe3+, Fe = +3; O2-, O = -2

3. The oxidation number of oxygen is –2. Exception: In H2O2 it is –1.

4. The oxidation number of hydrogen is +1 except when it is bonded to metals in binary compounds. In these cases, its oxidation number is –1.

6. The sum of the oxidation numbers of all the atoms in a molecule or ion is equal to the charge on the molecule or ion.

5. Group IA metals are +1, IIA metals are +2 and fluorine is always –1.

HCO3-

Oxidation numbers of all the atoms in HCO3

- ?

How to write oxidation numbers using rules?:Biochemhelp

Find the oxidation state of Cr in K2Cr2O7

Element K Cr O Total Charge of the compound has to

equal ZERO

Subscript

Oxidation State of 1

atom

Sum of oxidation

states0

Oxidation-Reduction Reactions“REDOX” reactions

1. Determine the oxidation number of the boldface underlined element in the following formulas:

NaClO4 NH4+

HCO3-

Concept Check1. Give the oxidation number for the following

atoms:

Cr3+ Cr = _____ O2 = _____

Cl-1 Cl = _____ Mg = _____

2. Give the oxidation number for the following atoms:

AgNO3 N = _____

NH3 N = _____

Concept CheckGive the oxidation numbers for the atoms in

(NH4)2CrO4

NH4NO3

Redox Reactions1. OXIDATION — loss of electron(s) by a substance.

Increase in oxidation number. (Mg Mg2+ + 2e-)

2. REDUCTION — gain of electron(s) by a substance. Decrease in oxidation number. (S+ 2e- S2-)

3. OXIDIZING AGENT — electron acceptor and the substance is reduced. (Non-metals, Cl atom gains 1 e-)

4. REDUCING AGENT — electron donor and the substance is oxidized. (Metals, Na atom loses 1 e-)

How would you classify most metals and non- metals?

You can’t have one… without the other!

1. Reduction (gaining electrons) can’t happen without an oxidation to provide the electrons.

2. You can’t have 2 oxidations or 2 reductions in the same equation. Reduction has to occur when there is oxidation.

LEO the lion says GER!ose

lectrons

xidation

ain

lectrons

eduction

GER!

Another way to remember

OIL RIGOxidation is loss

(oxidation number increases)

Reduction is gain(oxidation number decreases)

LEOOXIDATION

1. Loss of e-2. Increase in

oxidation number

3. Reducing Agent

GERREDUCTION

1. Gain of e-2. Decrease or

reduction in oxidation number

3. Oxidizing agentZn + 2HCl ZnCl2 + H2

Reaction of zinc with hydrochloric acid - YouTube

Examining Redox Reactions 1. After oxidation numbers have been

assigned, it can be determined that the reaction may or may not have been redox.

2. Is this reaction a redox reaction? Explain why or why not.

NaCl + AgNO3 AgCl + NaNO3

Redox ReactionsOxidation-Reduction Basics - YouTube

4 Fe(s) + 3 O2(g) 2 Fe2O3(s)

1. Assign oxidation numbers to all atoms.

2. Identify what is oxidized and what is reduced.

3. Identify the oxidizing and reducing agent.

Concept Check1. Give the oxidation number and identify what

is oxidized, reduced, and identify the oxidizing and reducing agents.

Cu + 2HNO3 Cu(NO3)2 + H2

Oxidation and Reduction half-reactions

Half-reaction1. Shows either the oxidation or reduction part of a

redox reaction.

2. Shows the electrons gained or lost.

3. Follow the law of conservation of matter:

a) Mass of atom(s) on reactant side = mass of atom(s) on the product side.

b) Conservation of charge: net charge must be the same on both sides of the equation, but does not necessarily equal zero.

Half ReactionsOxidation half-reaction

1. Shows an atom or ion losing 1 or more electrons while its oxidation number increases.

Fe(s) Fe3+(aq) + 3e-

For oxidation, the e- is on the product side.

Reduction half-reaction

1. Shows an atom or ion gaining 1 or more electrons while its oxidation number decreases.

Fe3+(aq) + 3e- Fe(s)

For reduction, the e- is on the reactant side.

Write the Half-reaction from an equation

Cu(s) + AgNO3(aq) Cu(NO3)2(aq) + Ag(s)

Step 1: Assign an oxidation number to each element. Step 2: Write a partial half-reaction to show the change in

oxidation state. (no e- yet)

Step 3: Now show the number of e- needed to explain how the oxidation number changed.

Step 4: Keep conservation of mass and charge for ½ reactions.

Step 5: There must be a balance between the number of electrons lost and gained. Balance by multiplying through by the lowest common denominator.

Step 6: Once the e- lost and gained are equal, we can cancel the e- on both sides and add the 2 half reactions.

2Mg (s) + O2 (g) 2MgO (s)

2Mg 2Mg2+ + 4e-

O2 + 4e- 2O2-

Oxidation half-reaction (lose e-)

Reduction half-reaction (gain e-)

19.1

1. A half reaction does not occur by itself.

2. At least two such reactions must occur so that the electron released by one reactant is accepted by another in order to complete the reaction.

3. Thus, oxidation and reduction reactions must take place simultaneously in a system.

Half reactions

Zn(s) + Cu2+(aq) Zn2+

(aq) + Cu(s)

Zn(s) Zn2+(aq) + 2e- OXIDATION ½ reaction

Cu2+(aq) + 2e- Cu REDUCTION ½ reaction

Concept Check

Balance the following reactions using the half-reaction method.

Li(s) + CuSO4(aq) LiSO4(aq) + Cu(s)

Ag(s) + S(s) Ag2S(s)

AgS animation

The Galvanic or Voltaic CellZn/Cu

Cu/Ag

Cu

1.0 M CuSO4

Zn

1.0 M ZnSO4

Salt bridge – KCl in agarProvides conduction between half-cells

CellConstruction

Observe the electrodes to see what is occurring.

Cu

1.0 M CuSO4

Zn

1.0 M ZnSO4

Cu plates out or deposits on electrode

Zn electrode erodes or dissolves

cathode half-cellCu+2 + 2e- Cu

anode half-cellZn Zn+2 + 2e-

-+

What about half-cell reactions?

What about the sign of the electrodes?

What happened at each electrode?

Why?

Galvanic cell

• cathode half-cell (+)REDUCTION Cu+2 + 2e- Cu

• anode half-cell (-)OXIDATION Zn Zn+2 + 2e-

• overall cell reactionZn + Cu+2 Zn+2 + Cu

Spontaneous reaction that produces electrical current!

Now for a standard cell composed of Cu/Cu+2 and Zn/Zn+2, what is the voltage produced by the reaction at 25oC?

Standard ConditionsTemperature - 25oCAll solutions – 1.00 MAll gases – 1.00 atm

Cu

1.0 M CuSO4

Zn

1.0 M ZnSO4

cathode half-cellCu+2 + 2e- Cu

anode half-cellZn Zn+2 + 2e-

-+

Now replace the light bulb with a volt meter.

1.1 volts

H2 input1.00 atm

inert metal

We need a standard electrode to make measurements against!

The Standard Hydrogen Electrode (SHE)

Pt

1.00 M H+

25oC1.00 M H+

1.00 atm H2

Half-cell2H+ + 2e- H2

EoSHE = 0.0 volts

Electrolysis

Lead bromide

Electrolysis is the break-down of a substance by electricity

Electrolyte - a molten or aqueous solution through which an electrical current can flow.

Electrode - an electrical conductor which carries charge to or from a liquid undergoing electrolysis.

Electrolysis experiments

• Electrolysis only happens in: - molten ionic liquids or - aqueous solutions containing ions.

• There must be a complete circuit. • A lamp or ammeter shows that

electricity is flowing around the circuit.

Electrolysis of zinc chloride

At the electrodes

Cathode (-)(negative electrode)

• Positive ions go here (cations).

• As metal ions are positive, they go to the cathode.

• Ions gain electrons. They are reduced and become neutral atoms.

Anode (+)(positive electrode)

• Negative ions go here (anions).

• As non-metal ions are negative, they go to the anode.

• Ions lose electrons. They are oxidised and become neutral atoms(which react together to form molecules).