Redox Reactions

Glenn V. LoDepartment of Physical SciencesNicholls State University

What is a redox reaction?

Redox is short for Reduction-Oxidation.Reduction: gain of electronsOxidation: loss of electronsA redox reaction is an “electron transfer” reaction

Reduction or Oxidation?

State if the following is a reduction or oxidation? F2 molecule becomes two fluoride ions O + 2e- O2-

Na Na+ + e-

Mg atom becomes Mg2+ ion

Oxidation: more positive chargeReduction: more negative charge

Half or Full?

Reduction and oxidation are “half reactions.” If something is reduced, something else is oxidized.Chemical equation for a redox reaction should reflect the fact that the following are equal: Number of electrons lost in oxidation half Number of electrons gained in reduction half

Why is the following considered unbalanced? Ag+ + Cu Ag + Cu2+

Recognizing Redox Reactions

You should be able to recognize reactions involving an elemental substance as redox reactionsNot all redox reactions involve an elemental form of a substance

Combination Reactions

Reactions involving an elemental substance are redox reactionsExample: combination of two elements to form a compound 2 Na + Cl2 2 NaCl Na is oxidized to Na+ ion Cl2 is reduced to two Cl- ions

Displacement Reactions

Reactions involving an elemental substance are redox reactionsExample: displacement of an element in one compound by another 2 Na + 2 HOH 2 NaOH + H2

Na is oxidized to Na+ ion What’s reduced?

Also called “single replacement”Application: extraction of precious metals

Combustion Reactions

Reactions involving an elemental substance are redox reactionsExample: combustion reactions (reaction with O2, leading to formation of oxides) Mg + 2 O2 2 MgO (also

combination) CH4 + 2 O2 2 CO2 + 2 H2O

Electron transfer?

How could there be electron transfer in CH4 + 2 O2 2 CO2 + 2 H2O

In molecules, atoms are sharing electrons.However, sharing is generally not “equal”. In CO2, electrons shared between C and O are “polarized towards O.” In O2, electrons are equal shared (why?)From O2 to CO2, O atoms may be thought of as having “gained electrons”

Decomposition

Decomposition reactions involve the breakdown of ONE reactant into two or more products.If one of the decomposition products is an element, the reaction is redox. Ex. 2 H2O 2 H2 + O2

Non-redox reactions

Double replacement reactionsPrecipitation AgNO3 + HCl AgCl + HNO3

Ag+ + Cl- AgCl

Acid-Base Neutralization HCl + NaOH NaCl + H2O H+ + OH- H2O

Non-redox reactions

Combination of two compounds to form another compound CaO + H2O Ca(OH)2

Decomposition, when none of the products is an elemental substance. CaCO3 CaO + CO2