Redox Reactions Glenn V. Lo Department of Physical Sciences Nicholls State University

What is a redox reaction? Redox is short for Reduction-Oxidation. Reduction: gain of electrons Oxidation: loss of electrons A redox reaction is an “electron transfer” reaction

Reduction or Oxidation? State if the following is a reduction or oxidation? F 2 molecule becomes two fluoride ions O + 2e -  O 2- Na  Na + + e - Mg atom becomes Mg 2+ ion Oxidation: more positive charge Reduction: 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 + Cu 2+

Recognizing Redox Reactions You should be able to recognize reactions involving an elemental substance as redox reactions Not all redox reactions involve an elemental form of a substance

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

Displacement Reactions Reactions involving an elemental substance are redox reactions Example: displacement of an element in one compound by another 2 Na + 2 HOH  2 NaOH + H 2 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 reactions Example: combustion reactions (reaction with O 2, leading to formation of oxides) Mg + 2 O 2  2 MgO (also combination) CH O 2  2 CO H 2 O

Electron transfer? How could there be electron transfer in CH O 2  2 CO H 2 O In molecules, atoms are sharing electrons. However, sharing is generally not “equal”. In CO 2, electrons shared between C and O are “polarized towards O.” In O 2, electrons are equal shared (why?) From O 2 to CO 2, 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 H 2 O  2 H 2 + O 2

Non-redox reactions Double replacement reactions Precipitation AgNO 3 + HCl  AgCl + HNO 3 Ag + + Cl -  AgCl Acid-Base Neutralization HCl + NaOH  NaCl + H 2 O H + + OH -  H 2 O

Non-redox reactions Combination of two compounds to form another compound CaO + H 2 O  Ca(OH) 2 Decomposition, when none of the products is an elemental substance. CaCO 3  CaO + CO 2