A reducing agent (also called a reductant or reducer) is the element or a compound in a
redox(reduction-oxidation) reaction (see electrochemistry) that reduces another species. In doing so, it becomes "oxidized", and is therefore the electron donorin the redox. For example consider the following reaction:
: [Fe(CN)6] 4- + 1/2 Cl2 → [Fe(CN)6] 3- + Cl-The reducing agent in this reaction is
ferrocyanide: it donates an electron, converting to ferricyanide, simultaneous with the reduction of chlorineto chloride.
organic chemistry, reduction refers to the addition of hydrogen to a molecule. For example, benzeneis reduced to cyclohexanein the presence of a platinum catalyst::C6H6 + 3 H2 → C6H12In organic chemistry, good reducing agents are reagents that deliver H2.
What makes a strong reducing agent?
Strong reducing agents easily lose (or donate) electrons. Atoms with relatively large atomic radii tend to be better reductants. In such species, the distance from the nucleus to the
valence electrons is so long that these electrons are not strongly attracted. These elements tend to be strong reducing agents. Good reducing agents tend to consist of atoms with a low electronegativity, the ability of an atom or molecule to attract bonding electrons1, and relatively small ionization energies serve as good reducing agents too. "The measure of a material to oxidize or lose electrons is known as its oxidation potential"2. The table below shows a few reduction potentials that could easily be changed to oxidation potential by simply changing the sign. Reducing agents can be ranked by increasing strength by ranking their oxidation potentials. The reducing agent will be the strongest when it has a more positive oxidation potential and will be a weak reducing agent whenever it has a negative oxidation potential. The following table provides the reduction potentials of the indicated reducing agent at 25° C. Also remember the useful mnemonic devices, "OIL RIG," which means Oxidation Is Loss (of electrons) and Reduction Is Gain (of electrons), or "LEO the lion says GER," which means Loss of Electrons is Oxidation and Gain of Electrons is Reduction. Oxidizing Agent Reducing Agent Reduction Potential (v) Li+ + e- = Li -3.04 Na+ + e- = Na -2.71 Mg2+ + 2e- = Mg -2.38 Al3+ + 3e- = Al -1.66 2H2O(l) + 2e- = H2(g) + 2OH - -0.83 Cr3+ + 3e- = Cr -0.74 Fe2+ + 2e- = Fe -0.41 2H+ + e- = H2 0.00 Sn4+ + 2e- = Sn2+ +0.15 Cu2+ + e- = Cu+ +0.16 Ag+ + e- = Ag +0.80 Br2 + 2e- = 2Br- +1.07 Cl2 + 2e- = 2Cl- +1.36 MnO42- + 8H+ + 5e- = Mn2+ + 4H2O +1.49
In order to tell which is the strongest reducing agent, change the sign of its respective reduction potential in order to make it oxidation potential. The bigger the number the stronger a reducing agent it is.
For example if one were to list Cu, Cl-, Na and Cr in order, one would get their reduction potential, change the sign to make it oxidation potential and list them from greatest to least. One will get Na, Cr, Cu and Cl-; Na being the strongest reducing agent and Cl- being the weakest one.
A few good common reducing agents include active metals such as potassium, calcium, barium, sodium and magnesium and also, compounds that contain the H- ion, those being NaH, LiAlH4 and CaH2.
Also, some elements and compounds can be both reducing or
oxidizing agents. Hydrogen gas is a reducing agent when it reacts with non-metals and an oxidizing agent when it reacts with metals.
2Li(s) + H2(g) -->2LiH(s) hydrogen acts as an oxidizing agent because it accepts an electron donation from lithium, which causes Li to be oxidized.
Half Reactions 2Li(s)0 -->2Li(s)+1 + 2e-::::: H20(g) + 2e- --> 2H-1(g)
H2(g) + F2(g) --> 2HF(g) hydrogen acts as a reducing agent because it donates its electrons to fluorine, which allows fluorine to be reduced.
Half Reactions H20(g) --> 2H+1(g) + 2e-:::::F20(g) + 2e- --> 2F-1(g)
Importance of reducing and oxidizing agents
Reducing agents and oxidizing agents are the ones responsible for
corrosion, which is the “degradation of metals as a result of electrochemical activity”3. Corrosion requires an anodeand cathodeto take place. The anode is an element that loses electrons (reducing agent), thus oxidation always occurs in the anode, and the cathode is an element that gains electrons (oxidizing agent), thus reduction always occurs in the cathode. Corrosion occurs whenever there’s a difference in oxidation potential. When this is present, the anode metal will begin deteriorating given that there is an electrical connection and the presence of an electrolyte.
Example of redox reaction
The formation of iron(III) oxide;:4Fe + 3O2 → 2Fe2O3
In the above equation, the
Iron(Fe) has an oxidation number of 0 before and 3+ after the reaction. For oxygen(O) the oxidation number began as 0 and decreased to 2−. These changes can be viewed as two " half-reactions" that occur concurrently:
#Oxidation Half Reaction: Fe0 → Fe3+ + 3e−
#Reduction Half Reaction: O2 + 4e− → 2 O2−
Iron III (Fe) has been oxidized because the oxidation number increased and is the reducing agent because it gave electrons to the oxygen (O).Oxygen (O) has been reduced because the oxidation number has decreased and is the oxidizing agent because it took electrons from iron (Fe)
Common reducing agents
Lithium aluminium hydride(LiAlH4)
Hydrazine( Wolff-Kishner reduction)
Zinc-mercury amalgam(Zn(Hg)) ( Clemmensen reduction)
Common reducing agents and their products
Agent Product H2 Hydrogen H+, H2O metals metal ions C CO2 carbon dioxide hydrocarbons CO2 carbon dioxide, H2O
* [http://www.usm.maine.edu/~newton/Chy251_253/Lectures/OxidtionReduction/OxiationReduction.html Table summarizing strength of reducing agents]
*"Chemical Principles: The Quest for Insight", Third Edition. Peter Atkins and Loretta Jones pg. F76
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Look at other dictionaries:
Reducing agent — Re*du cing a gent, n. (Chem.) a substance that causes reduction of another substance in a chemical reaction, as by donating electrons or adding hydrogen atoms; as, lithium hydride is a powerful reducing agent. [PJC] … The Collaborative International Dictionary of English
reducing agent — n a substance that reduces esp a substance (as hydrogen, sodium, or hydroquinone) that donates electrons or a share in its electrons to another substance called also reductant compare OXIDIZING AGENT * * * a substance capable of donating… … Medical dictionary
Reducing agent — Reducing agent. См. раскислитель. (Источник: «Металлы и сплавы. Справочник.» Под редакцией Ю.П. Солнцева; НПО Профессионал , НПО Мир и семья ; Санкт Петербург, 2003 г.) … Словарь металлургических терминов
reducing agent — n. any substance that reduces another substance, or brings about reduction, and is itself oxidized in the process … English World dictionary
reducing agent — reduktorius statusas T sritis automatika atitikmenys: angl. reducer; reducing agent vok. Reduktionsmittel, n; Reduktor, m rus. редуктор, m pranc. détendeur, m; réducteur, m … Automatikos terminų žodynas
reducing agent — reduktorius statusas T sritis chemija apibrėžtis Dalelė, atiduodanti elektronus. atitikmenys: angl. reducer; reducing agent; reductant rus. восстановитель … Chemijos terminų aiškinamasis žodynas
reducing agent — reduktorius statusas T sritis chemija apibrėžtis Redukuojančioji medžiaga. atitikmenys: angl. reducer; reducing agent; reductant rus. восстановитель … Chemijos terminų aiškinamasis žodynas
reducing agent — reduktorius statusas T sritis chemija apibrėžtis Įtaisas dujų arba skysčių slėgiui reguliuoti ir palaikyti. atitikmenys: angl. reducer; reducing agent; reductant rus. редуктор … Chemijos terminų aiškinamasis žodynas
reducing agent — noun a substance capable of bringing about the reduction of another substance as it itself is oxidized; used in photography to lessen the density of a negative or print by oxidizing some of the loose silver • Syn: ↑reducer, ↑reductant •… … Useful english dictionary
reducing agent — noun Date: circa 1817 a substance that reduces a chemical compound usually by donating electrons … New Collegiate Dictionary