establishing standard reduction...

1 June 14 Oxidation Reduction

Establishing Standard Reduction Potential

Dr. Fred Omega Garces Chemistry 201 Miramar College

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Objective

The main objective of this experiment is to establish the reduction potentials of four metals relative to an arbitrarily chosen metal, and then arrange them into a table that has the form of a table of standard reduction potentials. The identity of an unknown metal will be determined based on its standard reduction potential.

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Oxidation-Reduction: A Reaction

Oxidation: When a substances loses electrons. Reduction: When a substance gains electrons. Consider:

Ca(s) + 2H+(aq) D Ca2+

(aq) + H2(g)

The neutral Ca(s) has lost two e- to 2 H+ to become Ca2+. We say Ca has been oxidized to Ca2+

At the same time 2 electrons are gained by 2 H+ to form H2 . We say H+ is reduced to H2 .

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Redox Reaction with Air

Consider the reaction of Ca with O2: 2Ca(s) O2(g) g 2CaO(s) Ca is easily oxidized in air. On the left we see freshly cut Ca. On the right we see Ca with a dull surface. On the surface, there is a coat of CaO.

Again, Ca(s) gains electron and is oxidized to Ca+2 And the neutral O2 has gained electrons from the Ca to become

O2- in CaO. We say O2 has been reduced to O2-.

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Electron Transfer and Terminology Lose electrons: Oxidation Gain electrons: Reduction.

GER

Leo

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It Takes Two: Oxidation-Reduction In all reduction-oxidation (redox) reactions, one species is reduced at the same time as another is oxidized.

Oxidizing Agent: The species which causes oxidation is called the oxidizing agent. The substance which is oxidized loses electrons to the other. The oxidizing agent is always reduced

Reducing Agent: The species which causes reduction is called the reducing agent. The substance which is reduces gains electrons from the other. The Reducing agent is always oxidized

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Oxidation of Metals with Acids

It is common for metal to produce hydrogen gas when they react with acids. For example, the reaction between Mg and HCl:

Mg(s) + 2HCl (aq) D MgCl2(aq) + H2(g) .

In this rxn, Mg is oxidized and H in HCl is reduced.

Note the change in oxidation state for these specie: Mg0 g Mg+2 in MgCl2

& H+ in HCl g H0 in H2

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Redox Rxn with Acid It is possible for metals to be oxidized with salt:

Fe(s) + Ni(N03)2 (aq) g Fe(N03)2 (aq) + Ni (s) . Molecular Equation

The net ionic equation shows the redox chemistry well:

Fe(s) + Ni+2(aq) g Fe2+(aq) + Ni (s)

Net ionic Equation In this reaction iron has been oxidized to Fe2+ while the Ni+2 has been reduced to Ni0.

What determines whether the reaction occurs ?

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The Activity Series

Metals can be placed in order of their tendencies for losing electrons. This is called the activity series.

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Competition For e- Transfer

Consider: Na, Mg, Al, Metallic character decreases left to right. Metal tend to give up electrons.

Now consider the reaction: Na + AlCl3 g ??? (NaCl + Al)

To determine if the reaction occurs, the question is to determine which metal has a greater affinity for electrons (or which is willing to lose e- ).

Na is more willing to lose e- than Al Al is more willing to accept e- (less metallic) Conclude: The reaction occurs.

3Na + AlCl3 g 3NaCl + Al

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Reading Activity Table

A metal in the activity series can only be oxidized by a metal ion below it. In our example, Na

is oxidized by Al.

The metals at the top of the activity series are called active metals.

The metals at the bottom of the activity series are called noble metals.

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Example: Silver and Copper If we place Cu into a solution of Ag+ ions, will copper plate out of solution ? Cu(s) + 2AgNO3(aq) g ? [Cu(NO3)2(aq) + 2Ag (s)] or Cu (s) + 2Ag+ (aq) g ? [Cu2+

(aq) + 2Ag (s)]

Which metal is active? Which is noble ? g Cu g Ag

∴ Therefore, Cu 2+ ions is be formed because Cu is above Ag in the activity series.

Copper Cu g Cu2+ + 2 e-

Silver Ag g Ag+ + e- X

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Example: Redox Reaction Based on the activity series, what is the outcome of the following reaction ? b) Ag(s) + PbNO3 (aq) g ? c) Cr (s) + NiSO4 (aq) g ? e) H2 (g) + CuCl2 (aq) g ? f) Ba (s) + H2O (l) g ? b) Ag vs. Pb , Pb is more active, rxn not occurs

c) Cr vs. Ni , Cr is more active, rxn occurs Cr (s) + NiSO4 (aq) g Ni (s) + CrSO4 (aq)

d) H2 vs. Cu , H2 is more active, rxn occurs

H2 (g) + CuCl2 (aq) g 2HCl (aq) + Cu (s) e) Ba vs. H2 , Ba is more active, rxn occurs

Ba (s) + H2O (l) g H2 (g) + Ba(OH)2 (aq)

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Equipment & Chemicals

Equipment Chemicals

Filter paper 11.0 cm diameter cm x 1cm metal : Cu, Zn, Pb, Ag, & Fe

Steel Wool 1 M nitrate solutions: Cu2+, Zn2+, Pb2+, Ag+, & Fe2+

Forcept 1M NaNO3

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Procedure ..

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Data Table ..

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Data Table ..

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Data Table ..

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Summary

Redox - Oxidation/Reduction reaction Oxidation- Lose electron (LEO) Reduction- Gain electron (GER) Activity Series- Table showing elements’ relative ease

of oxidation. Active M- Metal which prefers to lose e- and

there fore prefer the oxidized form. Noble M- Metal which do not lose e- and

therefore prefers the zeroth state.