Electrochemistry

Redox ReactionsJune 13th 2012

Mr. Dvorsky

• Oxidation and Reduction are historical terms• Oxidation reactions don’t necessarily involve

oxygen• Reduction reactions don’t involve making things

smaller

Oxidation: The Loss of Electrons

• Chemists originally defined oxidation as any chemical reaction in which an atom or a compound reacted with molecular oxygen.

• As chemists made more observations and analyzed more data, they began to see similarities between reactions of atoms and compounds with oxygen and reactions of the same atoms and compounds with elements other than oxygen.

Oxidation: The Loss of Electrons

Consider the reaction between magnesium and oxygen:2Mg(s) + O2(g) 2MgO(s) -the product is magnesium oxide

-recall, magnesium oxide is an ionic compound containing Mg ions (2+) and oxide ions (2-). Magnesium must lose two electrons (become positively charged) while oxygen gains two electrons (become negatively charged).-Even in solid form, the elements exist as separate magnesium ions: 2Mg(s) + O2(g) 2Mg2+ + 2O2-(s)

Oxidation: more than just oxygen

• Now compare the reaction between magnesium and chlorine. The equation is:

• Mg(s) + Cl2(g) Mg2+ + 2Cl-(s)• The product is magnesium chloride, also ionic. In the

reaction magnesium atoms must lose two electrons and become ionized while two chlorine atoms must gain one electron each and become negatively charged ions

• When solid magnesium reacts with oxygen or chlorine, it loses two electrons and becomes a magnesium ion

• This shows the modern definition of oxidation which means the loss of electrons.

Reduction: The Gain of Electrons

• When metal is extracted from raw ore, the mass of metal is much smaller than ore

• The process of extracting metal was called reduction (red. In amount of mass)

• The chemical rxnsInvolved in removingmetals were called reductions.

Reduction: The Gain of Electrons

• If one atom or ion loses electrons in a reaction, then another atom or ion must gain electrons

• Extracting iron from iron oxide ore involves a reaction in which Fe gains electrons –since it was part of the “reduction” process, historically it was given the name reduction reaction.

• The modern definition now applies to all cases where an atom or ions gains electrons in a reaction.

LEO the lion says GER

Loss of electrons is oxidation

Gain of electrons is reduction

Redox Reactions

• If one atom or ion is oxidized in a reaction, than another atom or ion is reduced.

• oxidation-reduction reactions are those in which electrons are gained by one atom/ion and lost by another atom/ion

• Abbreviated redox reaction

Recall from grade 10 science…

• You learned about different types of reactions, e.g. neutralization, single and double displacement, etc.

• Redox reactions are not a separate type but fit into several categories.

• The examples we looked are earlier were synthesis reactions. –since decomp. Is the reverse, redox can be those as well.

-Single displacement are always redox. -The reactants and products in such a reaction are often visibly different. -Consider a zinc strip placed in copper (II) sulfate solution -The single displacement rxn taking place:Zn(s) + CuSO4 Cu(s) + ZnSO4(aq)

• We can write that equation as an ionic equation (an equation in which all soluble ionic compounds are written as ions)

Zn(s)+ Cu2+(aq) + SO42-(aq) Cu(s) + Zn2+(aq)+ SO4

2-(aq)

The sulfate ions are spectator ions, meaning that they are ions not involved in chemical reaction. By omitting the spectator ions, you obtain the following net ionic equation:Zn(s)+ Cu2+(aq) Cu(s) + Zn2+(aq)

• This equation shows us how to track the electrons

• In the reaction of zinc atoms with copper (II) ions, the zinc atoms lose electrons and undergo oxidation – the zinc atoms are oxidized.

• The copper (II) ions gain electrons and undergo reduction-the copper(II) ions are reduced.

Hooray, more terms!

• Since electrons are transferred from zinc atoms to copper (II) ions, the copper (II) ions are responsible for the oxidation of the zinc atoms – they are the oxidizing agent.

• The zinc atoms are responsible for the reduction of copper (II) ions – they are the reducing agent.

Summary

A redox reaction can also be defined as a reaction between an oxidizing agent and a reducing agent

The Spontaneity of Redox Reactions

• The reaction with zinc and copper (II) sulfate we just examined did proceed spontaneously in the forward reaction but not reverse.

• Copper atoms cannot reduce zinc ions. Zinc atoms are a stronger reducing agent

Oxidizing Agent Reducing Agent

Stronger oxidizing agent Weaker reducing agent

Cu2+ Cu

Zn2+ Zn

Weaker reducing agent Stronger reducing agent

Predicting spontaneity

• Metal ions can act as oxidizing agents because they can remove electrons from certain metal atoms.

• Metal atoms can act as reducing agents because they can donate electrons to certain metal ions.

• Through experimentation, chemists have determined the relative strengths of many ions as oxidizing agents and atoms as reducing agents.

Predicting spontaneity

• You can use the table to predict whether ions of one element and atoms of another element will spontaneously undergo a redox reaction.

• A stronger reducing agent loses electrons more readily than a weaker reducing agent does.

• A stronger oxidizing agent gains electrons more readily than a weaker oxidizing agent does.

The process for predicting spontaneity of redox reactions: see on board