Chemistry 30 Unit 7 Electrochemistry Chapter 13

Section 13.1, p. 558 - 567 Electrochemistry is the study of how electrons are transferred from one chemical species to another. Electrochemical chemical reactions involve a loss of electrons by one reactant and a gain of electrons by another reactant.

Electrochemical reactions may be either:   a) Exothermic Reactants  Products + electrons Example: corrosion of metals batteries b) Endothermic Reactants + electrons  Products Example: producing metals from ores charging batteries

What happens when a strip of zinc metal is placed in a solution of copper (II) nitrate? Non-ionic Equation:   Zn(s) + Cu(NO3)2(aq)  Cu(s) + Zn(NO3)2(aq) Total Ionic Equation: Zn(s) + Cu2+(aq) + 2 NO3–(aq)  Cu(s) + Zn2+(aq) + 2 NO3–(aq)

Zn(s) + Cu2+(aq)  Cu(s) + Zn2+(aq) Net Ionic Equation: Zn(s) + Cu2+(aq)  Cu(s) + Zn2+(aq) We look to the total ionic equation to visualize what species are present for the reaction. Zn(s) + Cu2+(aq) + 2 NO3–(aq) Cu(s) + Zn2+(aq)+2 NO3– (aq) Zn metal Cu metal being coated Cu2+(aq) and NO3–(aq) ions Zn2+(aq) and NO3–(aq) ions

Which species does not take part in the chemical reaction? NO3–(aq)   Therefore, the only reaction that exists is: Zn(s) + Cu2+(aq)  Cu(s) + Zn2+(aq)

Writing Half-Reactions Given: Zn(s) + Cu2+(aq)  Cu(s) + Zn2+(aq) What happens to the zinc metal? Zn(s)  Zn2+(aq) + 2 e – What happens to the copper (II) ion?  Cu2+(aq) + 2 e –  Cu(s)

For any metallic atom to become an ion, it must lose electrons For any metallic atom to become an ion, it must lose electrons. For any metallic ion to become a metal, it must gain electrons. Each of the reactions above is a half reaction, which shows either a loss of electrons or a gain of electrons. A half reaction that shows a loss of electrons is called an OXIDATION half reaction. (L.E.O.) A half reaction that shows a gain of electrons is called a REDUCTION half reaction. (G.E.R.)  

OIL RIG “Oxidation Is Loss” “Reduction Is Gain” LEO the lion says GER “Loss of Electrons is Oxidation” “Gain of Electrons is Reduction”

The species that loses electrons in an oxidation half reaction is said to be OXIDIZED (LEO) and is also called the REDUCING AGENT. The species that gains electrons in a reduction half reaction is said to be REDUCED (GER) and is also called the OXIDIZING AGENT.

Writing Complex Half-Reactions Evidence shows that water molecules, hydrogen ions, and hydroxide ions play an important part in the half-reactions. Therefore, when writing half-reactions, water molecules, hydrogen ions or hydroxide ions may have to be included. For reactions occurring in an acidic solution, you will add a water and a hydrogen ion to write a half reaction. For reactions occurring in a basic solution, you will add a hydroxide ion as well. Follow the examples and notes below:

Summary of Writing Half-Reaction Equations: For acidic solutions: Write the chemical formulas for the reactants and products. Balance all atoms, other than O and H. Balance O by adding H2O(l). Balance H by adding H+(aq). Balance the charge on each side by adding e- and cancel anything that is the same on both sides. Balancing with basic solutions are NOT part of the curriculum! So don’t worry about it! 

Homework Practice # 7 – 12, p. 564 – 566 Section Questions # 1 – 7, p. 567