Electrochemistry Ch.19 & 20 Using chemical reactions to produce electricity

Objectives  Distinguish between oxidation and reduction  Identify the path taken by electrons in an electrochemical cell  Describe how current flows in a voltaic cell

Oxidation and Reduction  Any chemical process in which elements undergo changes in oxidation number is an oxidation-reduction reaction.  This name is often shortened to Redox reaction.  Reactions are carried out in electrochemical cells called: half cells

Reduction Reaction in which the oxidation state of an element decreases  When an atom or ion gains one or more electrons  Cu 2+ (aq) +2e -  Cu (s)  The species being reduced is called the oxidizing agent  Reduction takes place at the cathode (+):metal electrode placed in a half cell

Oxidation Reaction in which the atoms or ions increase in oxidation state  When an atom or ion losses one or more electrons.  Zn (s)  Zn e-  The species being oxidized is called the reducing agent.  Reduction takes place at the anode (-).

Redox Reactions O I L R I G XIDATIONXIDATION SO S of e - EDUCTIONEDUCTION SA I N of e -

 In a redox reaction, number of electrons lost = number of electrons gained Total Redox Reaction of Cu & Zn: Zn (s) + Cu 2 + (aq)  Zn 2+ (aq) + Cu (s)

Assigning Oxidation Numbers General Rules  Assign the most electronegative element first with the value it would have as an anion  Next, decide the oxidation number of the cation (same as usual), if an ionic compound. Or determine the o.n. of the left most element. Always leave the middle element last. The charge the atom would have in a molecule (or an ionic compound) if electrons were completely transferred

Assigning Oxidation Numbers General Rules (continued):  The oxidation numbers of all elements present should add up to zero, if a neutral compound. Or add up to the charge if an ion. Specific Rules:  Pure elements always have an oxidation number of zero.

Assigning Oxidation Numbers Specific Rules (continued):  Fluorine always is _____. It’s the most e.n. element.  Oxygen can be +2 or -2, except in perioxide compounds; Then it is -1.  Hydrogen can be +1 or -1 (only -1 with a metal)

Electron Pathway in an Electrochemical Cell

How are electrochemical processes carried out? Both oxidation and reduction must occur in an electrochemical reaction.  Half-cell: A single electrode (anode or cathode) immersed in a solution of its ions An electrochemical cell will contain 2 half-cell  Electrons can be transferred from one half-cell to the other through an external connecting wire called a circuit

Electricity is the movement of electrons along a given path. The circuit is a closed loop path, so the movement of electrons through the wire is balanced by the movement of ions in solution.

Electrochemical Cell 19.2 spontaneous redox reaction anode oxidation cathode reduction

Voltaic cells (galvanic cells) use spontaneous oxidation-reduction reactions to convert chemical energy into electrical energy. The most common application of voltaic cells is in batteries.

Batteries 19.6 Leclanché cell Dry cell Zn (s) Zn 2+ (aq) + 2e - Anode: Cathode: 2NH 4 (aq) + 2MnO 2 (s) + 2e - Mn 2 O 3 (s) + 2NH 3 (aq) + H 2 O (l) + Zn (s) + 2NH 4 (aq) + 2MnO 2 (s) Zn 2+ (aq) + 2NH 3 (aq) + H 2 O (l) + Mn 2 O 3 (s)