Transcript electrochem1 (2)

Chapter 20
Electrochemistry
ADAPTED FROM
Chemistry, The Central Science, 10th edition
Theodore L. Brown; H. Eugene LeMay, Jr.; and Bruce E. Bursten
Oxidation-Reduction
Reactions
• Definitions:
• An oxidation occurs
when an atom or ion
loses electrons.
• A reduction occurs
when an atom or ion
gains electrons.
Oxidation-Reduction
Reactions
One cannot occur
without the other.
A point to emphasise
Electrochemical Reactions
In electrochemical reactions, electrons are
transferred from one species to another.
Keeping track of the transferred
electrons!
In order to keep track
of what loses
electrons and what
gains them, we
assign oxidation
numbers (or is it
oxidation states?).
Oxidation numbers vs states
• A measure of the degree of oxidation of an
atom in a substance (OS)
• The oxidation number is the charge the
central metal atom ( in coordination
compounds) would have if all the ligands
were removed along with the electron
pairs that were shared with the central
atom
Oxidation and Reduction
• A species is oxidized when it loses electrons.
– Here, zinc loses two electrons to go from neutral zinc
metal to the Zn2+ ion.
Oxidation and Reduction
• A species is reduced when it gains electrons.
– Here, each of the H+ gains an electron and they
combine to form H2.
Oxidation and Reduction
• What is reduced is the oxidizing agent.
– H+ oxidizes Zn by taking electrons from it.
• What is oxidized is the reducing agent.
– Zn reduces H+ by giving it electrons.
Assigning Oxidation Numbers
1. Elements in their elemental form have an
oxidation number of 0.
2. The oxidation number of a monatomic
ion is the same as its charge.
Assigning Oxidation Numbers
3. Nonmetals tend to have negative
oxidation numbers, although some are
positive in certain compounds or ions.
– Oxygen has an oxidation number of −2,
except in the peroxide ion in which it has
an oxidation number of −1.
– Hydrogen is −1 when bonded to a metal,
+1 when bonded to a nonmetal.
Assigning Oxidation Numbers
3. Nonmetals tend to have negative
oxidation numbers, although some are
positive in certain compounds or ions.
– Fluorine always has an oxidation number
of −1.
– The other halogens have an oxidation
number of −1 when they are negative;
they can have positive oxidation
numbers, however, most notably in
oxyanions.
Assigning Oxidation Numbers
4. The sum of the oxidation numbers in a
neutral compound is 0.
5. The sum of the oxidation numbers in a
polyatomic ion is the charge on the
ion.
SAMPLE EXERCISE 20.1 What Chemical Reactions Occur in a Battery?
The nickel-cadmium (nicad) battery, a rechargeable “dry cell” used in battery-operated devices, uses the
following redox reaction to generate electricity:
Identify the substances that are oxidized and reduced, and indicate which are oxidizing agents and which are
reducing agents.
Solution
Analyze: We are given a redox equation and asked to identify the substance oxidized and the substance
reduced and to label one as the oxidizing agent and the other as the reducing agent.
Plan: First, we assign oxidation states to all the atoms in the reaction and determine the elements that are
changing oxidation state. Second, we apply the definitions of oxidation and reduction.
Solve:
Cd increases in oxidation state from 0 to +2 and Ni decreases from +4 to +2. Because the Cd atom increases in
oxidation state, it is oxidized (loses electrons) and therefore serves as the reducing agent. The Ni atom decreases
in oxidation state as NiO2 is converted into Ni(OH)2. Thus, NiO2 is reduced (gains electrons) and therefore
serves as the oxidizing agent.
Comment: A common mnemonic for remembering oxidation and reduction is “LEO the lion says GER”:
losing electrons is oxidation; gaining electrons is reduction.
SAMPLE EXERCISE 20.1 continued
PRACTICE EXERCISE
Identify the oxidizing and reducing agents in the oxidation-reduction reaction
Answer: Al(s) is the reducing agent; MnO4–(aq) is the oxidizing agent.
Keeping track of the transferred
electrons!
• In order to keep track of the exact
numbers of tranferred electrons, the
reaction equation must be balanced
• There mainly two ways of doing this – by
using Oxidation states or by using the ionelectron/ half rection method
Balancing Oxidation-Reduction
Equations
Perhaps the easiest way to balance the
equation of an oxidation-reduction
reaction is via the half-reaction method.
Balancing Oxidation-Reduction
Equations
This involves treating (the purpose is to
simplify the process only!) the oxidation
and reduction as two separate processes,
balancing these half reactions, and then
combining them to attain the balanced
equation for the overall reaction.