redox reactions
DESCRIPTION
Redox Reactions. Chapter 18. + O 2 . Oxidation-Reduction (Redox) Reactions. “redox” reactions: rxns in which electrons are transferred from one species to another oxidation & reduction always occur simultaneously we use OXIDATION NUMBERS to keep track of electron transfers. - PowerPoint PPT PresentationTRANSCRIPT
Redox Reactions
Chapter 18
+ O2
Oxidation-Reduction (Redox) Reactions
“redox” reactions: rxns in which electrons are transferred from one species to another
oxidation & reduction always occur simultaneously
we use OXIDATION NUMBERS to keep track of electron transfers
Rules for Assigning Oxidation Numbers:
1) the ox. state of any free (uncombined) element is zero. Ex: Na, S, O2, H2, Cl2, O3
Rules for Assigning Oxidation Numbers:
2) The ox. state of an element in a simple ion is the charge of the ion.
Mg2+ oxidation of Mg is +2
Rules for Assigning Oxidation Numbers:
3) the ox. # for hydrogen is +1
(unless combined with a metal, then it has an ox. # of –1)
Ex: NaOH (H bonded to O) v. NaH (H bonded to Na)
H = +1 H = -1
Rules for Assigning Oxidation Numbers:
4) the ox. # of fluorine is always –1.
Rules for Assigning Oxidation Numbers:
5) the ox. # of oxygen is usually –2.
Why USUALLY? Not -2 when it’s in a peroxide, such as hydrogen peroxide:
H2O2
Rules for Assigning Oxidation Numbers:
6) in any neutral compound, the sum of the oxidation #’s = zero.
Rules for Assigning Oxidation Numbers:
7) in a polyatomic ion, the sum of the oxidation #’s = the overall charge of the ion.
Rules for Assigning Oxidation Numbers:
**use these rules to assign oxidation #’s; assign known #’s first, then fill in the #’s for the remaining elements:
Examples: Assign oxidation #’s to each element:
a) NaNO3 Na = +1O = -2
Therefore, N = +5
Examples: Assign oxidation #’s to each element:
b) SO32-
O = -2, therefore S must = +4 to balance the charges and have an overall charge of 2-
-2+4
Examples: Assign oxidation #’s to each element:
c) HCO3-
-2+1 +4
Examples: Assign oxidation #’s to each element:
Do on your own:
d) H3PO4
e) Cr2O72-
f) K2Sn(OH)6
Definitions
Oxidation: the process of losing electrons (ox # increases)
Reduction: the process of gaining electrons (ox # decreases)
Oxidizing agents: species that cause oxidation (they are reduced)
Reducing agents: species that cause reduction (they are oxidized)
To help you remember…
OIL RIG Oxidation Is Loss Reduction Is Gain
Are all rxns REDOX rxns?You must determine this…
a reaction is “redox” if a change in oxidation # happens; if no change in oxidation # occurs, the reaction is nonredox.
Examples
MgCO3 MgO + CO2
+2 -2
MgCO3 is an ionic compound, so what is Mg’s charge in an ionic compound?
The carbonate ion CO32- is
the other ion, let’s figure out C because we already know O.
+4-2+2 -2+4
Is this a redox or nonredox reaction?
NONREDOX (no change in oxidation numbers)
Examples
Zn + CuSO4 ZnSO4 + Cu
Which oxidation numbers do we already know?
0, free element
0Break down this ionic compound into its ions
Cu and SO42-
So, Cu must be Cu2+
+2
O = -2 in SO42-, so S must
be…?
-2+6-2+2 +6
Is this a redox or nonredox reaction?
REDOX reaction
Examples
NaCl + AgNO3 AgCl + NaNO3
Redox or nonredox?
Examples
CO2 + H2O C6H12O6 + O2
-2+4 -2+1 0-2+10
What happened to Carbon? It went from +4 oxidation # to 0.
Was Carbon oxidized or reduced?
REDUCED
OIL RIG (oxidation is losing electrons so oxidation number increases, where as reduction is gaining electrons so oxidation number decreases)