Transcript electrochem ppt Notes

Electrochemistry
Oxidation-reduction reactions
Review
Ionic bonds : give and take of electrons
(example NaCl)
 Covalent bonds: electrons shared
(example CO2)
 All elements in the molecule become
STABLE (like a noble gas in electron
configuration) and NEUTRAL (balanced in
charge)

Oxidation Numbers
We can assign a specific charge to
elements involved in bonding
 For simplicity, we assign full charges to
elements even if they are sharing
(example H20…H is considered to be 1+
and O as 2-)
 (H 1+) x 2 + O 2- = H20
 (Oxygen has a tighter hold on the
electrons therefore is “negative”)

Building molecules
We can use oxidation numbers to predict
the ratios of elements or elements with
polyatomic negative ions in a stable and
neutral molecule.
 Example Pb(SO4)
 (SO 4) has a 2- charge ,therefore Pb must
be 2+
 Name = Lead II Sulfate
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Determining Oxidation Numbers
What about SO4 ?
 Oxygen is given oxidation number 2 4 oxygen atoms = 8- charge!
 What must S’s charge be so that SO4 has
a total net charge of 2-?
 S must have a 6+ charge 
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Practice!
Determine the oxidation numbers of each
element in the following examples:
 HCl
 PbO
 Fe2O3
 CuNO3
 NaOH
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OXIDATION
Process whereby an element becomes
MORE positive by losing electrons
 REMEMBER that there are protons in the
nucleus that are now unequal to the
number of electrons

REDUCTION
Process whereby elements become
MORE negative by gaining electrons
 Element now has MORE electrons than it
has positive protons in the nucleus and
has a net charge that is negative.

Oxidizing agent
This would be any element or molecule
that TAKES electrons allowing another
element to be oxidized (become more
positive by losing electrons)
 The oxidizing agent is reduced in the
process

Reducing agent
This would be any element or molecule
that GIVES electrons to another element
allowing it to be reduced- becoming more
negative by gaining electrons
 The reducing agent is oxidized in the
process

Example
Sodium reacts with carbon to produce
sodium carbide
 Identify the oxidation number of each
element
 Determine if any of the elements changed
oxidation number
 Determine who is acting as the agents of
change!

Writing half reactions
Once we identify which elements are
being oxidized and reduced, we can write
out “half reactions” for the transfer of
electrons involved
 Eg:
Na + C
Na4C
 Sodium is losing an electron= oxidized
 Carbon is gaining electrons = reduced

Na0
Na1+ + 1 e- (oxidized)
 C0 + 4 eC4- (reduced)
 We need to make sure that the number of
electrons given and taken is EQUAL!
 Therefore we will need 4 sodium atoms to
donate all of the electrons needed for
carbon
 This is often a good way to help balance
difficult reactions.

Try this!

Magnesium solid reacts with oxygen gas to
produce magnesium oxide
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STEP #1 -write a chemical equation
STEP #2 -assign oxidation numbers
STEP #3 -write “half reactions”
STEP #4 -label oxidation/reduction
STEP #5 -balance electrons
STEP #6 -balance the equation!
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Try these for homework!

Fe + CuS04
Cu + FeS04

H2 + 02
H20

Cu + AgN03
Cu(N03)2 + Ag

HNO3 + H2S
S + NO + H2O