SABIC CHEM 101 Chapter 4 - Part 4

SABIC CHEM 101 Chapter 4 - Part 4 -...

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Unformatted text preview: Types
of
Chemical
Reac1ons 
 Most reactions that take place in aqueous solutions can be grouped into one of three types: •  Precipita*on
Reac*ons
 •  Acid–Base
Reac*ons
 •  Oxida*on–Reduc*on
Reac*ons
 Chemical Reactions The reactions we have talked about so far – acid-base reactions & precipitation reactions Both take place in aqueous solutions and the charges of the ions DO NOT CHANGE – they simply swap partners. Let’s investigate another type of reaction where this is not the case… REDOX Reactions Let’s look at chemical reactions with full electron transfers: Cu + 2 AgNO3 Cu wire Ag+ ions Cu(NO3)2 + 2 Ag 2+ ions + Ag metal Cu Redox reactions are characterized by ELECTRON TRANSFER between substances (the actual charge or the charge distribution changes). Transfer leads to 1.  Increase in oxidation number of some element (lose electrons) = OXIDATION 2. Decrease in oxidation number of some element (gain electrons) = REDUCTION Redox
Reac1ons 
 -Often the simplest of chemical reactions, only require transfer of electrons Example: 2 Na(s) + Cl2(g) 2 NaCl(s) Redox
Reac1ons 
 -Often the simplest of chemical reactions, only require transfer of electrons Example: 2 Na(s) + Cl2(g) 2 NaCl(s) Write Half Reactions: 2 Na 2 Na+ + 2eCl2 + 2e- 2 Cl- Oxidation Reduction Na is oxidized, and it reduces Cl2 Cl2 is reduced, and it oxidizes Na Na is the reducing agent (reductant) Cl2 is the oxidizing agent (oxidant) *Sometimes it’s not obvious what’s oxidized or reduced. Then, we use the oxidation states, Zumdahl&Zumdahl Table 4.2 Oxida1on
States 
 Oxidation State provides a way to keep track of electrons in oxidation-reduction reactions. To assign oxidation numbers we treat all compounds as if they were completely ionic. There are some guidelines that help us determine the oxidation states of the atoms in our reactions. Oxida1on
State 
 1)  Any uncharged element: Oxidation number = 0 Na(S), Cl2(g), O2, and Hg(l) are all oxidation number = 0 2)  A monoatomic ion has an oxidation number equal to its charge: Cl1- Ox. No = -1 Na1+ Ox. No = +1 O2- Ox. No = -2 Ca2+ Ox. No = +2 Assigning
Oxida1on
States 
 3) Some elements because of their electronegativity USUALLY have the same oxidation number: O usually is -2 (unless combined with F) F always is -1 in compounds H usually is +1 in compounds Assigning
Oxida1on
States 
 4) For a neutral polyatomic formula the sum of the oxidation numbers = 0 CH4 C + 4H = 0 BF3 B + 3F =0 Assigning
Oxida1on
States 
 5) For a charged polyatomic ion the sum of the ox numbers equals the charge on the ion: MnO41- Cr2O72- Mn + 4 O = -1 2Cr + 7 O = -2 Assigning
Oxida1on
States 
 1.  Oxida*on
state
of
an
atom
in
an
element
=
0
 2.  Oxida*on
state
of
monatomic
ion
=
charge
of
the
 ion
 3.  Oxygen
=
-2
in
covalent
compounds
 4.  Hydrogen
=
+1
in
covalent
compounds
 5.  Fluorine
=
-1
in
compounds
 6.  Sum
of
oxida*on
states
=
0
in
compounds

 7.  Sum
of
oxida*on
states
=
charge
of
the
ion
in
ions
 Examples 
 
Find
the
oxida*on
states
for
each
of
the
 elements
in
each
of
the
following
compounds:
 •  •  •  •  •  K2Cr2O7
 CO32‐
 MnO2
 PCl5
 SF4
 What is the oxidation number of P in H3PO4? What is the oxidation number of Mn in MnO4¯? What is the oxidation number of Cr in Cr2O72-? What is the oxidation number of C in C3H8(propane)? Examples 
 Redox
Reac1ons 
 Redox
Characteris/cs
 •  Transfer
of
electrons
 •  Transfer
may
occur
to
form
ions
 •  Oxida*on
–
increase
in
oxida*on
state
(loss
 of
electrons);
reducing
agent
 •  Reduc*on
–
decrease
in
oxida*on
state
(gain
 of
electrons);
oxidizing
agent
 Corrosion of aluminum 0 +3 2 Al(s) + 3 Cu2+(aq) 2 Al3+(aq) + 3 Cu(s) +2 0 UA GenChem Examples 
 Which
of
the
following
are
oxida*on‐reduc*on
 reac*ons?

Iden*fy
the
oxidizing
agent
and
the
 reducing
agent.
 a) Zn(s)
+
2HCl(aq)
→
ZnCl2(aq)
+
H2(g)
 b) Cr2O72‐(aq)
+
2OH‐(aq)
→
2CrO42‐(aq)
+
H2O(l)
 c) 2CuCl(aq)
→
CuCl2(aq)
+
Cu(s)
 Examples 
 Balancing
Oxida1on‐Reduc1on
Equa1ons 
 In an oxidation-reduction reaction the number of electrons gained and lost must be equal. We there for need to be able to balance redox equations 1.  Write
the
unbalanced
equa*on.
 2.  Determine
the
oxida*on
states
of
all
atoms
in
the
 reactants
and
products.
 3.  Show
electrons
gained
and
lost
using
“*e
lines.”
 4.  Use
coefficients
to
equalize
the
electrons
gained
 and
lost.
 5.  Balance
the
rest
of
the
equa*on
by
inspec*on.
 6.  Add
appropriate
states.
 Balancing
Oxida1on‐Reduc1on
Equa1ons 
 •  Balance
the
reac*on
between
solid
zinc
and
 aqueous
hydrochloric
acid
to
produce
 aqueous
zinc(II)
chloride
and
hydrogen
gas.
 1.
What
is
the
unbalanced
equa/on?

 •  Zn(s)
+
HCl(aq)
→
Zn2+(aq)
+
Cl–(aq)
+
H2(g)

 2.
What
are
the
oxida/on
states
for
each
atom?
 •  Zn(s)
+
HCl(aq)
→
Zn2+(aq)
+
Cl–(aq)
+
H2(g)
 






0







+1
–1












+2













–1













0
 Balancing
Oxida1on‐Reduc1on
Equa1ons 
 3.
How
are
electrons
gained
and
lost?
 
 

 
 
1
e–
gained
(each
atom)
 •  Zn(s)
+
HCl(aq)
→
Zn2+(aq)
+
Cl–(aq)
+
H2(g)
 






0







+1
–1












+2













–1













0
 






















2
e–
lost
 •  The
oxida*on
state
of
chlorine
remains
unchanged.
 Balancing
Oxida1on‐Reduc1on
Equa1ons 
 4.
What
coefficients
are
needed
to
equalize
the
electrons
 gained
and
lost?
 
 

 
 
1
e–
gained
(each
atom)

×
2
 •  Zn(s)
+
HCl(aq)
→
Zn2+(aq)
+
Cl–(aq)
+
H2(g)
 






0







+1
–1












+2













–1













0
 






















2
e–
lost
 •  Zn(s)
+
2HCl(aq)
→
Zn2+(aq)
+
Cl–(aq)
+
H2(g)
 Balancing
Oxida1on‐Reduc1on
Equa1ons 
 5.
What
coefficients
are
needed
to
balance
the
remaining
 elements?

 •  Zn(s)
+
2HCl(aq)
→
Zn2+(aq)
+
2Cl–(aq)
+
H2(g)

 Examples 
 Classify the following reactions as acid-base, precipitation, redox and/or combustion reactions: FeCl2(aq) + Na2S (aq) FeS(s) + 2NaCl(aq) 2Na(s) + 2H2O(l) 2NaOH(aq) + H2(g) HNO3(aq) + LiOH(aq) LiNO3(aq) + H2O(l) 2C2H6(s) + 7O2(g) 6H2O(l) + 4CO2(g) UA GenChem FeCl2(aq) + Na2S (aq) FeS(s) + 2NaCl(aq) Precipitation reaction 2Na(s) + 2H2O(l) 2NaOH(aq) + H2(g) Redox reaction HNO3(aq) + LiOH(aq) LiNO3(aq) + H2O(l) Acid-base reaction 2C2H6(s) + 7O2(g) 6H2O(l) + 4CO2(g) Combustion and redox reaction UA GenChem Homework 
 Do the following Chapter 4 problems: 79, 83, 87 ...
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