Title: Chem 150 Unit 4 - Chemical Properties I Chemical Reactions
1Chem 150Unit 4 - Chemical Properties IChemical
Reactions
- The chemical properties of molecules describe
processes that involve the making and breaking
the stronger covalent bonds that hold molecules
together. As a consequence, the compositions of
the the molecules participating in a chemical
process change. These processes are called
chemical reactions and can be expressed using
balanced chemical equations.
2Introduction
- Chemical reactions involve the rearrangement of
the atoms within and between molecules that
results in the formation of new molecules. - This process involves the making and breaking of
covalent bonds. - An important concept in these processes is that
all of the atoms present before a reaction are
also present after the reaction - This a concept allows us to describe chemical
reactions using chemical equations
3Introduction
- If you need to review how to create and balance
chemical equation, take a look at Section 6.1 in
Raymond. - We will focus on some reactions that are
important in biological chemistry, including - Oxidation/Reduction reactions
- Reactions involving water
- We will also look at the different forms of free
energy that can be used to predict the directions
and rates of chemical reactions.
4Question
- When you are driving along in your automobile,
octane in the gasoline is reacting with oxygen
from the air to produce carbon dioxide and water.
Write a balanced chemical equation that can be
used to describe this reaction
2C8H18 25 O2 --gt 16 CO2 18 H2O
5Oxidation and Reduction
- In Unit 1 we discussed some of the strategies
that atoms use to obtain 8 valence electrons. - See Unit IElaboration - The Octet Rule
- See Unit 1Elaboration - Compounds
6Oxidation and Reduction
- When metal atoms combine with non-metal atoms,
they transfer electrons from the metal to the
non-metal to form ionic compounds - Sodium, Na (s), is a soft grey metal.
- Chlorine, Cl2 (g), is toxic green gas.
- Sodium chloride, NaCl (s), is a crystalline white
solid comprising sodium ions, Na, and chloride
ions, Cl.
7Oxidation and Reduction
- Reactions that involve the transfer of electrons
from one atom to another are called
oxidation/reduction reactions. - The atom losing the electrons is oxidized.
- In the previous example, the sodium is oxidized
- The atom gaining the electrons is reduced.
- In the previous example, the chlorine is reduced
- While the two processes can be separated, one
cannot occur without the other.
8Oxidation and Reduction
- Reactions that involve the transfer of electrons
from one atom to another are called
oxidation/reduction reactions. - The reactant that takes away the electrons is the
oxidizing agent. - In the previous example, the chlorine is the
oxidizing agent. - The chlorine took the electrons away from the
sodium. - The reactant that donates the electrons is the
reducing agent. - In the previous example, the sodium is the
reducing agent. - The sodium gave the electrons to the chlorine.
9Oxidation and Reduction
- In oxidation and reduction, metals can also
transfer electrons between themselves - Copper, Cu (s), is a reddish metal.
- Silver(I) nitrate, AgNO3 (aq), a colorless
aqueous solution containing silver(I) ions, Ag
ions and nitrate ions, NO3-. - copper(II) nitrate, Cu(NO3)2 (aq), a green
aqueous solution containing copper(II) ions,
Cu2, ions and nitrate ions, NO3-. - Silver, Ag (s), a silvery metal
10Oxidation and Reduction
- Oxidation and Reduction
- The atom losing the electrons is oxidized.
- In the previous example, the copper is oxidized
- The atom gaining the electrons is reduced.
- In the previous example, the silver(I) ion is
reduced
11Questions (Clickers)
- Zinc reacts with copper(II) sulfate according to
the equation - Is the zinc being
- Oxidized?
- Reduced?
12Questions (Clickers)
- Zinc reacts with copper(II) sulfate according to
the equation - Is the copper(II) ion being
- Oxidized?
- Reduced?
13Questions (Clickers)
- Zinc reacts with copper(II) sulfate according to
the equation - Which reactant is the oxidizing agent?
- Zinc
- Copper(II) ion
14Questions (Clickers)
- Zinc reacts with copper(II) sulfate according to
the equation - Which reactant is the reducing agent?
- Zinc
- Copper(II) ion
15Oxidation and Reduction
- Oxidation/reduction reactions can also occur when
no ions or metals are involved. - This can occur when molecular compounds composed
of nonmetals react with one another to form other
molecular compounds, - And
- Polar covalent bonds are produced or eliminated
- Or
- Double or triple bonds are produced or eliminated
- See Unit 1Elaboration - Polarity
16Oxidation and Reduction
- The combustion of an organic molecule to produce
carbon dioxide and water is an example - The products of this reaction contain polar
covalent bonds in which the electrons are being
drawn away from - The carbon atom in CO2
- The hydrogen atoms in H2O
- The carbons and the hydrogens are being oxidized.
- The oxygen is being reduced.
17Oxidation and Reduction
- An easier way to assess whether a reaction is an
oxidation/reduction reaction or not is to look
for the following
18Oxidation and Reduction
- Applying these rules to the combustion of
methane - The carbon is being oxidized because it gains
bonds to oxygen. - The carbon is also being oxidized because it is
losing bonds to hydrogen. - The hydrogens are being oxidized because they
gain bonds to oxygen.
19Oxidation and Reduction
Rules for Assigning Oxidation Numbers The
oxidation number of an atom is zero in a neutral
substance that contains atoms of only one
element. Thus, the atoms in O2, O3, P4, S8, and
aluminum metal all have an oxidation number of
0. The oxidation number of monatomic ions
is equal to the charge on the ion. The oxidation
number of sodium in the Na ion is 1, for
example, and the oxidation number of chlorine in
the Cl- ion is -1. The oxidation number of
hydrogen is 1 when it is combined with a
nonmetal. Hydrogen is therefore in the 1
oxidation state in CH4, NH3, H2O, and HCl.
The oxidation number of hydrogen is -1 when it is
combined with a metal. Hydrogen is therefore in
the -1 oxidation state in LiH, NaH, CaH2, and
LiAlH4. The metals in Group IA form
compounds (such as Li3N and Na2S) in which the
metal atom is in the 1 oxidation state.
The elements in Group IIA form compounds (such as
Mg3N2 and CaCO3) in which the metal atom is in
the 2 oxidation state. Oxygen usually has
an oxidation number of -2. Exceptions include
molecules and polyatomic ions that contain O-O
bonds, such as O2, O3, H2O2, and the O22- ion.
The nonmetals in Group VIIA often form
compounds (such as AlF3, HCl, and ZnBr2) in which
the nonmetal is in the -1 oxidation state.
The sum of the oxidation numbers of the atoms in
a molecule is equal to the charge on the
molecule. The most electronegative element
in a compound has a negative oxidation number.
http//chemed.chem.purdue.edu/genchem/topicreview/
bp/ch19/oxred_1.phpassign
20Oxidation and Reduction
- An easier way to assess whether a reaction is an
oxidation/reduction reaction or not is to look
for the following
21Oxidation and Reduction
- Hydrogenation
- Another type of oxidation/reduction reaction is
the hydrogenation reaction - In this example, an alkene is reduced to an
alkane. - This is considered reduction, because the
hydrogen is bringing in additional electrons to
the molecule. - The alkane that is produced in this reaction is
considered saturated because it can no longer
absorb any more hydrogen atoms.
saturated
unsaturated
22Oxidation and Reduction
- Often chemist use a shorthand method of writing
equations like these - The equation shown on the previous slide can be
written as follows - One of the reactants, H2, is placed above the
reaction arrow - Technically, this equation is no longer balanced
- The shorthand method of writing a chemical
equation is used to emphasize what happens to a
key component of the reaction - In this case it is the alkene.
23Oxidation and Reduction
- Saturated vs Unsaturated Fats
24Oxidation and Reduction
- Saturated vs Unsaturated Fats
25Oxidation and Reduction
- Saturated vs Unsaturated Fats
26Oxidation and Reduction
- Saturated vs Unsaturated Fats
27Oxidation and Reduction
- Saturated vs Unsaturated Fats
Fat (Triacylglyceride)
28Oxidation and Reduction
- Dehydrogenation
- Oxidation/reduction also occurs when hydrogens
are taken away from a molecule. This is called
dehydrogenation. - The oxidation of succinic acid to fumaric acid
- This reaction takes place in the Citric Acid
Cycle. - We will discuss the Citric Acid Cycle in Unit 12.
- The FAD is an abbreviation for a large organic
molecule called Flavin Adenine Dinucleotide.
29Oxidation and Reduction
- The reaction equation on the previous slide also
illustrates another shorthand method of writing
equations, which used multiple reaction arrows. - The longhand form of this reaction equation is
30Oxidation and Reduction
- Dehydration example
- The oxidation of ethanol to form acetaldehyde
- This reaction occurs in liver after consuming
alcohol. - The NAD is an abbreviation for a large organic
molecule named Nicotinamide Adenine Dinucleotide.
31Questions (Clickers)
- In the following reaction, what is the ON of
carbon 1 vs. carbon 2? - 0 and -1
- 0 and 1
- -1 and 1
- None, this is not a redox reaction
1
1
1
1
2
1
-3
-2
1
1
-2
1
1
1
1
32Questions (Clickers)
- In the following reaction, what role is played by
the NAD - reducing agent
- oxidizing agent
- neither
33Reactions Involving Water
- While the major role for water in biology is a
physical one as the primary solvent in living
cell, it also plays a chemical role as a reactant
or product in some chemical reactions. - Reactions involving water as a reactant or
product - Acid-catalzyed hydrolysis
- Base-catalyzed hydrolysis
- Hydration
- Dehydration
34Nobel prize in chemistry-2008
- Chalfie, Chimomura and Tsien-Green Fluorescent
Protein-GFP
GFP mouse
Now red fluorescent protein cat
35Nobel prize in chemistry-2008
- Chalfie, Chimomura and Tsien-Green Fluorescent
Protein-GFP
A whole bunch of new fluorescent proteins-Roger
Tsien
Brainbows!
36Reactions Involving Water
- Hydrolysis
- In the hydrolysis reaction, water (hydro) is used
to split (lyse) another molelcule. - In this case, water is being used to split an
ester into a carboxylic acid plus and alcohol.
37Reactions Involving Water
- Hydrolysis example
- The hydrolysis of the ester bond in the
neurotransmitter acetylcholine. - Again, shorthand notation is being used
- The H2O reactant is placed above the reaction
arrow, - The H below the arrow indicates an acid catalyst
is used.
38Reactions Involving Water
- Hydrolysis
- Hydrolysis can also be catalyzed using a base
(OH-). - Because one of the products of the hydrolysis is
a carboxylic acid, in base catalyzed hydrolysis
the base undergoes a second acid/base reaction
with the carboxylic acid to produce a carboxylate
ion. - The base catalyzed hydrolysis of esters is also
called saponification - We will be discussing acids and bases in Unit 6
39Reactions With Water
- Hydrolysis example
- The base catalyzed hydrolysis of fats produces
soap and glycerol
Fat
40Reactions With Water
- Hydrolysis example
- The base catalyzed hydrolysis of fats produces
soap and glycerol
Soap
Glycerol
41Reactions Involving Water
- Hydration
- In the hydration reaction water is also split,
but instead of being used to split another
molecule, it is added to another molecule to
produce a single product. - The water it is added to either an alkene or
alkyne - The hydration of an alkene produces an alcohol.
42Reactions Involving Water
- Hydration
- This can also be written in shorthand as
- The H below the reaction arrow is used to
indicate that this is an acid-catalyzed reaction. - The shorthand is used to emphasize what happens
to the key reactant.
43Reactions Involving Water
- Hydration example
- On an earlier slide a reaction from the Citric
Acid Cycle was shown, which involved the
dehydrogenation of succinic acid to produce
fumaric acid. - The sequent reaction in the Citric Acid Cycle is
an example of a hydration reaction
44Reactions Involving Water
- Dehydration
- In the dehydration reaction is the reverse of the
hydration reaction. - The water it is removed from an alcohol
- The dehydration of an alcohol produces an alkene.
45Reactions Involving Water
- Dehydration example
- The Citric Acid Cycle also provides a good
example of a dehydration reaction. - A dehydration reaction followed by a hydration
reaction is used to move a hydroxyl group from
one carbon to an adjacent carbon in citric acid
46Free Energy and Reaction Rates
- In Unit 3 we discussed how changes in the free
energy can be used to predict whether a process
is spontaneous (favorable) or nonspontaneous (not
favorable)
?G lt 0 spontaneous
?G gt 0 nonspontaneous
47Free Energy and Reaction Rates
- The same principles can be applied to chemical
reactions to predict whether they are favorable
or not
? ? B
? ? B
?G gt 0 nonspontaneous
?G lt 0 spontaneous
A
?
?
A
48Free Energy and Reaction Rates
- Just because a reaction is spontaneous, does not
mean that it will occur at an observable rate. - For example, diamond and graphite are two
different forms of pure carbon. The reaction that
converts diamond to graphite is actually a
favorable one - This does not make diamonds a bad investment for
fear that they will turn into pencil lead. - Why?
Diamond ? Graphite
?G lt 0 spontaneous
Diamond
Free Energy (G)
Graphite
Progress ofreaction
49Free Energy and Reaction Rates
- There is is a hill that for most reactions the
reactants must climb and go over to before they
can go on to become product.
? ? B
A
?
50Free Energy and Reaction Rates
- The height of this hill is called the activation
energy, Eact. - The activation energy has no effect on the
overall change in the free energy for the
reaction.
? ? B
Eact gt 0
A
?G lt 0 spontaneous
?
51Free Energy and Reaction Rates
- Diamonds are still good investment because the
activation energy for the conversion of diamond
to graphite is very high.
Diamond ? Graphite
Eact gt 0
Diamond
?G lt 0 spontaneous
Graphite
52Free Energy and Reaction Rates
- The reaction rate (speed) of a reaction is
determined by the height of the hill. - The higher the activation energy, the slower the
reaction rate.
53Free Energy and Reaction Rates
- There are several ways that reactants can be
pushed over the hill to speed up the reaction
rate. Two of these include - Increase the temperature of the reactant
molecules. - This increases the kinetic energy, which
increases the motion of the reactant molecules.
This increases the frequency with which they will
collide with one another to react. - Increase the concentration of the reactant
molecules. - This increases the number of reactant molecules.
This also increases the frequency with which they
will collide with other reactant molecules.
54Free Energy and Reaction Rates
- There is a third way to speed up the reaction
rate and that is to lower the height of the hill. - This is done using catalysts, which provide an
alternative pathway over the hill for the
reactants.
? ? B
Eact gt 0 without catalyst - with catalyst
A
?G lt 0 spontaneous
?
55Free Energy and Reaction Rates
- Catalysts speed up a reaction, but are not
produced or consumed in a reaction. - In the reaction equation, their presence in
indicated above or below the reaction arrow. - They have not effect on the change in free energy
for the reaction, ?G. - They cannot be used to make an unfavorable
reaction favorable.
56Free Energy and Reaction Rates
- In biological systems, catalysts are called
enzymes. - Most enzymes are proteins.
- Nearly every reaction that takes place in a
living cell has an enzyme associated with. - Enzymes bind the reactants, facilitate the
reaction, and then release the products.
57Free Energy and Reaction Rates
See Jmol Model of Hexokinase
58The End