LaurenHill Chemistry
8.
An Introduction to Reaction Rates
Definition
What is the rate of a reaction?
The rate of a reaction is the amount of reactant that disappears or
product that appears per unit time. Mathematically it is the ratio of
the change in the amount of a substance to the change in time.
Example 1
Given: Mg(s) +
2 HCl(aq) 
MgCl2(aq)
+
H2 
a.
A student places a piece of magnesium in acid and measures 0.12 moles of
hydrogen gas after 2.0 minutes. Six minutes after having added the Mg, a total of
0.36 moles of hydrogen were collected. Find the average rate of production, in
moles per second, of hydrogen gas between 2.0 and 6.0 minutes.
b.
Find the average rate of disappearance of hydrochloric acid between 2.0 and 6.0
minutes.
Example 2
If in the following reaction
Mg(s) +
2 HCl(aq)  MgCl2(aq) +
H2 
HCl decomposed at a rate of 10.0 mg per second, how many minutes
would it take to produce 50.0 g of hydrogen gas?
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III. Rates of Chemical Reactions
Example 3
A student places 5.0 moles of hydrogen and 5.0 moles of iodine into a 2.0
L container and measures the amount of HI formed according to:
H2(g)
+

I2(g)
Here’s her data:
Time
0.0
1.0
(minutes)
Moles of
0.0
4.0
HI present
2 HI (g)
2.0
3.0
4.0
5.0
6.0
6.0
7.4
8.4
9.0
9.2
Graph the amount of leftover H2 versus time and find the average rate of
hydrogen consumed.
Time
(minutes)
Moles of HI
present
Moles of
hydrogen
that reacted
0.0
1.0
2.0
3.0
4.0
5.0
6.0
0.0
4.0
6.0
7.4
8.4
9.0
9.2
Moles of
hydrogen
left over
Notice that since the
points on the graph
generate a curve, the
rate at which
hydrogen disappears
keeps changing.
That's why we refer
to the rate we
calculated as an
average rate. To get
the instantaneous rate
(the rate at any given
instant) we would
have to draw a
tangent line to the curve at that given moment, and obtain the slope of the tangent line. If you
imagine successive tangent lines drawn to the curve from left to right, the slopes of the tangents
become progressively more gentle. In other words, the rate at which hydrogen disappears keeps
decreasing with time.
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LaurenHill Chemistry
Exercises
1.
Hydrogen gas reacts with iodine according to the following equation:
+
a.
From the point of view of HI, how would the rate of the reaction be
expressed?
How could you express the rate of the reaction from the point of view of
hydrogen gas?
If you had the necessary data, would you obtain the same number for both
(a) and (b)? Why? Or why not?
b.
c.
2.

H2(g)
I2(g)
2 HI (g)
Experimental rate data was collected for the following reaction:
2 XY 
2 X + Y2
The graph expresses the concentration of product X over the course of time.
Concentration of Product X Versus Time
Concentration
of X (moles/L)
1
1
Time (minutes)
a.
What was the average rate of formation of product Y2 in the first 8.0
minutes? Express in moles per liter per minute and in moles per liter per
second.
b.
At what rate was XY decomposing between the 5.0th and 10.0th minute?
c.
At what rate was Y2 forming between the 5.0th and 10.0th minute?
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III. Rates of Chemical Reactions
d.
Use the graph to complete the following table. Assume a 1.0 L flask.
Time
0
1
2
3
4
5
6
7
8
e.
3.
Amount of XY
that reacted
(moles)
0
Amount of XY
remaining (moles)
Amount of Y2
forming(moles)
12
0
Using the above table, graph the concentration of remaining XY versus
time.
N2(g) + 3 H2(g) 
Consider the reaction:
2 NH3(g)
Under a certain set of conditions, the rate of formation of ammonia was found to
be 24 L/minute. At what rate was hydrogen being consumed?
4.
a.
The electrolysis of water produces hydrogen gas according to the
following equation:
2 H2O(l)

2 H2(g) +
O2(g)
A chemist wants 24.0 g of oxygen using an apparatus that decomposes 45.0 mL of
water per hour at room temperature and pressure. How many minutes will it take
to make that much gas?
b.
If under a different set of conditions it took an hour to make 200.0 g of
oxygen gas, at what rate was the water decomposing in moles/minute?
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LaurenHill Chemistry
9.
Combustion and Activation Energy
A.
Characteristics of Combustion



Combustion involves a reaction between a fuel and oxygen.
The products of a combustion reaction always include an oxide that binds to a victimized
atom within the fuel (we’re dramatizing here: the victim is simply an atom that lost electrons
to the electron-hungry oxygen).
If there is hydrogen within the molecules of the fuel, water will also be produced.
Example 1:
Predict what will form from the combustion of the following:
Fuel
Products of combustion
Aluminum ( Al)
Methane (CH4)
Gasoline ( C8H18)
coal (mixture of S + C)
wood (mixture of [C6H10O5]n and [C10H12O3]n )

Because combustion forms very stable products, combustion reactions are exothermic.
Example 2:

Draw a reaction profile (enthalpy versus reaction-progress) for a combustion
reaction.
Although combustion reactions eventually release heat, they also need heat to get started. The
temperature at which oxygen molecules smash into fuel molecules with enough energy so
that electrons are stolen and products are formed is known as the kindling point.
Substance
Paper
Wood (varies with type)
Kindling Point (oC)
232
190-266
Cotton
Methyl alcohol
Natural gas(depends on composition)
266
464
482-632
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III. Rates of Chemical Reactions
B.

Activation Energy
The little bump in the graph on the previous pager represents the amount of energy
needed to reach the kindling point. Many non-combustion reactions also need a jumpstart , and in general it is known as the activation energy, Ae.1 The activation energy
can be calculated by :
Ae = H maximum – H reactants
Example 3:
Calculate the activation energy for the following:
E (kJ)
120
80
20
Reaction progress
C.
Factors Affecting the Rate of Combustion: The Fire Triangle
Fire needs three essential elements: oxygen, fuel and heat. If any of the components at the
vertices is missing, there will not be a fire. These factors also affect the rate of
combustion, or simply put, how fast something burns. Surface area is an additional factor.



The nature of the fuel used.
The concentration of oxygen
Heat
Surface area of fuel (will provide heat sooner by
burning faster)
Oxygen
Fuel
Example 1
Why do we dig or place rocks around a campfire?
Example 2
Why don’t you attempt to start a fire by lighting a big log?
Heat
1
Activation energy is officially defined as the energy that a reaction must absorb before it occurs. Even if a
reaction will go on to lose energy overall, it still has to absorb a certain amount of activation energy to get
going..
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LaurenHill Chemistry
Example 3
Use the fire triangle to explain why adding water to a wood fire is a good
idea.
Exercises
1.
Write a balanced equation for the combustion of pentane. Pentane is C5 H12. Identify the fuel and
the oxide, and include heat on the appropriate side of the equation.
2.
Draw a reaction profile in which H reactants = 100 kJ, the activation energy is 50 kJ, and H = -25
kJ.
3.
Which of the following is not necessary for combustion to take place?
A.
C.
4.
a fuel
match
B.
D.
oxygen
a sufficiently high temperature
Carbon burns in air to produce CO2 and H2O. Under what conditions would you expect the fastest
rate of combustion?
A.
B.
C.
D.
Chunks of carbon are heated and allowed to burn naturally in air.
Forcing hot air over the carbon as it burns.
Powdering the carbon and allowing it to burn naturally.
Powdering the carbon and forcing hot air over it.
5.
How does the foam from a fire extinguisher help put out the fire?
6.
A cook started a fire by forgetting about the oil he was warming. He then pulled the pan off the
stove and threw baking soda in the pan.
Explain why the cook acted the way he did. Why did he not use water?
7.
Olive oil has a lower kindling point than corn oil. Which one is the more practical cooking oil?
8.
According to the manufacturer, the average rate of combustion of a certain
type of candle made of paraffin, C25H52 , is 8.33 x10-4 mol/min. This type of
candle is sold in four sizes: 25, 50, 75 or 100 g. You wish to use only one
candle of this type to provide 4 continuous hours of light. What is the smallest
one you can buy for this purpose ?
9.
From a molecular point of view, why does a combustion reaction need to reach a kindling point
before a fire starts?
10.
From a molecular point of view, why does surface area increase the rate of a reaction?
11.
Calculate the activation energy.
E (kJ)
133
83
22
Reaction progress
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III. Rates of Chemical Reactions
10.
Factors Affecting Rates of Reactions
A.
The Nature of Reacting Substances


Ionic bond changes occur faster than covalent bond changes.
Examples of ionic bond include precipitation and
neutralization reactions.
In general, if more bonds have to be broken and reformed in order for the reaction to
take place, the reaction will be slower.
Example 1:
Which of the following room temperature reactions are relatively slow?
a)
C6H6 +
7.5 O2  6 CO2
b)
Ag+1(aq) +
F-1(aq) 
c)
NaOH(aq)
+
d)
N2(g) +
3 H2(g) 
B.
The Concentration of Ions or Gases

+
3H2O
AgF (s)
HCl(aq) 
NaCl(aq)
+
H2O
2 NH3(g)
Increasing the concentration of ions or gases (increasing pressure also works)
increases the rate of a reaction. Why?
Crowding makes it more likely for molecules to collide with other ones.
Example 1
How could you speed up reaction 1d?
Example 2
How do you increase the rate at which Mg reacts with HCl(aq)?
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LaurenHill Chemistry
C.

Surface Area
Increasing surface area increases the rate because it increases the number of
effective collisions between molecules as it exposes more reacting
molecules to each other.
Example 1
D.
Which will react faster, and how much faster: A Zn cube or a Zn cube split
in two? How much faster if the cut is at 90o to the face of the cube?
Presence of Catalyst or Inhibitor
A catalyst is a chemical that speeds up a chemical reaction, but the catalyst itself
is not used up in the reaction. It lowers the activation energy of a reaction.
An inhibitor does the opposite; it slows down or prevents a reaction from
occurring. It raises the activation energy of a reaction.
Example 1
A potato contains catalase that can break down hydrogen peroxide. What
would you observe with and without the catalyst.
Example 2
The breakdown of ozone is catalyzed by the presence of Cl.
a)
How does this take place?
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III. Rates of Chemical Reactions
b)
Draw the reaction profile for the catalyzed and uncatalyzed reactions.
Example 3:
An apple’s browning reaction (caused by oxidation of phenols) is inhibited
by acid. What would you observe with and without the inhibitor?
Example 4:
The growth of bacteria can be inhibited by the presence of penicillin.
a)
How does this take place?
b)
Draw the reaction profile for the
inhibited and uninhibited reactions.
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LaurenHill Chemistry
Exercises
1.
Rate the following room temperature reactions as fast or slow, and justify your
choice.
a)
b)
c)
d)
NaBr(aq)
+ HgNO3 (aq) 
HgBr(s)
2 KOH(aq)
+ H2SO4 (aq) 
2 H2O(l)
N2(g) + 3 H2(g) 
2 NH3(g)
H2(g) +
I2(g)

2 HI (g)
2.
Given: H2(g)
+
I2(g)

+
+
NaNO3(aq)
K2SO4 (aq)
2 HI (g)
Why does increasing the concentration of hydrogen gas increase the rate at which
it reacts with iodine? Draw both low-concentration and the high-concentration
situations.
3.
What cooks faster? 2 kg of sliced potatoes or 2 kg of whole potatoes? Why?
4.
How much slower will a spherical piece of Zn react with acid compared to an
identical piece that has been sliced into 4 equal pieces? (The answer is not 4 x!).
Assume that all other factors affecting the rate remain equal.
Area of a sphere = 4r2.
5.
What happens to the activation energy of a reaction to which a catalyst has been
added? Show on a graph.
6.
Draw a reaction profile for an endothermic reaction, and then for that same
reaction, show the effect of an inhibitor on the graph.
7.
How does Cl act as a catalyst in the destruction of ozone?
8.
How does penicillin act as an inhibitor in fighting bacteria?
9.
Why would a giant(say 100 cm long) grasshopper never be able to breathe? (hint:
grasshoppers, like all insects, breathe only through their skins.)
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III. Rates of Chemical Reactions
E.
Temperature

As mentioned before, temperature increases the number of effective collisions between
molecules. This idea is based on the collision theory. It states that reactions only occur when
molecules collide with sufficient energy to rearrange atoms into different molecules.

As we saw in the enzyme example, the angle at which molecules approach themselves is also
important, but if we have more collisions at a higher temperature, we also increase the
likelihood that molecules will hit each other with the proper orientation.

What we called Hmax on the “energy versus progress of reaction” is the highest potential
energy point in the chemical reaction. That belongs to a temporary, unstable in-between
product known as the activated complex.

Since temperature is only an average of the kinetic energy of molecules,
not all molecules at a given temperature have the same kinetic energy. Distribution of kinetic
energies of molecules graphs show the relative number of molecules that possess different
amounts of energy.
INEFFECTIVE COLLISION
We have OH- and CH3Cl molecules hitting at the
wrong angle and with insufficient energy. No
CH3OH product is created
EFFECTIVE COLLISION
Here the collision is effective because the angle is
right and the energy is sufficient.(higher
temperature) An activated complex (in- between product with Hmax)is created . Note that with a
higher energy you are more likely to get the right
angle just from the increased motion. The CH3OH
product is created.
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LaurenHill Chemistry
(1)
(2)
The total area under the curve represents all molecules at a given temperature T1.
The area under the curve and to the left of the activation energy (vertical line)
represents those molecules that will not react at that given temperature.
(3)
The area under the curve and to the right(shaded) of the activation energy
represents those molecules that will react at that given temperature.
Example 1
Redraw a curve on the same graph for T2 where T2 > T1.
Activation Energy
Number of Molecules
T1
Energy
Example 2
How would you show the effect of an inhibitor on the same graph? Of a
catalyst?
Activation Energy
Number of Molecules
T1
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III. Rates of Chemical Reactions
Exercises
1.
Why is the activation energy in a “Distribution of Kinetic Energies" graph
sometimes referred to as the threshold energy? Threshold = lowest limit at
which something can be observed.
2.
What does increasing the temperature do to the number of molecules whose
energy exceeds the activation energy?
3.
What kind of chemical lowers the threshold energy?
4.
Draw a “Distribution of Kinetic Energies" graph for the decomposition of
hydrogen peroxide at room temperature (very slow reaction). Using a broken
line, use the same graph to show what happens when the powder from a dry cell
battery (MnO2) catalyzes the reaction.
5.
When you cut an apple and leave it on your counter, the phenolic compounds in
an apple oxidize with the help of a catalyst, and the apple turns brown. However,
adding CuSO4 will inhibit the reaction, and the apple will not change colour.
Again represent both situations on the same graph, using a broken line for the
inhibited reaction.
6.
a.
b.
c.
Estimate the percent of molecules that are reacting at T1 .(count squares!)
Estimate the percent of molecules reacting at T2 .
Which temperature is the higher one?
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LaurenHill Chemistry
7.
a.
Match the temperature with the correct curve.
b.
In the diagram below, what is constant for all three curves?
Number of molecules
8.
Use a drawing to show the difference between effective and ineffective collisions
between hydrogen and oxygen molecules.
11.
Rate Constants and Reaction Mechanisms
A.
Rate Law
One-way reactions are dependent only on the concentration of reactants, not
products. In such cases, the rate at which C( the product) forms in the reaction:
A( g) + B( g) C( g) can be expressed as
Rate = k[A]m[B]n,
where [A] = concentration of A in moles/L;
[B] = concentration of B in moles/L;
m and n are determined experimentally, and k can be found from the resulting
graph.
Example 1
Suppose that for the reaction 2A + B C, experiments revealed that m = 2
and n =1, so that
Rate at which C forms = k[A]2[B]1,
What would happen to the rate at which C formed if…
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III. Rates of Chemical Reactions
a)
[A] tripled and [B] remained constant?
b)
[B] tripled and [A] remained constant?
c)
If under certain conditions it took 9.0 minutes for 1.0 mole of C to appear,
how long would it take if the reaction was repeated with the same amount of
B but twice the concentration of A?
d)
What must have happened to the concentration of A if the rate tripled (3.0
times bigger) even though we increased the concentration of B by a factor of
4.0.
B.
Reaction Mechanisms
The rate expression can rarely be predicted from the overall reaction, because the overall
reaction does not reveal how the reaction actually takes place. The series of steps that
actually lead to the deceptively simple overall result is known as the reaction
mechanism.
Consider the overall reaction between nitrogen dioxide and carbon monoxide:
NO2( g) + CO( g)  CO2( g) + NO( g)
Experiments reveal that the rate at which CO2 forms is given by:
Rate = k [NO2]2
In other words the concentration of the reactant CO is almost irrelevant ( as long as it’s
not zero!). But how can that be?
The actual mechanism will shed light on this mystery.
NO2 + NO2  NO3 + NO (very slow)
NO3 + CO  CO2 + NO2 (very fast)
Overall: NO2 + NO2+ NO3 + CO NO3 + NO + CO2 + NO2
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LaurenHill Chemistry
Or canceling the common compounds on each side we get
NO2 + CO  CO2 + NO
The rate at which CO2 forms will be influenced by the slow step, not the fast
step. It’s like if you take 2 seconds to wolf down your burger and fries but another 35
minutes to eat the rest of your meal: the yucky brussel sprouts and other vegetables, then
the rate at which you finish your meal is determined by the rate at which you eat the
yucky stuff.
So the slow step is NO2 + NO2  NO3 + NO, so now it’s understandable why
CO plays an unimportant role and that
Rate = k [NO2]2
Example 1
Reaction
2 NO(g)+ Br2(g) 2 NOBr(g)
Hyp 1
Hyp 2
2 NO(g)+ Br2(g) 2 NOBr(g)
NO(g)+ Br2(g)  NOBr2(g) fast
NO(g) + NOBr2(g) 2 NOBr(g) slow
Give rate expressions consistent with each hypothesis.
Example 2 Draw a reaction profile for a 3-step reaction in which the second step is
the slow one.
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III. Rates of Chemical Reactions
Exercises (For Solutions: http://www.emsb.qc.ca/laurenhill/science/rate7soln.html)
1.
The rate at which water is formed from hydrogen peroxide is given by
Rate = k[H2O2(aq)][I-1 aq)]
a.
What will happen to the rate if the concentration of iodide is halved and the
peroxide concentration does not change?
b.
What would you have to do to the concentration of iodide if the rate
remained constant but the concentration of peroxide tripled?
2.
The rate at which HBr forms from its constituent elements is given by:
Rate = k[H2( g)][Br2( g)]0.5
a)
If it took 2 hours for two moles of HBr to appear, how long would it take
if the reaction was repeated by quadrupling the concentration of each reactant?
b)
If the rate tripled and the concentration of hydrogen was doubled, what
was done to the concentration of bromine?
3.
Explain why the reaction mechanism is so important in determining the rate of a
reaction.
4.
In an experiment that involved measuring the rate of a chemical reaction, 6.35
grams of solid copper reacted with a 1.0 mol/L solution of nitric acid. The
reaction lasted 1 min 40 s and occurred at room temperature.
What is the reaction rate in moles of copper per second (mol/s)?
5.
While studying the rate of various chemical reactions, a student measured the rate at
which certain metals react with different acids. One of the experiments involved
combining a strip of solid magnesium, Mg(s), with a hydrochloric acid solution, HCl(aq).
The student made the following observations:
- Mass of the magnesium strip used
1.78  102 g
- Atmospheric pressure in the room
101.3 kPa
- Room temperature
25.0C
- Temperature of the acidic solution
25.0C
- Duration of the reaction
6 min 40 s
This chemical reaction is represented by the following equation:
Mg(s) + 2 HCl(aq)  MgCl2(aq) + H2(g)
Under these conditions, what is the average rate of production of H2(g) in ml/min?
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LaurenHill Chemistry
7.
Draw a reaction profile for a 4-step reaction in which the second step is the slow
one.
8.
“Find the order with respect to E” means find x for rate = k[E]x[F]y
12.
The Role of Rates in Nature and Technology
A.
Examples From Nature
1.
In chloroplasts plants use photosynthesis to convert water and carbon dioxide into
sugars and oxygen. The overall reaction is:
6 CO2 + 6 H2O  C6H12O6 + 6 O2
The sequence of reactions leading up to the above overall reaction is complicated, but
it would never happen on its own if the rate was not catalyzed by chlorophyll
molecules.
 When light strikes chlorophyll molecules, they lose electrons, which are picked
up by a molecule that helps link up CO2 molecules.
 The electrons are eventually returned to the chlorophylls. The molecule that
ultimately loses its electrons for the sake of chlorophyll is water:
2 H2O  4 H+ + O2 + 4 e-
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III. Rates of Chemical Reactions
Chloroplast

Note that the above reaction releases oxygen, which is why plants release O2
while growing. The reaction also concentrates H+1 on one side of the membrane in
those little green disks shown in the above figure. That allows an energy carrying
molecule, ATP, to be made, and the energy is invested in the production of
sugars.
Note, that after all is said and done, chlorophyll is available again to absorb more
light, and to start the whole cycle again. True catalysts speed up reactions without
being consumed.
Example
Try to summarize all of the above with a simple diagram.
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LaurenHill Chemistry
2. Enzymes, as we have seen, are biochemical catalysts. Without them there
would be no life. They catalyze everything from the synthesis of protein in our
skin to the breakdown of all food molecules in our digestive system.
Example
Given:
sucrose + enzyme  glucose + fructose + enzyme.
Summarize the above with a simple diagram.
3. In hibernating animals, body chemistry can be altered so that the organism
can survive at lower temperatures. This lowers the amount of oxygen and glucose
that the hibernator needs.
B.
Examples From Technology
1.
Refrigeration lowers the rate of oxidation and it makes it difficult for bacteria and
mold to reproduce.
Why?
2.
Preservatives such as BHT in cereal packaging out-compete the cereal for
oxygen, which is desirable because food goes stale when it oxidizes.
Preservatives, however, are not true inhibitors if they are consumed in the
reaction with oxygen.
3.
Catalytic Converters use rhodium and platinum to attract the pollutants CO and
NO2 onto their surface. These are then broken down according to:
2 NO2  N2 + 2 O2
CO + 0.5 O2  CO2
Example
The actual converter is shaped like a honey comb with hundreds of cells
that are coated with catalyst. Why?
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III. Rates of Chemical Reactions
4.
Detergents: They consist of soaps, bleaching agents and enzymes used to
breakdown proteins and starch molecules found in stubborn stains.
Exercises
Photosynthesis
1.
TRUE? or FALSE?
a.
The reaction catalyzed by chlorophyll is the one that converts carbon dioxide
and water into sugars.
Chlorophyll is not a true catalyst because it is consumed in photosynthesis.
The oxygen that is released by plants comes from the breakdown of water.
Ultimately electrons are returned to chlorophyll by the breakdown of water.
b.
c.
d.
Enzymes
2.
b.
a.
What is an enzyme?
How are the following 3 molecules related to what an enzyme does?
3.
Look up aspirin. Why is it an inhibitor?
Hibernation
4.
To be a true hibernator, an animal must be able to
lower its body temperature to near freezing and then
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generate enough warmth to revive itself and wake up again. Most of the true
hibernators are bats and rodents such as ground squirrels, marmots, and dormice.
Do such organisms need more oxygen and glucose while hibernating? Or less?
Explain.
Refrigeration
5.
Why does food spoil more slowly at lower temperatures?
Converters, Preservatives and Detergents
6.
What role does surface area play in catalytic converters?
7.
a).
Why are BHT and SO2 (used to preserve the colour of white wine) not true
inhibitors?
b)
In what way are they similar to inhibitors?
8.
How do some detergents make use of biological catalysts?
9.
In 1953 it was discovered that aluminum and titanium allowed ethylene to
polymerize (make long chains) into polyethylene at normal atmospheric pressure
Previously high pressures were necessary for the reaction to occur. How did Al
and Ti make the reaction more feasible? ( What were Al and Ti acting as?)
Extra Questions from an Industrial Point of View
10.
Cracking is the name given to breaking up large hydrocarbon molecules from
petroleum into smaller and more useful bits. This is achieved by using high
pressures and temperatures without a catalyst, or lower temperatures and
pressures in the presence of a catalyst.
From industry’s point of view, what advantage would the second option have
if the catalyst is not too expensive?
11.
One possible catalyzed reaction involving the petroleum hydrocarbon C15H32
might be:
Or, showing more clearly what happens to the various atoms and bonds:
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III. Rates of Chemical Reactions
This is only one way in which this particular molecule might break up. The ethene
and propene are important materials for making plastics or producing other
organic chemicals. The octane is one of the molecules found in petrol (gasoline).
a) Why is the catalyst shown on the side of the arrow? In other words, why is it
not included among the reactants?
b) Flashback: which of the three products would have the highest per mole heat
of combustion and why?
12.
The enzyme lactase is used in the manufacture of ice cream. Lactase converts the
milk sugar lactose into the sugars glucose and galactose. Why do you think they
go through the expensive trouble of adding an enzyme? Think of two ways that it
pays off.
13.
Read the culinary chemist article on the next page and identify all the key
chemistry concepts that we’ve covered so far.
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Becoming A Culinary Chemist
My first brush with molecular gastronomy
came decades ago with attempts to
convince my Italian mother that the shape
of pasta noodles actually affects their taste.
Although all forms are made with
semolina(from durum wheat), their shapes
affect texture which is part of the taste
experience. Fusilli, bucatini and farfalle also have a range of surface to volume ratios. It is plausible that
this leads to noticeable differences in salt absorption and causes varying amounts of sauce to cling to
them. Anyone who has cooked has definitely noticed that the time of cooking varies inversely with the
surface area of noodles. As the time decreases, it creates a narrower window to capture that optimal al
dente texture. The al dente texture itself is caused by a network of coagulated protein in which starch is
embedded. If overcooked, the greater absorption of water causes more starch to coagulate. The
continuous network of protein then breaks down into discrete masses, and the pasta becomes soft and
sticky.
The next set of common reactions that could catch the fancy of a chemist are of the Maillard type. With
the addition of heat, the amino group (NH2) of amino acids attacks the carbonyl group (C=O) of a reducing
sugar eventually leading to a range of brownish and appetizing compounds. Here are some examples of
baking and cooking products that include compounds from Maillard reactions:1) Bread crust2) Boiling of
maple syrup3) Roasting of almonds, coffee or cocoa nuts4) Beer-making (where’s the heat you may
wonder? It’s the heat of fermentation.)5) Baking of cookies6) Browning of sauce on meat.The first
compound formed from the amino attack is an N-substituted glycosylamine. But the hexagonal ring of this
molecule then breaks up, undergoes another rearrangement with the help of a pH-change to produce an
Amadori compound.What happens to this type of compound depends on pH, but in either case the NH 2
group is lost forming ketones.(A compound with C=O group sandwiched between carbons.) In the next
stage, these compounds are split into some of the brown compounds that we taste and smell. While
Maillard reactions are taking place, amino acids can decompose into Schiff bases that eventually produce
cereal like flavours and those of roasted nuts, bread and meat.
Unless a sweet sauce is added to meat, the browning seen upon cooking is not a Maillard reaction. Rather
it results from the oxidation of the Fe2+in myoglobin to the Fe3+ state. This is part of the reaction where
the myoglobin protein is denatured to hemichrome.And when a cooked leaf loses its green colour, it is
because chlorophyll has lost its Mg2+ ion. Like most reactions it depends on enzymes. In this case if you
want to maintain the green colour, a little baking soda can be added. (not too much or you’ll gain both
colour and bitterness) The higher pH from HCO3 - prevents the enzyme from converting chlorophyll into
pheophytin.
If you are a non-meat eater, you won’t mind me switching the topic from meat to seafood. Astaxanthin is
a compound related to the carotene in carrots. It is pink and found in shrimp. Normally while the shrimp is
alive, the pink colour of astaxanthin is not evident because it is bound to a protein, which changes its
colour. But the heat of cooking uncoils the protein, unsheathing the same pigment that keeps flamingos
feathers pink.
A similar explanation applies to the blue/green to red colour change for cooked lobsters. Astaxanthin's
structure is similar to that of carotene. But the extra C=O group in astaxanthin increases the alternating
single-double bond network, which makes it easier for electrons to get excited to higher energy levels.
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III. Rates of Chemical Reactions
Compared to carotene, astaxanthin needs less energy or that of a longer wavelength for electronic
excitation. This is consistent with the fact that astaxanthin reflects color of a longer wavelength: pink
instead of orange. By the way, if you find a flamingo feather, keep it. Astaxanthin sells for $ 7000/kg! Of
course a feather will have a negligible fraction of astaxanthin, so look for a lost flamingo instead.We often
distort reality by trying to operate by simple rules of thumb. This certainly applies to vitamins in fruits and
vegetables. Cooking certainly reduces the concentration of vitamins in food, but it does not destroy them,
as shown in the table below: Effect of Heat on Vitamins(Source: USDA)
Food(100g)
carrots
red
peppers
broccoli
Vit A
Raw(IU)
in Vit A in
Cooked(IU)
28129
17202
61%
5.9
3.6
61%
2760
48%
190
163
86%
1967
66%
93
65
70%
5700
3000
retention
Vit C in Vit C in
Raw(mg) cooked(mg)
retention
And where in the fruit and vegetable are the vitamins located? Does the peel contain a lot of nutrients?
The answer varies. In the case of the potato, the peel does have fiber and minerals, and baking with the
peel prevents the escape of some vitamin C during the cooking process. I have not been able to verify the
claim that most of the vitamin C is in the flesh just underneath the peel. In the case of mangos, vitamin A
is distributed evenly throughout the orange pulp, which is coloured by similar beta carotene molecules.
But the peel, especially when ripe, has antioxidants, carotenes and vitamin C. Apple peels are not devoid
of nutrients either: they contain minerals (K+, Mg2+), antioxidants and fiber.
Finally a great example of another endothermic reaction in the kitchen: cooking avocados. Some
compounds in the avocado will be converted into bitter alkaloids with heat. But if the avocados are added
towards the end of a recipe to minimize the amount of heat absorbed, then the amount of bittertasting products will be kept to a minimum.
References
Barham, Peter. Molecular Gastronomy. Chem Rev. 2010 April 14; 110(4): 2313–2365
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2855180/ Atkins, P.W. Molecules. Scientific American Library. 1987
McGee, Harold. On food and cooking: the science and lore of the kitchen. Simon&Schuster. 2004
C.M. Ajilaa, S.G. Bhata and U.J.S. Prasada Rao. Valuable components of raw and ripe peels from two
Indian mango varieties. Central Food Technological Research Institute
United States Department of Agriculture usda.org http://www.plantphysiol.org/cgi/reprint/39/6/1056.pdf
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