LaurenHill Chemistry

LaurenHill Chemistry

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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?

III. Rates of Chemical Reactions

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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) 2 HI (g)

Here’s her data:

Time

(minutes)

0.0 1.0 2.0 3.0 4.0 5.0 6.0

Moles of

HI present

0.0 4.0 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) 0.0 1.0 2.0 3.0 4.0 5.0 6.0

Moles of HI

present 0.0 4.0 6.0 7.4 8.4 9.0 9.2

Moles of

hydrogen

that reacted

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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Exercises

1. Hydrogen gas reacts with iodine according to the following equation:

H2(g) + I2(g) 2 HI (g)

a. From the point of view of HI, how would the rate of the reaction be

expressed?

b. How could you express the rate of the reaction from the point of view of

hydrogen gas?

c. If you had the necessary data, would you obtain the same number for both

(a) and (b)? Why? Or why not?

2. 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

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?

1

1

Concentration

of X (moles/L)

Time (minutes)


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d. Use the graph to complete the following table. Assume a 1.0 L flask.

Time Amount of XY

that reacted

(moles)

Amount of XY

remaining (moles)

Amount of Y2

forming(moles)

0 0 12 0

1

2

3

4

5

6

7

8

e. Using the above table, graph the concentration of remaining XY versus

time.

3. Consider the reaction: N2(g) + 3 H2(g) 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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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 Kindling Point (oC)

Paper 232

Wood (varies with type) 190-266

Cotton 266

Methyl alcohol 464

Natural gas(depends on composition) 482-632


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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 jump-

start , 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:

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)

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?

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..

Oxygen

Fuel Heat

E (kJ)

Reaction progress

120

80

20


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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. a fuel B. oxygen

C. match D. a sufficiently high temperature

4. Carbon burns in air to produce CO2 and H2O. Under what conditions would you expect the fastest

rate of combustion?

A. Chunks of carbon are heated and allowed to burn naturally in air.

B. Forcing hot air over the carbon as it burns.

C. Powdering the carbon and allowing it to burn naturally.

D. 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)

Reaction progress

133

83

22


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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 + 3H2O

b) Ag+1(aq) + F-1

(aq) AgF (s)

c) NaOH(aq) + HCl(aq) NaCl(aq) + H2O

d) N2(g) + 3 H2(g) 2 NH3(g)

B. The Concentration of Ions or Gases

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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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 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?

D. 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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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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Exercises

1. Rate the following room temperature reactions as fast or slow, and justify your

choice.

a) NaBr(aq) + HgNO3 (aq) HgBr(s) + NaNO3(aq)

b) 2 KOH(aq) + H2SO4 (aq) 2 H2O(l) + K2SO4 (aq)

c) N2(g) + 3 H2(g) 2 NH3(g)

d) H2(g) + I2(g) 2 HI (g)

2. Given: H2(g) + I2(g) 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 = 4r2.

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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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 EFFECTIVE COLLISION

We have OH- and CH3Cl molecules hitting at the

wrong angle and with insufficient energy. No

CH3OH product is created

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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(1) The total area under the curve represents all molecules at a given temperature T1.

(2) 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.

T1

Num

ber of Molecules

Activation E

nergy

Energy

Example 2 How would you show the effect of an inhibitor on the same graph? Of a

catalyst?

T1

Num

ber of Molecules

Activation E

nergy


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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. Estimate the percent of molecules that are reacting at T1 .(count squares!)

b. Estimate the percent of molecules reacting at T2 .

c. Which temperature is the higher one?


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7. a. Match the temperature with the correct curve.

b. In the diagram below, what is constant for all three curves?

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…

Number of molecules


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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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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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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 1.78 102 g

- Atmospheric pressure in the room 101.3 kPa

- Room temperature 25.0C

- Temperature of the acidic solution 25.0C

- 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?

http://www.emsb.qc.ca/laurenhill/science/rate7soln.html


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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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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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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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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.

b. Chlorophyll is not a true catalyst because it is consumed in photosynthesis.

c. The oxygen that is released by plants comes from the breakdown of water.

d. Ultimately electrons are returned to chlorophyll by the breakdown of water.

Enzymes

2. a. What is an enzyme?

b. 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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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 NH2 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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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) Vit A in Raw(IU)

Vit A in Cooked(IU)

retention

Vit C in Raw(mg)

Vit C in cooked(mg)

retention

carrots 28129

17202 61% 5.9 3.6 61%

red peppers

5700 2760 48% 190 163 86%

broccoli 3000

1967 66% 93 65 70%

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 bitter- tasting 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

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2855180/ -Atkins

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2855180/ -Atkins

http://www.plantphysiol.org/cgi/reprint/39/6/1056.pdf

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