12.1 ACTIVATION ENERGY

12.1 ACTIVATION ENERGY

Web Activity: Simulation—Collisions and Reactions (Page 526)

The correct orientation is when the I in CH

I collides with the K. The incorrect orientation is when the K collides with the H in CH

I.

Incorrect orientation:

Correct orientation:

Practice (Page 526)

1. Two conditions that must be met for two molecules to react are: the molecules must have sufficient energy, and they must have the correct orientation at the moment of collision.

2. Scientific reasons for understanding how chemical reactions occur include: a better

understanding of energy in bonding, predicting and explaining orientations of molecules for effective collisions, and predicting and explaining effective catalysts for certain reactions.

Technological reasons for understanding how chemical reactions occur could include:

industries being able to improve their products and efficiency, such as oil refining (catalytic cracking), petrochemical production (polymerization), and other chemical technologies being made “cleaner” (for example, through the use of catalytic converters).

3. (a) N (g) 2 O (g)



o 2 NO (g)

'

H

 66.4 kJ (b)

(c) The electrons or charge in the lightning are more effective in initiating the reaction

because the temperature and pressure are much higher during the strike. An alternative

hypothesis is that the electrons or charge in the lightning strike may act as a catalyst, or

the electrons may cause other nearby molecules to become ions and act as a catalyst in

the reaction to increase the rate of the reaction.

Web Activity: Simulation—CollisionReaction Theory (Page 529)

Points that are reviewed include:

Section 1 – Molecular Collisions Section 2 – Activation Energy

Section 3 – Activation Energy and Temperature

Section 4 – Fraction of Molecules with Activation Energy Section 5 – Orientation of Molecules in a Collision Section 6 – Transition States or Activated Complexes

Web Activity: Web Quest—Neurotransmitters and Nerve Agents (Page 530)

[Students should create an informational poster for this activity, outlining how nerve agents work and how persons who have been exposed to nerve agents should be treated. Information such as the following should be included.]

Action of acetylcholine on nerves and muscles

Ɣ Acetylcholine (ACh) is an ester of acetic acid and choline. Its chemical formula is

CH

COOCH

CH

N

(CH

)

. ACh is released from a presynaptic cell into the synaptic cleft between individual nerve cells. The ACh travels through the cleft and links up with receptors (AChR) on the post-synaptic cell, which promotes a nerve impulse to cause, for example, a muscle fibre to be stimulated and to contract.

Role of enzymes in chemical reactions

Ɣ Enzymes are biological catalysts, which lower the activation energy of the reaction. In an enzyme-catalyzed reaction, the substance to be acted upon (the substrate, S) binds reversibly to the active site(s) of the enzyme (E). During this temporary bond, the enzyme facilitates the change of the substrate into a new product. During this process, the enzymesubstrate complex is considered an activated complex.

E + S o ES o E + P

Once the reaction is over, the product and the enzyme break apart and the enzyme is free to

catalyze another reaction.

(Source: http://en.wikipedia.org/wiki/Image:Activation2.png, 2007.) Action of acetylcholinesterase on acetylcholine using a chemical reaction

Ɣ Acetylcholinesterase is a complex molecule with one major bonding site and a secondary site, for bonding with acetylcholine. The reaction is believed to be as follows:

The reaction pathway could also be expressed as follows:

A description of inhibitors

Ɣ Inhibitors block the action of an enzyme, often by binding with it so it is unavailable to bind with substrate molecules. In this way the enzyme is inactivated and the reaction that it catalyzes is effectively stopped.

How nerve agents act as inhibitors to acetylcholinesterase, and what happens to the effectiveness of acetylcholinesterase when bonded with a nerve agent

Ɣ Some nerve agents contain a phosphorus atom, which bonds strongly to the AChE,

effectively inactivating it. Once the enzymes are inactivated, since each AChE is normally

capable of catalyzing over 15,000 reactions per second, the body rapidly loses its ability to

break down ACh. Only a relatively small amount of nerve toxin is required to cause serious

illness or death in the victim. The longer the nerve agent is bonded to the AChE, the more

long-lasting the bond becomes. Each nerve toxin has a limited amount of time during which

certain chemical treatments (known as oximes) can readily remove the nerve toxin and reactivate the enzyme. The effects of tabun can be reversed within approximately 14 h and the effects of sarin within about 4 h, while the effects of soman are irreversible after as little as 2 min. After these intervals, the nerve toxin and AChE combination cannot be separated and victims will take much longer to recover, assuming they receive treatment.

Symptoms of exposure to a nerve agent

Ɣ There are many symptoms of short-term exposure, many of which are not directly indicative of nerve toxin poisoning: increased production of saliva, pinpoint pupils, eye pain, blurred vision, runny nose, shortness of breath, nausea, cramping, involuntary urination, slowed heart rate, fatigue, muscle weakness and twitching, partial paralysis, convulsions, loss of

consciousness, and coma. Obviously the greater the exposure (time or amount of toxin), the sooner the onset of symptoms and the more severe the effects. Inhalation gives the most rapid rate of absorption of nerve agent (due to the large amount absorbed in the lungs). Nerve toxins can be absorbed through the skin, although symptoms will appear much more slowly and with initially less intensity.

Ɣ Long-term exposure to small doses of nerve toxin is being investigated, as the sickness of some troops from the US Gulf War may be from exposure to sarin. Survivors of the Tokyo terrorist attack in 1995 have shown long-term effects such as decreased memory recall, fatigue, headaches, and visual disturbances.

Treatment of exposure to a nerve agent, including information on the action of atropine and HI-6

Ɣ Military personnel, when under the threat of nerve-agent exposure, carry an auto-injector that contains atropine and an oxime, such as HI-6 or obidoxime. Emergency responders should also be supplied with and shown how to use auto-injectors, for use when there has been a known release.

The atropine in the auto-injector does not do anything to the nerve agent or the AChE, but it does block the AChR sites on the post-synaptic cell. This means that, as ACh is building up in the synapse, it cannot activate the blocked receptors. This will relieve some of the symptoms, but it cannot reactivate the enzyme and is therefore a temporary solution.

The oxime in the auto-injector, if administered in time (depending on the nerve agent), can break the bond between the phosphorus and the AChE and reactivate the enzyme. If too much time has elapsed, the oxime will not be able to effectively remove the nerve agent and the nerve will have to create more AChE. This process may require several weeks.

A victim of exposure to nerve agents can be given multiple treatments from auto- injectors. Further treatment includes an anti-convulsant drug (diazepam). If exposure to the nerve agent has lasted too long and the oximes are not effective in removing the nerve agent from the AChE, then long-term hospitalization will be required.

Section 12.1 Questions (Page 531)

1. A successful reaction of two molecules involves a collision between molecules that have

sufficient kinetic energy to equal the activation energy. The molecules must also collide with

the correct orientation (positioning). Reactants collide ineffectively until a sufficient quantity

of energy equal to the activation energy is added. Once this is reached, the reactants form an

activated complex and the reaction begins. From there the chemical potential energy of the

product molecules decreases until (if endothermic) the final potential energy is greater than

the initial or (if exothermic) the potential energy is less than the initial.

2. Qualitative and quantitative evidence for different rates of reaction led to the hypothesis of activation energy. Controlling for temperature and concentration (with no catalyst), the rate of reaction appeared to be dependent on the nature of the reactants. Even reactions that are exothermic sometimes do not react until a specific quantity of energy is added. This implies that some kind of “energy barrier” (dependent on the reactants present) has to be overcome before the reaction can take place.

3. (a) The reaction is endothermic, as indicated by the chemical potential energy of the products being greater than the chemical potential energy of the reactants.

(b) (i) the forward reaction activation energy (ii) the forward reaction enthalpy change (¨

H)

(c) As the reactant entities, R, speed towards each other, some will combine to form an activated complex, if they have the correct orientation and sufficient kinetic energy. To form the activated complex, the kinetic energy of the reactant molecules must be converted to potential (bond) energy within the activated complex. The activated complex breaks into the product entities, P, converting the potential (bond) energy into kinetic energy. Since this reaction is endothermic, there will be less kinetic energy and more potential energy in the products than in the reactants.

4. Enthalpy change and activation energy are similar in that they both measure the difference in chemical potential energies between molecules at different stages in a reaction. They are different in that activation energy measures the chemical potential energy difference between the reactants and the activated complex, while enthalpy change measures the chemical potential energy difference between the reactants and final products. [For the reverse reaction, the activation energy changes but only the sign of the enthalpy change changes.]

5. (a) For the reaction of hydrogen and oxygen to produce water, the hydrogen and water must collide with the correct orientation, and they need sufficient kinetic energy (which could be supplied by a spark or a flame) to overcome the activation energy for the reaction.

(b)

(c) and (d)

6. Purpose

The purpose of this investigation is to test a hypothesis about activation energies of reactions of the alkali metals—sodium, potassium, and cesium—with water.

Problem

What is the order of activation energies, from highest to lowest, for sodium, potassium, and cesium?

Hypothesis

The activation energy decreases from sodium to potassium to cesium when they are reacted with water. The quicker and more violently the alkali metal reacts with water, the lower its activation energy should be.

Prediction

When reacted with water, sodium will be the slowest to react (indicating that it has the largest activation energy), potassium has the second slowest (having an intermediate activation energy), and cesium will react the fastest (having the smallest activation energy).

Design

Equal masses and shapes of the three alkali metals are placed in separate equal volumes of distilled water. The time for the metal to completely react in the water is measured. Variables are:

Ɣ manipulated: alkali metal

Ɣ responding: time for the metal to completely react

Ɣ controlled: volume of water, mass of each metal, surface area of each metal, initial temperatures of the distilled water

Materials

Ɣ eye protection Ɣ sodium metal (1.00 g)

Ɣ lab apron Ɣ potassium metal (1.00 g)

Ɣ -three 50 mL beakers Ɣ cesium metal (1.00 g)

Ɣ gloves Ɣ distilled water

Ɣ forceps Ɣ razor blade or scalpel

Ɣ centigram balance Ɣ wire screen

Ɣ stopwatch Ɣ 50.0 mL graduated cylinder

Ɣ safety shield Ɣ fume hood

CAUTION: The alkali metals are dangerously reactive materials; handle with care in a fume hood, behind a safety shield, using eye protection.

Procedure

1. Prepare the workspace by setting up the safety shield in a fume hood.

2. Prepare three beakers with 40.0 mL of distilled water in each and place them behind a safety shield.

3. Cut samples of each metal, remove excess oil with a paper towel and quickly adjust the mass to obtain a 1.00 g sample, each with approximately equal surface area.

4. Drop one metal into one beaker and start the stopwatch immediately. Place the wire screen on the beaker.

5. Once the metal has completely disappeared, stop the stopwatch and record the time in a table.

6. Repeat steps 4 and 5 for each of the remaining metals.

7. Dispose of all solutions into the sink with lots of running water.

7. (a) and (b)

Extension

8. There are many techniques that Aboriginal peoples used to start fires. All methods involve adding or creating a set amount of heat energy to overcome the activation energy of the combustion material. Most techniques involved using friction between two wooden items, which increased the temperature of a small area that was covered with dry and finely divided combustible straw, moss, grass, or wood strips. To maximize the friction, bow drills could be used. This technology involved a bow with its string wrapped around a spindle, which twists quickly back and forth against another piece of wood. Blowing on the embers would increase the flow of oxygen to the area, which would also increase the rate of combustion.

9. Some examples of ignition temperatures are: gasoline (280 qC), acetone (465 qC), ethanol (390 qC), propan-2-ol (isopropyl alcohol) (399 qC), and methane (537 qC). Each of the chemicals forms a different activated complex with oxygen, and each of those activated complexes has a different chemical potential energy. The higher the chemical potential energy of the activated complex (compared with the reactants), the higher the activation energy will be. The higher the activation energy, the higher the ignition temperature, as more energy is required to form the activated complex and start the reaction.

10. (a) Spoiling of food is a chemical reaction that requires activation energy to occur. A lower temperature would mean a lower average kinetic energy of the reactant molecules (the food), or the molecules within bacteria and fungi. This lower temperature drastically slows down the reaction rate of the molecules of the food itself and/or the life processes within the bacteria and fungi. Also, most enzymes (as catalysts) work best at

temperatures around 20 to 40 qC; below this they are not effective.

(b) [Students have a wide variety of choices for cold storage technologies including:

refrigeration; vacuum flasks or Dewar flasks; and cryopreservation with liquid nitrogen.]

(c) [Student answers will vary, but should include an introduction, body text, and a

summary, as well as a variety of illustrations. The article should include the scientific

theory of reaction rates and activation energy.]