Unit I: Reaction Kinetics

Expressing Reaction RatesReactions proceeding at different rates

Reactions can occur: quickly, slowly, not at allReaction rate = amount of product formed

time interval

amount of reactant usedtime interval

∆amount, ∆t

amount can be a volume or a mass "∆" means the change in

NB: In Chem 12, let’s assume that rxn rates always have a positive value. We will use absolute value

Ex-1: If 16 g of HCl are used up after 12 min, what is the average reaction rate?Rate of using HCl: 16 g = 1.3 g/min

12 min

Ex-2: If a reaction between CaCO3 and HCl produces 245 ml of CO2 (g) in 17s, what is the average reaction rate?Rate of producing CO2 : 245 mL = 14 ml/s

17s

Types of Equations (Chem 11 Review)i) Word Equation:

Aqueous sodium hydroxide and aqueous iron (III) chloride yields aqueous sodium chloride and solid iron (III) hydroxide

ii) Balanced chemical equation:

3NaOH(aq) + FeCl3(aq) 3NaCl(aq) + Fe(OH)3(s)

iii) Complete ionic equation: Aqueous solutions break into anions and cations3Na+(aq) + 3OH-(aq) + Fe3+(aq) + 3Cl-(aq) 3Na+(aq) +3Cl-(aq) + Fe(OH)3(s)

iv) Net Ionic equation: Removal of spectator ions3OH-(aq) + Fe3+(aq) Fe(OH)3(s)Spectator ions: Na+, Cl-

Exercises Heb p. 2 # 1-5

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Kin Bkt p.6#1, p.10#11, p.13#4-5Calculations Involving Measuring Reaction RatesColgur’s Notes: p.2-4 Big Question: What is changing? What is used up? What is produced?

Noticing each substance’s state.Using the triangles from Chem 11Using a road map, to convert the mole amount from one substance into another using a mole bridge.

Demo: Limestone with 6M HClBest fit curveDrawing a tangent at 5.0 minutes and at 9 daysKin Bkt p.6-7#2-3, p.11#14, p.12, p.13#6Rev Bkt p.2-4#3-5, p.8#15

Monitoring Reaction Rates (Colgur’s Notes: p.6-8)- properties which can be monitored (measured at specific time

intervals) in order to determine rx. rate.

Note : Must consider - physical states (s), (l), (g), (aq) - coefficients of gases - heat (endo or exo?)

- Do demo with Cu & HNO3discuss colour, mass, conc., pressure (volume) change

1.) Colour changes- only in reactions where coloured reactant is consumed or new

coloured product formed.

Ex-1 Cu(s) + 4HNO3(aq) Cu(NO3)2(aq) + 2H2O(l) + 2NO2(g) + heat

copper clear blue clear brown- in this case could measure - intensity of blue

- intensity of brown gas - penny disappearing

Ex-2 Cu(NO3)2(aq) + Zn(s) Cu(s) + Zn(NO3)2(aq) blue grey reddish colourless- as this reaction proceeds the blue colour fades

Complete ionic form:

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Cu2+(aq) + 2NO3-(aq) + Zn(s) Cu(s) + Zn2+(aq) + 2NO3-(aq)

net ionic: Cu2+(aq) + Zn(s) Cu(s) + Zn2+(aq) [Cu2+ is blue!]

- colour intensity can be measured quantitatively using a spectrophotometer

(see Hebden p. 4)rate = colour intensity

time

2.) Temp changes

- in exothermic reaction temperature of surroundings will _____increase__(hot)________ - in endothermic reaction temperature of surroundings will _____decrease__(cold)______

- measured in insulated container (calorimeter)rate = temp time

3.) Pressure changes (constant volume or sealed container)

- If more moles of gas (coefficient) in products pressure will go upZn(s) + 2HCl(aq) H2(g) + ZnCl2(aq) O m.o.g. 1 m.o.g. (mol of gas)

- If more m.o.g. in reactants - pressure will ______decrease_______

N2 (g)+ 3H2 (g) 2NH3 (g) 4 m.o.g. 2 m.o.g.

rate = pressure (constant volume) time

- If equal m.o.g., pressure will not change:

NO2(g) + CO(g) CO2(g) + NO(g) 2 m.o.g. 2 m.o.g.

4.) Volume change (constant pressure eg. balloon, manometer, eudiometer)

eg.) if more gas is produced, volume of balloon will increase

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rate = volume (constant pressure) time

5.) Mass changes

- if only one solid is used up- could remove periodically and weigh it:

Mg(s) + 2HCl(aq) H2(g) + MgCl2(aq)(periodically remove Mg and weigh what is left)

- if one gas is produced and escapes, measure mass of what’s left in container (mass of container and contents)

eg) heat + CaCO3(s) CaO(s) + CO2(g)

rate = mass of container & contents time

Note: it’s not practical to measure masses of (aq) substances separately since they are mostly water.

eg) Ca(s) + 2HNO3(aq) H2(g) + Ca(NO3)2(aq) We could use a pH meter to measure [H+]

6.) Changes in molar concentration of specific ions

eg) Mg(s) + 2HBr(aq) H2(g) + MgBr2(aq)complete ionic eqn: Mg(s) + 2H+(aq) + 2Br -(aq) H2(g) + Mg2+(aq) +

2Br -(aq) ion molecule

- could monitor [ H+]: it will decrease using pH meter

eg.) rate = [Mg 2 + ] [ Mg2+] will increase timeNote: Does the [Br -] change? ___No___ Explain.

It is a spectator ion and it is found both as a reactant and as a product.

- the concentration of a specific ion can be measured:- using a pH meter for H+

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Only CO2 gas escapes. So as CO2 escapes, the mass of the container and contents will decrease.

- periodic samples taken and titrated to measure conc.- using a eudiometer to measure the volume of H2(g) in mL

7.) Changes in Acidity [H+]

- special case of #6rate = [H + ] time

pH is a measure of acidity

pH 0 7 14<----------------- ------------------>more acidic more basic(less basic) (less acidic)

neutral

if H+ is a reactant (or any acid HCl, HNO3 , H2SO4 : S.A.Strong Acid)[H+] will decrease so pH will INCREASE!

(less acidic)

rate = pH time

Read Hebden p. 4-5 Ex. 7-9 page 5) Kin Bkt p.8, p.9#7Rev Bkt p.1-2

Fast and Slow Reactions:Fast reactions:1)Reactions between gaseous species

O2(g) + CH4(g) explosion (homogeneous rxn)2)Reactions between species that are miscible in liquid phase

C2H5OH(l) + CCl4(l) (homogeneous rxn)Reasons for fast reaction:

.speed of gaseous particles

.close proximity of liquid particles.Aqueous ions have fastest reaction rates because:

.close proximity to each other (particles are near each other)

.ability of ions to move through solvent (water).strong electrostatic (positive-negative) attractionsAgNO3(aq) + HCl(aq) AgCl(s) + HNO3(aq) (fast homogeneous rxn)

Rates:

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Measured with a pH meter

aqueous ions > gases or liquids >>> solids (fast) (slow)

Factors Affecting Both Homogeneous & Heterogeneous Rxn RatesColgur’s Notes p.9-112 Types of Reactions:

homogeneous and heterogeneous reactions (not mixtures!!)

.homogeneous rxn: reactants are in same phase( liquid, solid or gas).2 gases: g and g Ex: H2(g) + O2(g).2 substances dissolved in water: aq + aq Ex: AgNO3(aq) +

HCl(aq).2 miscible liquids (dissolved in each other) (water + alcohol)

.heterogeneous rxn: reactants are in more than one phases and l (sodium and water) Ex: Na(s) + H2O(l)g and l (hydrogen and water) Ex: H2(g) + H2O(l)g and s (combustion of wood-burning wood using oxygen gas)

Ex: C6H12O6(s)+ O2(g)l and l [immiscible; don't dissolve in each other: oil(l) + water(l)]

Big Question: What does it take to have a chemical reaction?.Resulting from effective collisions between particles.Not all collisions are successful.Effective collison: sufficient speed and alignment (correct geometry)

1) Temperature Temperature: average kinetic(moving) energy of particles.increase in T increases average velocities of gas molecules.in solution, increasing T of solvent increases rate at which most solutes dissolve

solvent: what does the dissolving (water)solute: what is dissolving (solid)Ex: sodium chloride(solute) dissolves in water(solvent).When T increases, the reaction rate increases

re: Hebden p. 18-19, Colgur’s Notes p.16-21.few molecules travel at very high speed .most molecules travel too slowly to produce effective collisions.T1: lower temperature with few effective collisions.energy threshold: minimum energy needed for collisions between atoms to form new molecules .T2: temperature is increased

.more molecules have high K.E.

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.more effective collisions > E (threshold energy)

.reaction rate is greater than T1 Analogy: Using the Bell Curve in evaluating university studentsRev Bkt p.5#9

Rule-of-thumb:(only for slow reactions).increase T of 10° C doubles reaction rate for slow reactions

ex: starch + iodine purple coloured complex (slow rxn)T1= 25°C Rate: 0.500 mol/sT2 = 35°C Rate: 1.000 mol/s

.When doubling a reaction rate, the increase is due to:.increased number of molecules with sufficient energy to react.not due to increased number of collisions

.it doesn't double the number of collisions.Changing temperature affects kinetic energy and number of collisions

Heb p.7#11, p. 19#29-32

2) Concentration 2 NO(g) + O2(g) 2NO2(g) + 114 kJ T = 25°C

Exp 1: 2 mol 1 mol reaction: 1 minuteExp 2: 2 mol 2 mol faster reaction

More molecules produce more collisions# collisions/sec increases, reaction rate increases.Rate of chemical reaction increases as the concentration of reactants is increased. More sites available for the particles to react.

3) Pressure .For a gas, concentration expressed as partial pressure

When pressure of a gaseous reactant increases, more reactant compressed into given volume (reactant concentration increases)A decrease in the volume of a reaction container increases the pressure (and its rate).

Reactant pressure increases, rate increasesDemo: Asking all students to move to one side of the classroom and increase the pressure by asking them to move more closely to each other

4) Reactants’ Nature Chem Rxn: Reactants Products

Types of Families/Groups:Most reactive metals: alkali metals or group 1

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Most reactive non-metals: halogens or group VII or group 17

Consider the following chemical reaction:Rxn of alkalis with water

Alkali + 2H2O(l) Alkali Cation+(aq) + 2OH-(aq) + H2(g) + energyEx: Li(s) + 2H2O(l) Li+(aq) + 2OH-(aq) + H2(g) + energy Rate: Li < Na < K << Cs <<<<<<<<< Fr Slowest FastestWhy?Within a family, the reaction rate goes faster as we proceed down a group.Reason: Shielding effect, the larger elements from a same family have more inner electrons that prevent the attraction of the outer electron towards the nucleusIt is easier to remove an outer electron down a group. (faster rate)Ex: Li(3) 1s22s1 (2 inner electrons) Cs(55) 1s22s22p63s23p64s23d104p65s24d105p66s1 (54 inner electrons)

Core configuration: [Xe] 6s1

*You Tube Video: Brainiacs:Reactions of alkaline metalsDemo: Li, Na, K with water done outside (acid/base unit)

Ex: lithium with water: conducts electricity, temperature given off, LiOH produced after evaporationSodium reacts faster than lithium in waterPotassium reacts faster than sodium in water

5) Bonding Action (i)Bond strengths aqueous ions/charged ions > polar covalent > covalent

uneven partial chargeFe3+ >>> H2O >>> O2

Ions are fast because there are free flowing in a solution and have a strong electrostatic attractions

(ii)Number of bonds being broken (Heb p.7)

Ex: The following reaction is relatively slow because many chemical bonds have to be broken and then reformed:

5 C2O42- + 2 MnO4- + 16 H+ 10 CO2 + 2Mn2+ + 8H2O (slow)

On the other hand, the following reaction is fast because no bonds are broken or formed, only a single electron is transferred:

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Fe2+ + Ce4+ Fe3+ + Ce3+ (fast)

6) Catalysts catalyst: substance that increases the rate of a chemical reaction without being used up itself

**Demo: peroxide and manganese dioxide to produce oxygenex: 2H2O2(aq) 2H2O(l) + O2(g) + 108 kJ very slow reactionwith adding manganese (IV) oxide, reaction rate increases dramatically:

MnO22H2O2(aq) ------- 2H2O(g) + O2(g)

Demo: Elephant toothpaste

inhibitor: reduces a reaction rate by preventing the reaction from occurringex: poisons, antibiotics, preservatives(foods and medical preparations)

Factors Which Affects Only Heterogeneous Reactions (More than one phase)

7) Effect of Surface Area ( Only for solid reactants and heterogeneous rxns!!!!)**lab: Reaction of zinc with hydrochloric acid

Zn: bar, strip, pellet, powder with 3M HCl(aqueous solution) Zn: powder > granular >>> mossy.atoms of zinc in the interior of pieces cannot react until they are in direct contact with the acid.Increasing the surface of zinc will increase rate of reaction.cutting/breaking zinc chunks into smaller pieces

The greater the surface area, the greater the rate of reactionLooking for the reactant with most exposed sites and this reactant has the largest surface area**Demo-Lycopodium powder: powder(s) + flame O2(g) heterogeneous rxn

Examples of reactions with a solid (surface area): fires occur explosively in grain elevators and coal mines

fine combustible powder (grain or coal dust): when in suspension in air and powder can ignite(burst into

flame)dust(solid) + methane(gas) + spark explosion !!!

.Reactions occurring in solid phase are slow:.solid + solid reacting.reactants can't move freely

To summarize, factors which affect the reaction rate are:

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a) Temperature: for all reactionsReason: larger fraction of collisions with sufficient energy (bell curve with energy threshold)

b) Concentration(aqueous solutions) more particles present increasing the probability of successful collisions

c) Pressure increase or volume decrease (for gas only) gaseous particles move faster and collide more often at higher pressure

d) nature of reactants less bonds to break, few e- to transfer: more effective

e) surface area (for solids only) more sites for successful collisions

f) catalysts and inhibitors lowers the activation energy so that there is an easier pathway.

Heb p.9#15-17Kin Bkt p.10-11#12-13Rev Bkt p.4-5#6-8, p.9#16,18, p.10

Activation EnergyWhy are we still alive?

2N2(g) + 5O2(g) + 2H2O(l) 4HNO3(l) + 121 kJDemo: Adding concentrated nitric acid to a penny

Where could we find a cheap and plentiful source of oxygen gas?Where can you find lots of nitrogen gas?Where can you find large amount of liquid water?

Since exothermic reactions are spontaneous, oxygen, nitrogen and water will spontaneously combine to make nitric acid.

Surely, anyone who dives into nitric acid will need plastic surgery to heal the chemical burns to their body (if you survive), it is suicidal to plunge into nitric acid!

Since oceans have existed for billions of years and contain negligible amounts of nitric acid, some type of wall must prevent N2, O2 and water to form nitric acid.

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This reaction does not appear to occur or seems to have a very slow rateAll collisions don't result in formation of new products

Activation energy: minimum energy needed before a molecule or chemical species to react

Molecules need to overcome the energy barrier

Colgur’s Notes p.25-26

Thermochemical Reactions:Is a reaction exo or endothermic?Kin Bkt p.4, Colgur’s Notes p.15Does the energy increase or decrease from the reactants to the products?

Exothermic reactions occur spontaneously and that endothermic reactions require heat to be added in order to occur.

Reaction Heatsenthalpy (H): total kinetic and potential energy which exists in a system at constant pressureHeat: total kinetic energy of all the particlesEnthalpy = Heat∆H = H prod -H react = change in enthalpy during the course of a reactionH prod: com

bined enthalpies of all the products H react :combined enthalpies of all the reactants

Exothermic Reaction

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Sign of ∆H:a) If a reaction is exothermic ∆H < 0

H prod < H reactEx: fossil fuel combustion

b) If a reaction is endothermic ∆H > 0products have more energy than reactants

H prod > H react

Ex: 2N2+ O 2+ 164 kJ 2N2O or 2N2+ O 2 2N2O ∆H = 164 kJ

164kJ is absorbed by the N2 and O2 (the system) comes from the surroundings (such as its container, the air...)

The loss of energy from the surroundings to the system causes the surroundings to feel coolerENdothermic reaction: Heat ENters the system

b) If a reaction is exothermic ∆H < 0products have less energy than reactants

H prod < H react

Ex: H2 + Cl2 2HCl + 184 kJor H2 + Cl2 2HCl ∆H = -184 kJ

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The reactants lose energy to become productsThe 184 kJ of heat is given off by the H2 and Cl2 (the system) and is absorbed by the surroundings.The loss of energy causes the surroundings to feel warmerEXothermic reaction: heat EXits the system.

Memory Aid:+ → - ( we read from left to right)heat term on left side(reactants’ side), then ∆H = positiveheat term on right side(products’ side), then ∆H = negativeColgur’s Notes p.15Heb p. 16 #24-28Kin Bkt p.5, p.14-17Rev Bkt p.5-8#9-14, p.9#17

Reactants Becoming ProductsKin Bkt p.3, Colgur’s Notes p.22-24 On a PE diagram, the peak is when the actual reaction occursIf reactants < Ea , there is no reaction taking placeMolecules with insufficient energy just bounce off each other without reacting

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Activated complex: arrangement of atoms when the reactants are in the process of rearranging to form products. (transition molecule)

Activation energy: Ea = the minimum potential energy required to change the reactants into the activated complex.

2 molecules approach each other to convert KE into PE, 3 possibles cases:

Case 1: KE < PE to reach activation energy .molecules will come to a halt before reaction can occur,

.they move back away from each other (rebound)

.Ineffective collision

Case 2: KE = PE.approaching molecules will come to standstill.reaction is possible although not guaranteed.molecules may separate without reacting

Case 3: KE > PE.reacting molecules will be locked together.effective collision

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In order to have a successful collision, a very small portion of the molecules possesses the minimun energy required for a reaction to occur. The reaction rate will be very slow.If this minimum energy were lower, more molecules would be sufficient KE and the reaction rate would be faster.Colgur’s Notes p.21-23

2 requirements for successful reaction. Molecules must have:a) sufficient KEb) correct alignment (geometry, orientation)

reactant must be in the proper position to reactex: parallel to each other, in a specific orientation

Colgur’s Notes p.27-31Kin Bkt p.14-17Rev Bkt p.5-8#10-14, p.11-12Heb. p. 23#33-40

Forward and Reverse ReactionsThere is no apparent reason why molecules can't go over the hill from:

.left to right

.right to left.reactants can form products and products can re-form reactants:

Reactants ⇌ Products

We need to specify the amount of energy of the activated complex for the forward or for the reverse reaction

Ea(f) = activation energy for forward reactionEa(r) = activation energy for reverse reaction

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NB: Activation energy is always endothermic, energy must be added to get to the top of the energy hill.

Relationship between E a (f) and E a (r): Ea(f) = Ea(r) + ∆Ha) Endothermic reactions

Heb p.25 #41-45

Reaction MechanismsColgur’s Notes p.31-33def: actual sequence of steps which make up an overall reaction

The process must be carried out in several steps in order for each reactant particle to line up and break/make bonds forming the product particle.

In the following reaction: 5 C2O42- + 2 MnO4- + 16 H+ 10 CO2 + 2 Mn2+ + 8 H2Othe probability of 23 reactant particles colliding together all at once is 0.(5 + 2 + 16 = 23 reactant particles)

.A complex reaction cannot take place in a single step; there must be more than one step involved in breaking and forming multiple bonds between molecules. Heb p.26-27Let’s consider the following reaction:

4HBr + O2 2H2O + 2 Br2

Step 1: HBr + O2 HOOBr (slow) RDSStep 2: HOOBr + HBr 2HOBr (fast)Step 3: 2 HOBr + 2HBr 2H 2O + 2Br2 (fast)

Overall Rxn: 4 HBr + O2 2H2O + 2 Br2

The slowest step is called the Rate-determining step (RDS) or bottle-neck step (Step 1 in the above reaction)

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Step 1: HBr + O2 HOOBr (experimentally found to be slow) RDS

NB: (i)The dots mean the bonds are in the process of forming or breaking.(ii) A plain or double line means a single or a double bond(iii) In the activated complex, the bonds are in the process of breaking/forming

Note: Prediction of reaction mechanism will never be required. You may be asked questions about a given reaction mechanism but you will not be asked to dream up a reaction mechanism on your own.

Note:If the reaction were instantly frozen, very little HOOBr or HOBr will be present-they can't build up in concentration since they are made very slowly

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and they react very fast.

Elementary process: individual step in a reaction mechanism

In the reaction:4HBr + O2 2H2O + 2 Br2There are 3 steps or elementary processes

Notes:

a) The overall rate is unaffected by adding HOOBr or HOBr (intermediates) since they are not used in the rate determining step and are quickly eliminated from the reaction.b) To determine the overall reaction from the reaction mechanism, simply add up all

the steps in the reaction equations and cancel any species which occur on both sides of the final equation.

c) The HOOBr and HOBr are called reaction intermediates or simply intermediates.

They can exist indefinitely on their own, but happen to react quickly in the above mechanism.

d) The formula of an activated complex is found by adding up all the atoms involved in the 2 reacting molecules.

Difference between a catalyst and an intermediate:

Intermediate CatalystFirst Appears as: A product A reactantThen, it is being used up as:

A reactant A product

Heb p.28#46-53

Energy Diagram of a Reaction MechanismRecall the reaction mechanism:HBr + O2 HOOBr (slow)HOOBr + HBr 2HOBr (fast)HOBr + HBr H2O + Br2 (fast)Each step involves individual activated complex and activation energy

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Since they are 3 steps, there are 3 “humps” in the PE diagrams for 4HBr + O2 2H2O + 2 Br2

∆H is negative and this is an exothermic reaction

Colgur’s Notes p.31-40Kin Bkt p.22-29Heb p. 30 #54-55Rev Bkt p.13-15#25-28

Effect of Catalysts on the Activation EnergyCatalyst: substance which provides an overall reaction with an alternative mechanism having a lower activation energy.energy hump is lowered by adding a catalyst

.If Ea is lowered by adding a catalyst, more reactant molecules possess the minimum KE required to form the activated complex.More molecules can react, forward reaction rate increases.Reverse reaction rate is also increased since reverse activation energy is lowered

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Notes

a) A catalyst is not a substance which takes no part in a reactionA catalyst is an active participant which is regenerated in a later step of the reaction mechanism

b) ∆H for the overall reaction is the same for both the catalyzed and uncatalyzed reaction.

The initial and final PE remain the same; only the intermediate reaction details differ.

c) Both intermediate species and catalysts cancel out when individual steps are added to get the overall reaction.

Heb p.34 #56-61, p.36 #62-63

Difference between an intermediate and an activated complex:

Intermediate: lives indefinitely on its own, stable species

Activated Complex: short-lived, high energy transition species where reactants become products

What is the probability of making successful collisions between several reactant particles?

Demo: At the same time, 3 students have to throw a tennis ball to hit teacher's face shield

If the balls don't hit the shield at once, there is no chemical rxnThere is no correct orientation and alignment

There is a extremely low probability of having successful collisions between several reactant particles in one step.

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Impossible to have correct alignments and positions and timing in one step.

If this reaction is broken down into several steps, the likelihood of probable successful collisions is greater.

Possible to align particles with correct geometry and timing.Key: To have few bonds to break at once.

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