CI 10.1 The collision theory of reactionsReactions occur when particles of reactants collide with a certain minimum kinetic energy. (Activation enthalpy)

Low concentration Higher concentration

Any factor which increases the number of collisions will increase the rate of reaction. As particles approach and collide, K.E. is converted to P.E. and the P.E. of reactants rises.

In the above exothermic reaction, existing bonds start to stretch and break and new bonds form. Only Reactants with sufficient K.E. will overcome the activation enthalpy and form products.

Activation enthalpy

H

Reactants

Products

Enthalpy

Progress of reaction

X

At higher temperatures, more of the colliding particles have enough energy to react.

Enthalpy profiles

The curved line is the energy pathway for a pair of colliding molecules (called the energy or enthalpy profile for the reaction).X corresponds to the arrangement of atoms where old bonds are stretched and new bonds are starting to form.Eg. For chlorine atoms and ozone, the oxygen – oxygen bonds in ozone break and a new bond between oxygen and chlorine is forming.Cl + O3 Cl------O------O O ClO + O2

At X(this is a very unstable arrangement and only lasts for a very short time).

The enthalpy profile is shown as a single curve (this is only true for single-step reactions). Other reactions have several steps, so the enthalpy profile would show several steps, too.

do problems for 10.1 ‘Rates of Reactions’ p. 222-223 questions 1 to 4.Questions

Activation enthalpy

H

Reactants

Products

Enthalpy

Progress of reaction

X

1. For each of the following reactions, say how, if at all, you would expect its rate of reaction to be affected by the following factors:

a) Temperatureb) Total pressure of gas.c) Concentration of solutiond) Surface area of solid

a b CA Rate will

increase with temperature

Rate will increase with temperature

Rate will increase with temperature

B Rate of forward reaction not affected

Rate increases Rate of forward reaction not affected

C Increasing the concentration of acid will increase the rate

Solutions not involved

Increasing the concentration of peroxide will increase the rate

D The more finely divided the magnesium, the faster the rate

The more finely divided the catalyst the faster the rate

Solids not involved

2.) Both the acid and the enzyme can act as catalysts for the hydrolysis of a protein.3. a) The greater the concentration of reactants, the greater the

rate of collisions and hence the faster the reaction proceeds.3 b) A change of temperature has little effect. Most collisions result in a reaction.4 a) B and C b) A and D c) D d) Be) B f) D

CI 10.2: The effect of temperature on rate

For many reactions, increasing the temperature by 10oC will DOUBLE the rate of reaction.

Extending the collision theory of reactionsConsider the reaction in the Haber process for making ammonia:

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

The collision theory says that N2 and H2 will only react when they collide. The more frequently they collide, the faster the rate of reaction. Increasing the pressure brings H2 and N2 closer together, so they collide more often. Increasing the temperature makes molecules move faster, increasing the frequency of collisions.

How much more frequently do molecules collide, when temperatures rise?The average speed of molecules is proportional to the square root of the absolute temperature.Example: Temperature rise: 300 K to 310 K

Increase in average speed of molecules: (310/300)1/2 = 1.016

(this is 101.6%...so an increase of 1.6% faster)Particles must also react with a certain minimum kinetic energy.In the Haber process, (reaction between H2 and N2)at 300 K only 1 in 1011 collisions between H2 and N2 results in a reaction! Even at 800 K only 1 in 104 collisions results in a reaction.The collision theory says:Reactions occur when molecules collide with a certain minimum kinetic energy. The more frequent these collisions, the faster the rate of reaction.The energy needed to overcome the energy barrier is called the activation enthalpy, for the reaction. It is the energy needed to

start breaking bonds in the colliding molecules, so that collisions can lead to reactions.

The distribution of energies

Molecules are moving at different speeds, at any given time. Some have higher energies, some medium energies, others have lower energies.

Maxwell-Boltzmann distribution: distribution of kinetic energies in a gas, at a given temperature.

As the temperature increases, more molecules move at higher speeds and have higher kinetic energies.

Num

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f mol

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Kinetic energy (E)

300 K

Kinetic energy (E)

Num

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300 K

310 K

What is the significance of this for reaction rates?Eg. the activation enthalpy for a reaction is: Ea is +50 kJ mol-1.How many collisions have MORE energy than +50 kJ mol-1?

Only collisions with energies in this region can lead to a reaction.

At higher temperatures, a significantly higher proportion of molecules will have energies above +50 kJ mol-1.

300 K

Kinetic energy (E)

300 K

Num

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Num

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310 K

About twice as many molecules have enough energy to react –so the reaction goes twice as fast.

SL: What is removing the ozone?Several radicals remove ozone, in the stratosphere:Chlorine atom, ClBromine atom, BrChloromethane, CH3Cl Bromomethane, CH3Br

Several reactions occur in the stratosphere:

Chlorine-containing molecules absorb high energy solar radiation and break down to give chlorine atoms.

Reaction 6: Cl + O3 ClO + O2

(chlorine atoms react with ozone, forming radicals.)Reaction 7: ClO + O Cl + O2

(The ClO radical then reacts with oxygen atoms.)

We now have TWO reactions, competing to remove ozone:Reaction 4 O + O3 O2 + O2 andReaction 6 Cl + O3 ClO + O2

The concentration of Cl atoms in the stratosphere is much less than the concentration of O atoms. How significant is reaction 6?

do assignment 7, p.69

Kinetic energy (E)

From the oceans, burning coal / vegetation (eg forest fires); they are the source of Br and Cl in the atmosphere.

Chlorine atoms are particularly effective at removing ozone. A single atom can remove about 1 million ozone molecules.Add equations 6 and 7 together to produce the equation for the overall reaction caused by chlorine atoms.Comment on the result. What role are Cl atoms playing in the overall reaction?

(O + O3 O2 + O2 ) The radicals are regenerated, and so acting as catalysts.

It is important for chemists to know which reaction (6 or 7) is happening fastest, to understand whether oxygen atoms or chlorine atoms are responsible for the removal of ozone.

In fact, Cl atoms react 1500 faster with ozone, compared to O atoms. Even though Cl atoms have a much lower concentration in the stratosphere than O atoms, the fact that they can be regenerated in a catalytic cycle can have devastating effects.

Bromine is 100 times more effective at destroying ozone than chlorine, despite being much less concentrated than chlorine.

CI 10.5 How do catalysts work? Collision theory and enthalpy profiles can help us to understand how catalysts work. Bond breaking is endothermic (requires an energy input to overcome the activation enthalpy barrier). Bonds first stretch and then break, reactants are made and new bonds instantly form. If few molecules have enough energy to overcome this activation enthalpy, then the reaction will be slow.Catalysts provide an alternative reaction pathway for breaking and remaking bonds that has a lower activation enthalpy.

Catalysts and equilibrium

Catalysts only affect the RATE and not the position of equilibrium in a reversible reaction. The composition of the reaction mixture remains unchanged.

Homogeneous catalysts

A homogeneous catalyst forms an intermediate compound before breaking down to the final product and reforming the catalyst again. This is why the enthalpy profile shows TWO humps.

Activation enthalpy uncatalysed reaction

HReactants

Products

Enthalpy

Progress of reaction

Activation enthalpy catalysed reaction

HReactants

Products

Enth

alpy

Progress of reaction

Intermediate compound

Final product

CFCs act as homogeneous catalysts in the stratosphere, breaking down ozone. Cl atoms catalyse the reaction, forming the intermediate ClO:

Cl + O3 O2 + ClO intermediateClO + O Cl + O2

O3 + O O2 + O2 overall changeA single Cl atom can catalyse the reaction of many ozone molecules through a catalytic cycle.Industry uses mostly heterogeneous catalysts. However homogeneous catalysts can be more specific and controllable.

Eg. Methanol ethanoic acid Conversion is 99% with soluble rhodium compounds. Do problems for 10.5 p.243 questions 1 and 2.

SL: Other ways ozone is removedRadicals such as hydroxyl and nitrogen monoxide can destroy ozone, as well as chlorine and bromine. In general: X + O3 XO + O2

XO + O X + O2

Overall reaction: O + O3 O2 + O 2

Hydroxyl radicals (HO) form in the stratosphere when water molecules react with oxygen. The reaction with ozone is:

HO + O3 HO2 + O2

HO2 + O HO + O2

The reformed HO radicals can react with more ozone, in a catalytic cycle.Nitrogen monoxide (NO) forms nitrogen dioxide and dioxygen when it reacts with ozone. NO and NO2 are relatively stable radicals which can be collected in ordinary ways.

do assignment 8, p.71a) Write an equation to show the formation of HO radicals from

O atoms and water.H2O(g) + O(g) HO + HO

Rhodium (aq)

b) Write equations to show how nitrogen monoxide can destroy ozone in a catalytic cycle.

NO + O3 NO2 + O2

NO2 + O NO + O2

SL A4: The CFC storyIn the early 1970s there was concern about jet aircraft releasing NO in their exhausts. Levels were not significant at the time. In 1974 CFCs became a concern… read about it p.71-74 and do assignments 9 and 10.

CI 13.1 HalogenoalkanesAre man-made compounds with one or more halogen atoms (F, Cl, Br, I) attached to a carbon atom. The attached halogen changes the chemical properties of alkane chains…they are very unreactive, and so have been very useful to humans.

Naming halogenoalkanes (haloalkanes)(similar rules to naming alcohols, just add the halogen as a prefix):

halogens are in alphabetical order. lowest numbers possible are used.

CH3CH2CH2Cl is 1-chloropropaneCH3CHClCH2Cl is 1,2-dichloropropaneCH3CHBrCH2CH2Cl is 3-bromo, 1-chlorobutane.CH3CHICHBrCH2Cl 2-bromo,1-chloro,3-iodobutane2-bromo, 3-chloro, 1-iodopentaneCH2ICHBrCHClCH2CH3

Physical properties of halogenoalkanes immiscible with water The bigger the halogen atom /the larger the number of

halogen atoms the higher the boiling point.

Larger halogen atoms (Br or Cl) cause greater environmental damage than smaller halogen atoms (F); this is important when designing replacements for CFCs.

Chemical reactions of halogenoalkanesCarbon –halogen (C-Hal) bonds can break either homolytically or heterolytically.

Homolytic Fission forms radicals eg when a halogenoalkane absorbs radiation of the right frequency.

H H

H C Cl + hv H C + Cl

H HChloromethane methyl radical chlorine radical

Shorthand is: CH3-Cl + hv CH3 + Cl (occurs in stratosphere).

Heterolytic fission is more common in lab conditions using polar solvents such as ethanol or ethanol and water. The polar C-Hal bond can break, leaving a negative halide ion and positive carbocation.

CH3 CH3

CH3 C Cl + hv CH3 C+ + Cl-

H H2-chloro-2-methylpropane carbocation chloride ion(negatively charged substances may react with the positive carbocation causing a substitution reaction).

Importance of reaction conditions…for determining how bonds break

Eg. Bromoethane C-Br bonds break:

Heterolytically , forming ions when dissolved in a polar solvent (say a mixture of ethanol and water) BUT

Homolytically , in the gas phase at high temp. or when dissolved in a non-polar solvent, such as hexane.

Different halogens, different reactivity.All reactions with halogenoalkanes involve breaking the C-Hal bond. The C-F bond is the strongest (bond enthalpy 467 kJmol-1) and therefore the hardest to break, whereas the C-I bond is relatively weaker (228 kJmol-1) and therefore easier to break. C-Hal bonds get weaker, and so more reactive, down group 7. Chloro compounds are fairly unreactive and remain in the troposphere long enough to reach the stratosphere, where they react with and destroy the ozone layer.

Substitution reactions of halogenoalkanes

Halogenoalkanes can hydrolysed by hydroxide ions to form alcohols.

Eg. Bromobutane forms butanol:

CH3–CH2–CH2–CH2–Br + OH- CH3–CH2–CH2–CH2–OH + Br-

The C-Br bond is polar The oxygen atom on OH- is –vely charged. C–Br H–O

The partial positive charge on the carbon atom attracts the negatively charges oxygen of the hydroxide ion. A lone pair of electrons on the O atom forms a bond with the C atom as the C__Br bond breaks.

H H H H H H H H _H__C__C__C__C__Br H__C__C__C__C__O__H + Br H H H H H H H H _

Heterolytic fission results in IONS and not radicals.Curly arrows show the movement of electrons (full headed arrows for a pair of electrons…unlike radical reactions).

O H

Halogenoalkanes can give substitution reactions with hydroxide ions and other NUCLEOPHILES. Nucleophiles can donate a pair of electrons to a positively charged carbon atom to create new covalent bonds.

Some common nucleophiles:

Name Formula Structure showing lone pairs

Hydroxide ion OH- _

H__O

Ethanoate ion CH3COO- _CH3

__C__O O

Ethoxide ion C2H5O- _CH3CH2

__O

Water molecule H2O O H H

Ammonia molecule NH3 N H H H

Cyanide ion CN- _ N C

The carbon atom attacked by the nucleophile may be part of a carbocation and carry a full positive charge, or it may be part of a

neutral molecule (as in the above example with bromobutane) and carry a partial positive charge as a result of bond polarisation.

If X- represents a nucleophile, the nucleophilic substitution process is:

C–Hal C__X + Hal-

X

Water as a nucleophile

Nucleophiles may be neutral or have negative charge, so long as it has a lone pair of electrons which can form a bond to a carbon atom.Eg. Water has 2 lone pairs of electrons on the O atom. First it attacks the halogenoalkane (bromobutane in this case):

H H H H H H H H _H__C__C__C__C__Br H__C__C__C__C__O__H + Br H H H H H H H H H

O H H

The resulting ion loses H+ to form an alcohol:

H H H H + H H H H H__C__C__C__C__O H__C__C__C__C__O__H + H+

H H H H H H H H H

The overall equation for the reaction of water with a genera; halogenoalkane R__Hal is:

R__Hal + H2O + R__OH + H+ + Hal-

Ammonia as a nucleophile

A lone pair of electrons on the N (similar to water) attacks the halogenoalkane to produce an AMINE with an NH2 group:

R__Hal + NH3 R__NH3+ Hal- R__NH2 + H+ + Hal-

Using nucleophilic substitution to make halogenoalkanes

Halogenoalkane + OH- alcohol

Halogenoalkanes can be made via the reverse reaction of making alcohols; the nucleophile is Hal-.

Eg. 1-bromobutane is made using a nucleophilic substitution reaction between butan-1-ol and Br- ions, in the presence of a strong acid.Ist step: H+ ions bond to O atom on the alcohol:

H H H H H H H H + H__C__C__C__C__O H__C__C__C__C__O__H H H H H H H H H H

H+

This gives the C atom to which the O is attached a greater partial positive charge. It is now more readily attacked by Br- ions, forming bromobutane.

H H H H + H H H H H__C__C__C__C__O H__C__C__C__C__Br + H2O H H H H H H H H H

_Br

The overall equation for the reaction is:

CH3CH2CH2CH2OH + H+ + Br- CH3CH2CH2CH2Br + H2O

Activity A4.2 Problems for 13.1 pages 303- 304 questions 1- 9.