Chemistry - an illustrated guide to science (794128), страница 24
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890 kJof energy are released per mole ofmethane combusted.EA = activation energy2 Endothermic∆H = heat of reaction● Inan en dotherm ic reaction, energy istaken in, and the temperature of thereaction mixture decreases as thereaction proceeds. The products are ata higher energy than the reactants.● The energy taken in is due to anincrease in the enthalpy, !H, of thesystem. Therefore, !H is positive foran endothermic reaction.● The following equation represents thesteam reforming of methane:2 EndothermicEACH4 (g) + H2O(g) ➞ 3H2(g) + CO(g)!H = +20 6 kJ mol -1EnergyThis reaction is an endothermicreaction. 206 kJ of energy are taken inper mole of methane reformed.reactants∆H© Diagram Visual Information Ltd.products140CHEMICAL REACTIONSKey wordsdissociationenthalpyAverage bonddissociation energies1 Average bond enthalpy1 Average bond enthalpy● Bonddissociation energy is theenergy change when one mole ofbonds is broken.
It refers to a specificbond in a molecule. However, theexact value depends on the localenvironment of the bond. Forexample, if the C-H bonds in methaneare broken one after another, each willhave a different bond dissociationen thalpy:CH4 (g) ➞ CH3(g) + H(g)!H = +425 kJ mol -1CH3(g) ➞ CH2(g) + H(g)!H = +470 kJ mol -1CH2(g) ➞ CH(g) + H(g)!H = +416 kJ mol -1CH(g) ➞ C(g) + H(g)!H = +335 kJ mol -1this reason, in a moleculecomposed of more than one atom, it ismore useful to know the averageamount of energy needed to break aparticular bond.● Thetable at right utilizes the completecombustion of propane to illustratehow bond enthalpies can be used toestimate the enthalpy change in areaction.● 6,488 kJ mol-1 of total energy is takenin to break the bonds.● 8,542 kJ mol-1 of total energy is givenout when the bonds are formed.● The enthalpy change when 1 mole ofpropane is completely combusted is6,488 – 8,542 = 2,054 kJ mol-1.Average bondenthalpy / kJmol–1BondAverage bondenthalpy / kJmol–1C–C347C=O805C=C612H–Cl432C–Cl346H–H436C–H413N–H391C–N286O–H464C–O336O=O4982 Estimating the enthalpy change in a reaction.Complete combustion of propane.C3H8 (g) + 5O2(g) ➞ 3CO2 (g) + 4H2O(g)Energy is taken in to break bonds:BondAverage bonddissociationenergy / kJ mol–1Numberof bondsEnergy takenin / kJ mol–1C–HC–CO=O4133474988253,30 46942,490● For2 Estimating enthalpychangeBond6,488Total energy taken inEnergy is given out when bonds are formed:BondAverage bonddissociationenergy / kJ mol–1Numberof bondsEnergy takenin / kJ mol–1C=OO–H80 5464684,8303,712© Diagram Visual Information Ltd.Total energy taken inThe enthalpy change when 1 mole of propane is completely combusted is6,488 – 8,542 = 2,054 kJ mol –1.8,542141Catalysts: characteristics1 Characteristics of catalystsKey wordsStoichiometryThe overall stoichiometry of a reaction is unaltered.SpecificityCatalysts may alter the rate of one reaction but have no effecton others.Reaction mechanismA catalyst provides an alternative reaction pathway for areaction to take place.Chemical involvementA catalyst is chemically involved in a reaction.
It is consumedduring one step and regenerated in another. A catalyst doesnot undergo a net chemical change, but it may change itsphysical form.EquilibriumA catalyst speeds up the rates of both forward and backwardreactions, and this speeds up the rate at which equilibriumis attained.YieldCHEMICAL REACTIONSA catalyst does not alter the yield of a reaction.2 Increasing reaction rateManganesedioxidecatalystManganese dioxideremains at the endof the reactionactivation energyactive sitecatalysteffective collisionCatalysts● A catalystis a substance that alters therate of a chemical reaction but remainschemically unchanged by it.1 Characteristics ofcatalysts● Catalystsmay be classified ashomogenous or heterogeneous.Homogenous catalysts are in the samephase (solid, liquid, or gas) as thereactants; heterogeneous catalysts arein a different phase.● A large number of reactions arecatalyzed on the surface of solidcatalysts.
The surface provides activesites where reactions can occur. Thus,an increase in the surface area willincrease the effect of the catalyst.2 Increasing reaction rate● Hydrogenperoxide decomposes veryslowly on its own to form water andoxygen gas:2H2O2(aq) ➞ 2H2O(l) + O2(g)The rate of this reaction is greatlyincreased by adding manganesedioxide, MnO2.● Manganese dioxide acts as a catalystand remains unchanged after all of thehydrogen peroxide has decomposed.EA for catalyzed reactionExtra fraction of particles with E >EAfor catalyzed reactionEA for uncatalyzed reactionFraction of particles withE >EA for uncatalyzedreactionEAKinetic energy (E)EA3 Activation energies● A catalystlowers the minimum energy,or activation en ergy ( EA), required fora reaction to occur.
The frequency ofeffective collision s is, therefore,increased, resulting in an increase inthe rate of a reaction.© Diagram Visual Information Ltd.Number of particles with energy (E)3 Distribution of the kinectic energies of reacting particlesand the activation energies for catalyzed and uncatalyzedreactions142CHEMICAL REACTIONSKey wordscatalystequilibriumexothermicLe Chatelier’sprincipleoxidationoxidation statereductiontransition metalsvanadiumCatalysts: transitionmetals1 Transition metals and reaction catalyzedTransitionmetal/ compound1 Reaction catalyzed● Catalystsare often tran sition m etalsor transition metal compounds.Transitional metals are useful ascatalysts because or their ability toexist in different oxidation states.2 V20 5 as catalyst● Thecontact process is an importantstep in the manufacture of sulfuricacid (see pages 85 and 86).
Sulfurdioxide is oxidized to sulfur trioxide inthe presence of a van adiu mpentoxide, V2O5, catalyst.● This reaction involves the redu ctionand subsequent oxidation of thecatalyst. In the reduction reaction, theoxidation state of vanadium changesfrom + 5 to + 4. In the oxidationreaction, it changes back from + 4 to+ 5.3 Iron as catalyst● TheHaber process for themanufacture of ammonia uses finelydivided iron as the catalyst (see pages74 and 75):polymerization of ethene to poly(ethene)V2O5contact process in production of sulfuric acidFeHaber process on production of ammoniaNihydrogenation of alkenes in hardening of vegetable oilsCuoxidation of ethanol to ethanalPtoxidation of ammonia in manufacture of nitric acidV2O5(s)2SO2(g) + O2(g)2SO3(g)SO2 + V2O5SO3 + V2O42V2O4 + O22V2O53 Iron as catalyst in Haber processFe(s)(Energy profiles for the reaction N 2 + 3H2activation energy foruncatalyzed reaction = 668kJN2 + 3H22NH3!H = -92 kJ mol -1activation energy forcatalyzed reaction = 212kJ● This2NH3)catalyzed reactionuncatalyzed reaction70 060 050 0Energy content (kJ)© Diagram Visual Information Ltd.TiCl 32 Vanadium oxide as catalyst in contact processFe(s)reaction is exotherm ic.
Accordingto Le Chatelier’s prin ciple, a lowtemperature would produce moreammonia in the equ ilibriu m mixture,but it would take longer to reachequilibrium.● The catalyst does not alter the yield ofammonia in the equilibrium mixture,but it does increase the speed withwhich equilibrium is attained.
Using acatalyst, a reasonable rate of reaction isachieved at a lower temperature thanwould otherwise be the case.Reaction catalyzed40 030 020 010 0N2 + 3H202NH3–10 0heat of reaction = –92kJ–20 0–30 0143Oxidation and reductionCHEMICAL REACTIONSKey words1 OxygenOxidation is the addition of oxygen to a substanceReduction is the removal of oxygen from a substanceoxidationoxidation stateredox reactionreductionEvolving definition● Overtime, scientists have extendedthe definitions of oxidation andredu ction .1 Oxygen● Historicallythe terms oxidation andreduction were applied to reactionsinvolving either the addition or theremoval of oxygen.
For example:2 HydrogenOxidation is the removal of hydrogen from a substanceReduction is the addition of hydrogen to a substance2Cu(s) + O2 ➞ 2CuO(s)copper is oxidizedFe2O3(s) + 3CO(g) ➞ 2Fe(s) + 3CO2(g)iron is reduced2 Hydrogen● Theterms were extended to includethe removal or addition of hydrogen:CH3-CH3(g) ➞ CH2=CH2(g) + H2(g)ethane is oxidized3 Modern definitionCH3COH(l) + H2(g) ➞ CH3CH2OH(l)ethanal is reducedOxidation is the loss of electrons from a substanceReduction is the gain of electrons by a substance3 Modern definition● Theterms oxidation and reduction arenow used more widely to describechanges in oxidation state:Cu(s) ➞ Cu2+ (aq) + 2e-copper is oxidized to copper(II)Fe3+ (aq) + e- ➞ Fe2+ (aq)iron(III) is reduced to iron(II)definition covers all of thosereactions involving the gain or loss ofoxygen and other reactions that do notinvolve oxygen.● This4 Redox reactionoxidation4 Redox reactionMg(s) + Cu 2+ ➞ Mg 2+ (aq) + Cu(s)reductionthat involve a reductionmust also involve an oxidation.
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