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Factors affecting Rates

Mr. Shields Regents Chemistry L13 L05

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What affects Reaction What affects Reaction RatesRates

Let’s look at the factors that can affect how fasta reaction occurs. In other words, What affects reactionRate?

It turns out there are 5 things that are important:

1)Nature of the reactants2)Temperature3)Concentration4)Surface area5)Catalysts

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Nature of the ReactantsNature of the ReactantsThe basic question we must ask when we’re considering the Nature of Reactants is …

“ how complex are the bonds to be broken and how complex are the bonds that need to be formed”

And this in turn is affected by several other factors that affect the rate of reaction. Namely…

1) If there are a lot of bonds to be broken the reactionWill probably proceed more slowly than a rxn in whichOnly a few bonds need to be broken

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Nature of the ReactantsNature of the Reactants

2) HOMOGENEOUS RXNS are more likely to proceed more Quickly than HETEROGENEOUS RXNS

For Example …

Fe(s) + O2(g) Fe2O3(s) heterogeneous reaction

H2(g) + Cl2(g) 2HCl(g) homogeneous reaction (faster than heterogeneous rxn)

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Nature of the ReactantsNature of the Reactants3) If atoms or molecules can move freely about they are alsoMore likely to react more quickly

NOTE that in general reaction rates follow these trends …

a)Gas rxn rates > Liquid rxn rates >> Solid rxn rates

b) Ionic rxns >> inorganic rxns > Organic rxns (ions in sol’n) Fe(s) + S(s)

Let’s see why …

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Nature of reactantsNature of reactantscase (a): Gas rxn rates > Liquid rxn rates >> Solid rxn rates

- Higher molecular velocities (i.e. higher KE)- The higher the velocities the greater the number of

collisions/unit time

In case (b): Ionic rxns >> inorganic rxns > Organic rxns

- ions have a charge and quickly attract ions of opposite charge (nearly instantaneous - happens in solution)

- inorganic compounds form bonds by electron transfer. This is a faster process than bond formation by electron sharing (organic type compounds)

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TemperatureTemperature

Recall that as KE (i.e. Temperature) increases so do theNumber of molecules that have higher molecular velocities

KE = ½ mv2

Inc Temp (KE)

# of Molecules

T1 T2Energy exceeding EA

More molecules at T2With KE > EA

Collision theory says I need an effective collision for reactionTo occur. What 2 factors determine whether a collision Will be effective?

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TemperatureTemperature

The general rule ofThumb is that forEvery 10 deg increaseIn temperatureRxn rate increases2x

-Inc Velocity-Inc KE

Slow

Faster

Fastest

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ConcentrationConcentrationNature of reactants and temperature will affect rxn rate.Third on our list of factors affecting Reaction rate is CONCENTRATION

Why would a change in concentration (up or down) affectReaction rate?

Sure… referring back to Collision Theory, we know we needan affective Collision for reaction to occur.

AND …

Increasing the number of molecules will have several positiveImpacts.

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ConcentrationConcentration

By Increasing the Concentration of reactants you …

1)Decreases time between collisions

2)And there are now Greater number of molecules in a givenvolume with KE equal to or greater than the EA

Slower Rxn

Faster Rxn

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ConcentrationConcentration

Let’s consider what would happen if we put a match toSteel (Fe) wool in Air (O2 (18%), N2, CO2 etc)?

Fe + O2 (air) Fe2O3 + heat

Now, what would happen if we put a match to steel wool inpure O2?

The same thing but now the RATE is much greater

Why???

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Factors affecting RatesFactors affecting Rates

So far we’ve discussed three factors affecting ReactionRates:

1)Nature of reactants2)Temperature3)Concentration

We’re now going to discuss the last two factors affectingReaction Rate…

4) Surface area5) Catalysts

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Surface AreaSurface AreaEver go camping? Ever sit around a campfire?

Which burns faster on the fire, a large log or a log split into several smaller pieces?

Obviously the log split into several pieces. Why?

Of course … Because there’s more surface area to expose The wood to Oxygen needed for combustion.

If we expose more surface area we increase the frequencyOf collisions between the Oxygen and the wood

- more collisions means more effective collisions and therefore a faster reaction rate

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Surface AreaSurface AreaVery fine powder or dust can sometimes explodeSpontaneously when dispersed into the air (i.e. O2)

Exploding Grain elevators and coalMines are examples

The very fine grain or coal dust suspended in the airreacts with O2 at extremely high rates due the extremely Large surface area producing rapidly expanding gases …

Iron powder

In other words,An explosion!

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CatalystsCatalystsThe thing to remember about catalysts is

1)They effect the rate of rxn EQUALLY in the forward and reverse direction.

AND

2) They do not affect the final equilibrium concentrations

- They ONLY affect how quickly a reaction reaches Equilibrium Concentrations

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CatalystsCatalysts- Catalysts are also NOT CONSUMED in a chemical reaction

- they are simply expeditors speeding up the reaction- after reaction they can be recovered unchanged

In our bodies ENZYMES act as catalysts to speed up biochemical reactions.

Catalyst Gathering the Catalyst & & Reactants Reactants Product

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CatalystsCatalystsThe way catalysts perform their trick is to simply ALTERTHE REACTION PATHWAY

So what do we mean by“the reaction pathway”

Think of it this way …

A reaction must climb anEnergy hill (EA). The catalystmakes the hill theReactants must climbA lot smaller

No Catalyst

Takes less ETo get to the Other side

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CatalystsCatalystsThis change in the reaction pathway is accomplished by…

1) Creating an INTERMEDIATE between the REACTANTS and the CATALYST

2) The INTERMEDIATE then becomes PRODUCT and REGENERATES the catalyst

By creating this newIntermediate the EA

Is decreased