Lesson ThreeFactors that Affect the Rate of

Reaction.

Alfred Nobel amassed a fortune during his

lifetime from his 355 inventions, dynamite is the most famous.

In 1888, opened a newspaper to read his own obituary. As it was Alfred's brother Ludvig who had died, the obituary was eight years premature. Nobel s reflection on his life’s impacts inspired him to leave his fortune to create prizes for the"greatest benefit on mankind" in physics, chemistry, peace, physiology or medicine, and literature.

Nobel's instructions named a Norwegian Nobel Committee to award the Peace Prize.

Now, Norway's Nobel Committee was responsible

for awarding the Nobel Peace Prize and the Swedish institutions retained responsibility for the other prizes.

Alfred Nobel

nitroglycerin

3

Nature of Reactants

Table salt is an ionic compoundtransfer electrons from the metal to the non-metal to form an easily dissociated compound (just add water)

◦ Sodium chloride, NaCl (s), is a comprising sodium ions, Na+, and chloride ions, Cl–.

◦Cation (+) and anion (-) attract like magnets

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Nature of Reactants

Free energy (if you get energy or put it in) can be used to predict whether a process is spontaneous (favorable) or nonspontaneous (not favorable)

ΔG < 0spontaneous

ΔG > 0nonspontaneous

Which one would you

favor?

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Nature of ReactantsThe same principles can be applied to chemical reactions to predict whether they are favorable or not:

FreeEnergy(G)

Progress ofreaction

Α → B

A

Β

ΔG < 0spontaneous Free

Energy(G)

Progress ofreaction

Α → B

A

ΒΔG > 0nonspontaneous

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Nature of ReactantsJust because a reaction is spontaneous, does not mean that it will occur at an observable rate.

Diamond and graphite are two different forms of pure carbon.

The reaction that converts diamond to graphite is actually a favorable one

FreeEnergy(G)

Progress ofreaction

Diamond → Graphite

Diamond

Graphite

ΔG < 0spontaneous

Why can Beer’s say

diamonds are forever?

7

Nature of ReactantsThere is is a hill that for most reactions the reactants must climb and go over to before they can go on to become product.

FreeEnergy(G)

Progress ofreaction

Α → B

A

Β

ProductsReactants

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Nature of Reactants

The amount of energy you need to put is the activation energy, Eact.

◦ The activation energy has no effect on the overall change in the free energy for the reaction.

FreeEnergy(G)

Progress ofreaction

Α → B

A

Β

ΔG < 0spontaneous

Eact > 0

Products

Reactants

Energy you put-in

Energy you get (gain)

In class demo

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Nature of Reactants

So before we go calling DeBeers can you think of an anwers to why the reaction that converts diamond to graphite is actually a favorable one – but it doesn’t happen often (or quickly)

FreeEnergy(G)

Progress ofreaction

Α → B

A

Β

ΔG < 0spontaneous

Eact > 0

Products

Reactants

11

Nature of ReactantsThe activation energy for the conversion of diamond to graphite is very, very,very high.

FreeEnergy(G)

Progress ofreaction

ΔG < 0spontaneous

Eact > 0

Diamond → Graphite

Diamond

Graphite

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Nature of ReactantsThe reaction rate (speed) of a reaction is determined by the height of the hill.

◦ The higher the activation energy, the slower the reaction rate.

But WHY, How is this connected to the Nature of the reactants?

What takes energy to make and break?

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Activation energy: needed to break/make bonds

We can estimate by: Counting the number of bonds being broken and made

5C2O42- + 2MnO4

- + 16H+ → 10CO2 + 2Mn2+ + 8H2O

Many bonds need to be broken and made =slow reaction.

Fe2+ + Ce4+ → Fe3+ + Ce3+

This is a fast reaction because no bonds are being broken

( only 1 electron is being transferred)

Reactants with strong bonds = more Ea=slower reaction rate  Reactants with weak bonds =smaller Ea= faster reaction rate

In class demo

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Reaction Rates

We know two ways to speed up the reaction rate. We just haven't thought about them as overcoming activation energies but now we will.

◦ Increase the temperature of the reactant molecules.• This increases the kinetic energy, which increases the motion of

the reactant molecules. This increases the frequency with which they will collide with one another to react.

◦ Increase the concentration of the reactant molecules.• This increases the number of reactant molecules. This also

increases the frequency with which they will collide with other reactant molecules.

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Collide?We will dive into collision theory next class for now:

Increase the temperature of the reactant molecules.

• This increases the kinetic energy, which increases the motion of the reactant molecules. This increases the frequency with which they will collide with one another to react.

Increase the concentration of the reactant molecules.• This increases the number of reactant molecules. This also

increases the frequency with which they will collide with other reactant molecules.

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Reaction RatesThere is a third way to speed up the reaction rate and that is to lower the height of the hill.

◦ This is done using catalysts, which provide an alternative pathway over the hill for the reactants.

FreeEnergy(G)

Progress ofreaction

Α → B

A

Β

ΔG < 0spontaneous

Eact > 0without catalyst-with catalyst

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Reaction RatesCatalysts speed up a reaction, by lowering the activation energy but are not produced or consumed in a reaction.

◦ In the reaction equation, their presence in indicated above or below the reaction arrow.

They have not effect on the change in free energy for the reaction, ΔG.

They cannot be used to make an unfavorable reaction favorable.

If the number of bonds

broken/made remains the same how

might catalysts do this?

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Reaction RatesIn biological systems, catalysts are called enzymes.

◦ Most enzymes are proteins.

◦ Nearly every reaction that takes place in a living cell has an enzyme associated with.

◦ Enzymes bind the reactants, facilitate the reaction, and then release the products.

We will revisit ENZYMES over and over and over again!They are fascinating!

ENZYMES can do what chemical factories do with little /no effort!

Remember the Harper-Borsh process for ammonia.