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Why changes take placeWhy changes take place

Spontaneous processSpontaneous processTakes place ‘naturally’ with no apparent

cause or stimulus.

Nonspontaneous processNonspontaneous processRequires that something be done in order

for it to occur.

Spontaneous Nonspontaneous

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When will a reactionWhen will a reactionbe spontaneous?be spontaneous?

Spontaneity of a reaction can be determined by a study of thermodynamics.thermodynamics.

Thermodynamics can be used to calculate the amount of useful work that is produced by some chemical reactions.

The two factors that determine spontaneity are enthalpyenthalpy and entropyentropy.

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EnergyEnergy

EnergyEnergy - the ability to do work.

WorkWork - when a force is applied to an object.

There are several types of energy:• Thermal - heat• Electrical• Radiant - including light• Chemical• Mechanical - like sound• Nuclear

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EnergyEnergy

Energy can be classified as:

Potential energy Potential energy Stored energy - ability to do work.

Kinetic energyKinetic energyEnergy of motion - actually doing work.

Energy can be transferred from one object to another. It can also change form.

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Kinetic vs. potential energyKinetic vs. potential energy

PotentialEnergy

PotentialEnergy

Converting potentialConverting potentialto kinetic energyto kinetic energy

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Kinetic vs. potential energyKinetic vs. potential energy

KineticEnergy

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Energy and Energy and chemical bondschemical bonds

During a chemical reactionDuring a chemical reaction•Old bonds break. •New bonds are formed.•Energy is either absorbed or released.

ExothermicExothermic Energy is released. New bonds are more stable.

EndothermicEndothermic Energy is required. New bonds are less stable.

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ExothermicExothermic

En

erg

yReactants

Products

Since excess energy is released,the products are more stable.

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EndothermicEndothermic

Reactants

Products

Additional energy is requiredbecause the products are less stable.

En

erg

y

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EntropyEntropy

EntropyEntropy - a measure of the ‘disorder’ or randomness of a system.

Disorder is favored over order and may account for reaction occurring spontaneously even if it is endothermic.

solid gas

Increasedentropy

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Rate of changeRate of change

Not all spontaneous changes take place in a useful time period.

Some may require some initial energy to get them started.

2H2 (g) + O2 (g) 2H2O (l)

Others can be made faster by adding a catalyst.

2H2O2 (l) 2H2O (l) + O2 (g)

KineticsKinetics - the study of the rate of a reaction.

spark

I-

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Temperature, energy and heatTemperature, energy and heat

Temperature.Temperature. An intensive property of a material.

Thermal energy.Thermal energy. Energy of motion of molecules, atoms or ions. All materials have this energy if at a temperature above 0 K.

Heat.Heat. Thermal energy transfer that results from a difference in temperature. Thermal energy flows from warm objects to cool ones.

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Law of conservation of energyLaw of conservation of energy

““Energy cannot be created or destroyed in a Energy cannot be created or destroyed in a chemical reaction.”chemical reaction.”

During a reaction, energy can change from one form to another.

Example.Example. Combustion of natural gas.Chemical bonds can be viewed as potential energy. So during the reaction:

2CH4 (g) + 3O2 (g) 2CO2 (g) + 2H2O (l) + thermal energy + light

some potential energy is converted to thermal energy and light.