energy & rates of chem rxns ch 14.4 8th

8/8/2019 Energy & Rates of Chem Rxns Ch 14.4 8th

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ENERGY & RATES OF CHEMICAL

REACTIONSChapter 14 Section 4


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OBJECTIVES:

Compare exothermic and endothermic reactions.

Explain activation energy.

Interpret an energy diagram.

Describe the factors that affect the rate of a

reaction.


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CHEMICAL REACTIONS & ENERGY

Some reactions release energy

y Running a mile

Some reactions absorb energyy Eating a meal


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REACTIONS & ENERGY

Energy is needed to break chemical bonds in the

reactants

As new bonds are formed in the products, energyis released


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EXOTHERMIC REACTIONS

Chemical reaction in which energy is released

go out = exo; heat = thermic

Energy can be given off in several forms

y Light energy, electrical energy, light & thermal

energy

Written as a product:

2Na + Cl2 2NaCl + energy


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ENDOTHERMIC REACTIONS

A chemical reaction in which energy is taken in

go in = endo

Often written as a reactant

Ex: 2H2O + energy 2H2 + O2

Ex: photosynthesis

6CO2 + 6H2O + energy C6H12O6 + 6O2


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THE LAW OF CONSERVATION OF ENERGY

Neither mass nor energy can be created or

destroyed in chemical reactions

Energy CAN change forms

It can be transferred from one object to another

Total amount of energy is the same before andafter the reaction


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RATES OF REACTIONS

There must be enough energy to break the bonds

that hold the particles together in a molecule

The speed at which new particles form is calledthe rate of reaction


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ACTIVATION ENERGY

A boost of energy is needed to start a reaction =

activation energy

The smallest amount of energy that moleculesneed to react

Ex: striking a match

y All the reactant necessary to burny Striking a match causes friction, which adds heat


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SOURCES OF ACTIVATION ENERGY

Friction is one source of activation energy

Electric spark in an engine of a car

Light can also be a source (photosynthesis)


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FACTORS AFFECTING RATES OF

REACTIONS

Rate of reaction = how fast the reaction takes

place

4 factors affect the rate of a reaction:y Temperature

y Concentration

y Surface Area

y

Presence of an inhibitor or a catalyst


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TEMPERATURE

Higher temperature = faster rate of reaction

Particles of reactants move quickly

Particles collide with a lot of energy

Reactants are changed into products quickly


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CONCENTRATION

High concentration = fast rate of reaction

A measure of the amount of one substance

dissolved in another

Many particles to react per given volume

Small distance between particles


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SURFACE AREA

Amount of exposed surface of a substance

Increased surface area of solids = fast rate of

reaction

Grinding solid into powder increases surface area

More exposed particles


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INHIBITORS

A substance that slows down or hinders a

reaction

Ex: preservatives slow down the growth ofbacteria or fungi

Ex: some antibiotics are inhibitors (penicillin)


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CATALYSTS

A substance that speeds up a reaction withoutbeing permanently changed itself

It is NOT a reactant

Lowers the activation energy of a reaction whichallows the reaction to occur more quickly

Enzymes speed up reactions in our bodies

Catalytic converters in cars (make exhaust lessharmful)


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QUICKQUIZ:

Why does grinding a solid into a powder increase

reaction rate?

What is the difference between a reactant and a

catalyst?


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REFERENCE:

Holt Science and Technology: Physical Science.

New York: Henry Holt & Co, 2007. Print.

energy & rates of chem rxns ch 14.4 8th

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