Chapter 6: Chemical Reactions Chapter 6: Chemical Reactions Occur in Predictable Ways Occur in Predictable Ways

Science 10Science 10

Chemistry UnitChemistry Unit

p.254-283p.254-283

6.2 – Factors Affecting Reaction Rate 6.2 – Factors Affecting Reaction Rate

• Words to know:Words to know:• CatalystCatalyst• Catalytic converterCatalytic converter• Rate of reactionRate of reaction• Surface areaSurface area

p.272-283

• Often, controlling the rate of a chemical reaction is as important as Often, controlling the rate of a chemical reaction is as important as having the reaction occur in the first place.having the reaction occur in the first place.• Many factors can determine the rate a chemical reaction occurs.Many factors can determine the rate a chemical reaction occurs.

• TemperatureTemperature

• ConcentrationConcentration

• Surface areaSurface area

• CatalystCatalyst

See pages 272 - 273

A bicycle chain slowly rusts

Understanding Reaction Rate Understanding Reaction Rate

• To make a reaction happen quickly:To make a reaction happen quickly:• Increase the temperature Increase the temperature • Increase the concentration of reactantsIncrease the concentration of reactants• Increase the amount of surface area that Increase the amount of surface area that

reactsreacts• Add a catalyst, or remove an inhibitorAdd a catalyst, or remove an inhibitor

See pages 272 - 273

Understanding Reaction Rate Understanding Reaction Rate

• To make a reaction happen slowly:To make a reaction happen slowly:• Decrease the temperature Decrease the temperature • Decrease the concentration of reactantsDecrease the concentration of reactants• Decrease the amount of surface area that Decrease the amount of surface area that

reactsreacts• Remove a catalyst (if present), or add an Remove a catalyst (if present), or add an

inhibitorinhibitor

See pages 272 - 273

Understanding Reaction Rate Understanding Reaction Rate

TemperatureTemperature

• Temperature is a measure of the average Temperature is a measure of the average energy of molecules.energy of molecules.• The more energy molecules have, the higher The more energy molecules have, the higher

the temperature.the temperature.• When molecules have more energy, they move When molecules have more energy, they move

around more, bump into other molecules more, around more, bump into other molecules more, and therefore react faster.and therefore react faster.

See page 275

TemperatureTemperature

• The rate of reaction changes with the The rate of reaction changes with the temperature.temperature.• Higher temperature = faster reaction rate, and vice Higher temperature = faster reaction rate, and vice

versa.versa.• Sometimes we want slower reactions (we use a Sometimes we want slower reactions (we use a

fridge to prevent spoilage).fridge to prevent spoilage).• Sometimes we want faster reactions (we cook food Sometimes we want faster reactions (we cook food

to speed up the production of new molecules).to speed up the production of new molecules).

See page 275

2. 2. DecompositionDecomposition

• are the opposite of synthesis reactionsare the opposite of synthesis reactions• A compounds breaks down into two or more A compounds breaks down into two or more

products (often elements).products (often elements).

General Formula:General Formula:

AB AB A + B A + B

(where A and B represent elements)(where A and B represent elements)

ConcentrationConcentration

• Concentration refers to how many Concentration refers to how many molecules of a substance exist in a certain molecules of a substance exist in a certain volume.volume.• How much solute (what’s dissolved) is there in How much solute (what’s dissolved) is there in

a certain amount of solvent (what the substance a certain amount of solvent (what the substance is dissolved in.is dissolved in.

• Concentration is measured in mass per unit Concentration is measured in mass per unit volume (g/L).volume (g/L). See page 275

Changing the concentration of O2 changes the intensity of

flames.

ConcentrationConcentration

• Usually, the higher the concentration of reactants, the Usually, the higher the concentration of reactants, the faster the reaction occurs.faster the reaction occurs.• Since there are more molecules per unit volume in Since there are more molecules per unit volume in

high concentrations, there are more opportunities high concentrations, there are more opportunities

for molecules to collide and react.for molecules to collide and react.• Eg. a splint of wood glows brighter highly Eg. a splint of wood glows brighter highly

concentrated oxygen than in normal air, with a lower concentrated oxygen than in normal air, with a lower concentration of oxygen.concentration of oxygen.

See page 275

Surface AreaSurface Area

• Chemical reactions occur when and where atoms and Chemical reactions occur when and where atoms and compounds collide.compounds collide.• The more atoms and molecules there are to collide, the The more atoms and molecules there are to collide, the

higher the reaction ratehigher the reaction rate

See page 276

Steel wool (on the right) is made up of small strands of steel, and therefore has much

more surface area than an equivalent amount of

solid steel.

Surface AreaSurface Area

• Increasing surface area increases the rate of reaction.Increasing surface area increases the rate of reaction.• Since there are more atoms and compounds exposed to Since there are more atoms and compounds exposed to

react, more reactions take place.react, more reactions take place.• Surface area can be increased by creating smaller Surface area can be increased by creating smaller

pieces. pieces. Eg. A powdered substance has far more surface area Eg. A powdered substance has far more surface area than one, large chunk.than one, large chunk.

• The increase in surface area must also be exposed for The increase in surface area must also be exposed for reaction. reaction. Eg. a powder only reacts more quickly if it is spread Eg. a powder only reacts more quickly if it is spread into the air instead of lying on a pan.into the air instead of lying on a pan.

See page 276

CatalystsCatalysts

• Sometimes increasing the temperature or Sometimes increasing the temperature or concentration is not a desirable method to concentration is not a desirable method to increase reaction rate.increase reaction rate.• Changing these two variables may be impractical or Changing these two variables may be impractical or

dangerous.dangerous.• A catalyst is a chemical that allows a reaction to occur A catalyst is a chemical that allows a reaction to occur

more quickly without actually participating in the more quickly without actually participating in the reaction itself.reaction itself.

See pages 276 - 277

CatalystsCatalysts

• The catalyst speeds up the reaction rate, but The catalyst speeds up the reaction rate, but does not get used up as a reactant.does not get used up as a reactant.

• Catalysts often lower the amount of energy Catalysts often lower the amount of energy necessary to break the bonds in the reactants.necessary to break the bonds in the reactants.

• Enzymes are an example of biological Enzymes are an example of biological catalystscatalysts• Saliva has enzymes that help speed the Saliva has enzymes that help speed the

breakdown of starches when they enter the breakdown of starches when they enter the mouth.mouth.

Salivary amylase increases the digestion of starches.

Catalysts (continued)Catalysts (continued)

• A catalytic converter is a device installed in all cars to A catalytic converter is a device installed in all cars to decrease pollution.decrease pollution.• Car exhaust passes through the catalytic converter before Car exhaust passes through the catalytic converter before

leaving the car.leaving the car.• Catalysts found in the honeycomb-shaped filters in the Catalysts found in the honeycomb-shaped filters in the

converter help to change many of the pollutants.converter help to change many of the pollutants.

• Poisonous CO is changed into COPoisonous CO is changed into CO22

• Hydrocarbons are converted into COHydrocarbons are converted into CO22 & H & H22OO

• Nitrogen oxides are changed into NNitrogen oxides are changed into N22 & O & O22

• 2N2N22OO33 2N 2N22 + 3O + 3O22Take the Section 6.2 Quiz