Collision Theory of ReactionsCollision Theory of Reactions

A chemical reaction occurs when:A chemical reaction occurs when:• Collisions between molecules have sufficient Collisions between molecules have sufficient

energy to break the bonds in the reactants. energy to break the bonds in the reactants. • Molecules collide with the proper orientation.Molecules collide with the proper orientation.• Example: Bonds between atoms of the reactants Example: Bonds between atoms of the reactants

(N(N22 and O and O22) are broken and new bonds (NO) can ) are broken and new bonds (NO) can

form. form.

Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

Collision Theory of ReactionsCollision Theory of Reactions A chemical reaction does not take place if the: A chemical reaction does not take place if the:

• Collisions between molecules do not have Collisions between molecules do not have

sufficient energy to break the bonds in the sufficient energy to break the bonds in the reactants. reactants.

• Molecules are not properly aligned.Molecules are not properly aligned.

Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

N2 O2 N2 O2

Activation EnergyActivation Energy• The activation The activation

energy is the energy is the minimum energy minimum energy needed for a reaction needed for a reaction to take place.to take place.

• When a collision When a collision provides energy provides energy equal to or greater equal to or greater than the activation than the activation energy, product can energy, product can form.form.

Collision Theory in SummaryCollision Theory in Summary

Three requirements for a reaction to Three requirements for a reaction to occur:occur:

Collision—The reactants must Collision—The reactants must collide.collide.

Orientation—The reactants must Orientation—The reactants must align properly.align properly.

Energy—The collision must provide Energy—The collision must provide the energy of activation.the energy of activation.

Reaction Rate and Reaction Rate and TemperatureTemperature

• Increases when Increases when temperature rises temperature rises because reacting because reacting molecules move molecules move faster providing faster providing more colliding more colliding molecules with molecules with energy of energy of activation.activation.

Blizzard vs. the HeatBlizzard vs. the Heat

Reaction:Reaction:

Mg(s) + HCMg(s) + HC22HH33OO22(aq) (aq)

HH22(g) + Mg(C(g) + Mg(C22HH33OO22))22(aq)(aq)

acetic acid

magnesium acetate

Reaction Rate and Reaction Rate and ConcentrationConcentration

• Reaction rate is Reaction rate is affected by the affected by the concentration of concentration of the reactants.the reactants.

• By adding more By adding more reactants, this reactants, this increases the increases the number of number of collisions.collisions.

Pioneer Porta-StovePioneer Porta-Stove

Reaction:Reaction:

CHCH44(g) + O(g) + O22(g) (g)

COCO22(g) + H(g) + H22O(g)O(g)

methane

Reaction Rate and CatalystsReaction Rate and Catalysts

A catalyst: A catalyst: • Speeds up the rate Speeds up the rate

of a reaction.of a reaction.• Lowers the energy Lowers the energy

of activation. of activation. • Is not used up Is not used up

during the reaction.during the reaction.

Elephant ToothpasteElephant Toothpaste

Reaction:Reaction:

HH22OO22(l)(l)

HH22O(l) + OO(l) + O22(g)(g)

NaI

hydrogen peroxide

Learning CheckLearning CheckState the effect of each on the rate of reaction as:State the effect of each on the rate of reaction as:

1) increases 2) decreases1) increases 2) decreases 3) no change 3) no change

A. Increasing the temperatureA. Increasing the temperature

B. Removing some of the reactantsB. Removing some of the reactants

C. Adding a catalystC. Adding a catalyst

D. Placing the reaction flask in iceD. Placing the reaction flask in ice

E. Increasing the concentration of one of the E. Increasing the concentration of one of the reactantsreactants

1

2

1

2

1

Learning CheckLearning CheckIndicate the effect of each factor listed on the rate of Indicate the effect of each factor listed on the rate of

thethe

following reaction as: 1) increases 2) decreases following reaction as: 1) increases 2) decreases

3) none3) none

2CO(2CO(gg) + O) + O22((gg) 2CO) 2CO2 2 ((gg))

A. Raising the temperatureA. Raising the temperature

B. Adding OB. Adding O22

C. Adding a catalystC. Adding a catalyst

D. Lowering the temperatureD. Lowering the temperature

1

1

1

2