Kinetics:Kinetics:Reaction Rates and Reaction Rates and

Potential Energy Potential Energy DiagramsDiagrams

Honors ChemistryHonors Chemistry

Unit 9- Chapter 17Unit 9- Chapter 17

Collision Theory of Collision Theory of ReactionsReactions

A successful reaction depends upon the A successful reaction depends upon the effective effective collisionscollisions between molecules. between molecules.

Reactant molecules must collide with Reactant molecules must collide with enough enough energyenergy to break bonds to break bonds

Not every collision will result in a reaction.Not every collision will result in a reaction. The minimum energy required to break The minimum energy required to break

the bonds of the reactants and start a the bonds of the reactants and start a reaction is called the reaction is called the Activation EnergyActivation Energy

Examples of Activation Examples of Activation EnergyEnergy

Scraping a match on a rough surface Scraping a match on a rough surface to light it.to light it.

Using that lighted match to light a Using that lighted match to light a candle.candle.

Lightening strike causes oxygen (OLightening strike causes oxygen (O22) ) to form ozone (Oto form ozone (O33))

Reaction RatesReaction Rates

The rate of a reaction can be increased, The rate of a reaction can be increased, in general, by increasing the chances for in general, by increasing the chances for effective collisions between molecules.effective collisions between molecules.

Likewise, you can slow a reaction down Likewise, you can slow a reaction down by reducing the chances of collisions.by reducing the chances of collisions.

There are four main factors that affect There are four main factors that affect reaction rates.reaction rates.

1. 1. TemperatureTemperature

Higher temperature Higher temperature molecules move molecules move fasterfaster Molecules will have more Molecules will have more energyenergy

There will be more There will be more effectiveeffective collisions collisions– Molecules collide with the minimum energy Molecules collide with the minimum energy

(activation energy) more often.(activation energy) more often.

2. 2. ConcentrationConcentration

Higher Concentration = Higher Concentration = MoreMore particles particles More particles = more possible More particles = more possible

collisionscollisions More collisions = More collisions = fasterfaster reaction! reaction!

For a gas - Increase For a gas - Increase PressurePressure Volume Volume decreasesdecreases concentration concentration increasesincreases..

3. 3. Surface AreaSurface Area

Particle size determines surface areaParticle size determines surface area Smaller particles Smaller particles moremore surface surface

area.area. More surface area means more More surface area means more areaarea

for collisions to occurfor collisions to occur

MORE COLLISIONS = MORE COLLISIONS = FASTER FASTER REACTION!REACTION!

4. 4. CatalystsCatalysts A catalyst is a substance that increases A catalyst is a substance that increases

the rate of a reaction without being the rate of a reaction without being used up in the reaction.used up in the reaction.

It increases the rate of the reaction by It increases the rate of the reaction by loweringlowering the activation energy for a the activation energy for a reactionreaction

Low activation energy means = faster Low activation energy means = faster reactions reactions

There is a There is a greatergreater chance for an chance for an effectiveeffective collision if the activation collision if the activation energy is lower.energy is lower.

Potential Energy Potential Energy DiagramsDiagrams In a reaction mixture the reactants and products In a reaction mixture the reactants and products

contain potential energy. contain potential energy. This potential energy is also known as This potential energy is also known as enthalpyenthalpy

(symbol (symbol HH). During a chemical reaction the ). During a chemical reaction the enthalpy (or Potential Energy ) of the reactants enthalpy (or Potential Energy ) of the reactants changes as the reactants form new products.changes as the reactants form new products.

The The enthalpy changeenthalpy change ( (∆∆HH) for a reaction can ) for a reaction can be calculated from a potential energy diagram.be calculated from a potential energy diagram.

∆∆H = H(products) - H(reactants)H = H(products) - H(reactants)

∆∆H is measured in kilojoules per mole (kJ/ mol)H is measured in kilojoules per mole (kJ/ mol)

Exothermic ReactionsExothermic Reactions An An exothermic reactionexothermic reaction releasesreleases

heat, we can feel the heat given off heat, we can feel the heat given off to the surroundings as the reaction to the surroundings as the reaction happens.happens.

****Energy required to break reactant Energy required to break reactant bonds is bonds is lessless than the energy released than the energy released when product bonds form.when product bonds form.

A + B A + B C + D + Energy/heat C + D + Energy/heat The enthalpy change for an exothermic The enthalpy change for an exothermic

reaction is always negative. (-ΔH)reaction is always negative. (-ΔH)

Endothermic ReactionsEndothermic Reactions An An endothermic reaction endothermic reaction absorbsabsorbs

heat, we can feel a beaker become heat, we can feel a beaker become colder as a reaction proceeds as heat colder as a reaction proceeds as heat is taken in from the surroundings.is taken in from the surroundings.

****Energy required to break reactant Energy required to break reactant bonds is bonds is moremore than the energy than the energy released when product bonds formreleased when product bonds form

A + B + Energy/heat A + B + Energy/heat C + D C + D The enthalpy change for an The enthalpy change for an

endothermic reaction is always endothermic reaction is always positive. (+ΔH) positive. (+ΔH)

ΔHrxn = Heat energy released/absorbed in reaction

Reverse reactionActivation energy

Potential Energy Diagram with Potential Energy Diagram with CatalystCatalyst

With a catalyst – The Activation Barrier is Lowered

This Increases Reaction Rate

Note - Enthalpy , ΔH Remains the same!

Exothermic

∆H = (-)

Endothermic

∆H = (+)

Exothermic Reaction: ExampleExothermic Reaction: Example

The activation energy (Ea) for the forward reaction is shown by (a), what is the value of the forward activation energy?

Ea = 200 – 150 = 50 kJ/mole

The activation energy (Ea) for the reverse reaction is shown by (b): what is the value?

Ea (reverse) = 200 - 50 = 150 kJ/mole

The enthalpy change for the reaction is The enthalpy change for the reaction is shown by shown by (c)(c): What is the value of : What is the value of ∆∆H ?H ?

∆H = H products) – H(reactants)

= 50 – 150 = -100 kJ/mol

Kinetics Movie ClipKinetics Movie Clip