kinetics & equilibrium ted.ed. review a chemical equation describes a chemical change...
DESCRIPTION
Reaction Rates HHHHow fast does a reaction go to completion? –R–R–R–Reactions occur at different rates EX: Burning of hydrogen fast EX: Rusting of iron slow EX: Convert coal to diamond very slow RRRReaction rate – the “speed” of a reaction CCCCollision theory: a reaction can occur when molecules collide if: –T–T–T–They have enough kinetic energy –T–T–T–They are colliding frequently –T–T–T–They have the right orientationTRANSCRIPT
ReviewReview
A chemical equation describes a A chemical equation describes a chemical change (reaction).chemical change (reaction).
2NO(g) + O2(g) 2NO2(g)
BEFOREBEFORE
Reaction!Reaction!
AfterAfter
Reaction RatesReaction RatesHow fast does a reaction go to completion?How fast does a reaction go to completion?
– Reactions occur at different ratesReactions occur at different ratesEX: Burning of hydrogen EX: Burning of hydrogen fast fastEX: Rusting of iron EX: Rusting of iron slow slowEX: Convert coal to diamond EX: Convert coal to diamond very slow very slow
Reaction rateReaction rate – the “speed” of a reaction – the “speed” of a reaction Collision theoryCollision theory: a reaction can occur when : a reaction can occur when
molecules collide if:molecules collide if:– They have enough kinetic energyThey have enough kinetic energy– They are colliding frequentlyThey are colliding frequently– They have the right orientationThey have the right orientation
Collision TheoryCollision TheoryActivation Energy Activation Energy – the minimum – the minimum
amount of energy required to start a amount of energy required to start a reaction.reaction.– The energy of a molecular collision must The energy of a molecular collision must
be be at leastat least as much as the activation as much as the activation energy.energy.
– Anything less, the molecules will simply Anything less, the molecules will simply bounce apart without a reactionbounce apart without a reaction
Potential Energy DiagramPotential Energy DiagramAll reactions have an energy All reactions have an energy
profile:profile: Potential Energy Diagram
Reaction Coordinate
Ener
gy
Activation Energy
1.1. Reactants absorb energy in Reactants absorb energy in order to collide and reactorder to collide and react
2.2. The activated complex releases The activated complex releases energy to become productsenergy to become products
3.3. If the products have less energy If the products have less energy than the reactants, thenthan the reactants, then Energy was releasedEnergy was released Reaction is ExothermicReaction is Exothermic
reactants
Activated complex
products
Potential Energy DiagramPotential Energy Diagram
Potential Energy Diagram Reactions that gain more Reactions that gain more
energy than released are called energy than released are called endothermicendothermic
The energy of the products is The energy of the products is higher than the reactantshigher than the reactants If energy is absorbed, then If energy is absorbed, then
the reaction is endothermicthe reaction is endothermic
Reaction Coordinate
Ener
gy
Reactants
Products
+H
Activation Energy
Factors that Affect Rates of ReactionFactors that Affect Rates of Reaction
1.1. TemperatureTemperature – average kinetic energy – average kinetic energy– Reactions run faster at high temperatures.Reactions run faster at high temperatures.
Increasing temperature means faster moleculesIncreasing temperature means faster moleculesMore collisions per secondMore collisions per secondCollisions with greater energyCollisions with greater energyActivation energy can be reachedActivation energy can be reached
Factors that Affect Rates of ReactionFactors that Affect Rates of Reaction
2.2. ConcentrationConcentration– Increasing solute concentration increases the rate of Increasing solute concentration increases the rate of
reactionreaction– Higher concentration means more molecules collidingHigher concentration means more molecules colliding
Ex: Zn(s) + 2HCl(aq) Ex: Zn(s) + 2HCl(aq) → ZnCl ZnCl22(aq) + H(aq) + H22(g)(g)6.00 M HCl solution will “eat” Zinc much faster than 1.00 6.00 M HCl solution will “eat” Zinc much faster than 1.00
M HCl solutionM HCl solution– Explains why we usually mix reactants with one reactant in Explains why we usually mix reactants with one reactant in
great excess, it keeps reaction rate high for entire reaction.great excess, it keeps reaction rate high for entire reaction.
ConcentrationConcentration
Which bumper car ride will result in Which bumper car ride will result in more collisions per minute for the red more collisions per minute for the red car?car?
Factors that Affect Rates of ReactionFactors that Affect Rates of Reaction
3.3. Particle SizeParticle Size– The smaller the reactant particles, the The smaller the reactant particles, the
faster the reaction.faster the reaction.– Smaller particles have greater surface Smaller particles have greater surface
areaareaGrinding and/or dissolving solids will Grinding and/or dissolving solids will
increase the surface areaincrease the surface area
Particle SizeParticle SizeAluminum powder bears a warning that it Aluminum powder bears a warning that it
is an explosion hazard. Aluminum foil is an explosion hazard. Aluminum foil bears no such warning. Why is Al powder bears no such warning. Why is Al powder so dangerous when Al foil is not?so dangerous when Al foil is not?– Powdered Al has a Powdered Al has a muchmuch greater surface area greater surface area
exposed to oxygen.exposed to oxygen.– The oxidation of aluminum is highly The oxidation of aluminum is highly
exothermic.exothermic.– With powdered aluminum, a highly exothermic With powdered aluminum, a highly exothermic
reaction can occur at a much greater rate.reaction can occur at a much greater rate.
Particle SizeParticle Size
These two blocks have the same total volume, but the one on the right has been broken into 1000 equal pieces. This increases its surface area by 60 times.
1 meter
10 cm
Particle SizeParticle SizeAvailable to reactNot available to react
Factors that Affect Rates of ReactionFactors that Affect Rates of Reaction
4.4. CatalystCatalyst a substance that speeds up a reaction by a substance that speeds up a reaction by
lowering the activation energy of a lowering the activation energy of a reactionreaction– Not consumed in the reaction.Not consumed in the reaction.– Written above the reaction arrow.Written above the reaction arrow.
Example: Platinum metal is often used as Example: Platinum metal is often used as a catalyst to convert methanol vapors a catalyst to convert methanol vapors into formaldehyde.into formaldehyde.
CH4O Pt CH2O + H2
CatalystsCatalysts
Enzymes are proteins that act as Enzymes are proteins that act as biological catalysts.biological catalysts.– speed up many biochemical reactions speed up many biochemical reactions
that would take far too long to happen that would take far too long to happen naturally.naturally.
– Enzymes function best in specific Enzymes function best in specific temperature ranges.temperature ranges.Human enzymes work best around 37ºC.Human enzymes work best around 37ºC.Too high a temperature will 'denature' an Too high a temperature will 'denature' an
enzyme and destroy its functionalityenzyme and destroy its functionality
Catalysts and Activation EnergyCatalysts and Activation Energy
Reaction Coordinate
Pote
ntia
l Ene
rgy
Uncatalyzed
Catalyzed
Factors that Affect Rates of ReactionFactors that Affect Rates of Reaction
5.5. Chemical Nature of ReactantsChemical Nature of Reactantsbreaking bonds absorb energy and slows reactionsbreaking bonds absorb energy and slows reactionsReactants with many bonds react slowerReactants with many bonds react slower
Combustion of sucroseCombustion of sucroseCC1212HH2222OO1111 + 12 O + 12 O22 → 12 CO → 12 CO22 + 11 H + 11 H22OO
Many bonds to breakMany bonds to breakReacts slowlyReacts slowly
2Na + Cl2Na + Cl22 → 2NaCl → 2NaClFew bonds to breakFew bonds to breakReacts quicklyReacts quickly