KineticsTopic 6

• Rate of reactions–can be fast or slow–defined as the change in [concentration]

of the reactants or products per time• concentration of reactants decreases over

time– [reactants]/time

• concentration of products increase over time– [products]/time

• normally measure in Molarity per sec (M/s)–will continue until reaches equilibrium or

one of the reactants is used up

(reactants)

concentration of reactants decrease over timeconcentration of products increase over time

Experiments for measuring rate of reactions– over time, could measure the change of:

• for gaseous reactions– mass would go down as gas escapes– volume would increase at constant temp. and

pressure – pressure would increase at constant temp. and

volume• change in pH if acids and bases are involved• change in temperature• change in electrical conductivity

– if produces ions in solution, conductivity will increase

• using a spectrometer to detect color changes

http://2.bp.blogspot.com/-76Io28yn2Ys/TZnaOvfvedI/AAAAAAAAAAM/8qkS2oQ4FkM/s1600/Untitled.png

• usually involves a graph of properties over time

• usually a curve, and the reaction rate is determined from the slope of the line at a time (also known as a tangent)

• reaction rates tend to slow with time as reactants are converted to products

Analyzing data from rate experiments (CONCENTRATION, VOLUME, and MASS)

• reaction slows down with time because the

CONCENTRATION of the reactants decreases

• “rise over run”– .040M/200s

= .0002M/s

– .025M/400s

= .000063M/s

ExampleC4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq)

• the change in concentration of a reactant or product per unit of time

• [ ] refer to the concentration of the reactants or products

12

12 ][][

tt

ttimeatAttimeatARate

t

ARate

][

2NO2(g) 2NO(g) + O2(g)

[NO2]

t2[ ]

constantNO

t

product

productreactant

[NO2]

t

gas is being released in the

reaction so MASS decreases over time

reaction is creating gas so

VOLUME increases over time

Kinetic theory – the average energy of particles is

proportional to the temperature (Kelvin or Celsius)• all particles have same energy if the same

temperature– ex. 4He 20Ne 40Ar

»would all have the same temp. and energy– however, lighter particles (4He) would have

greatest speed than larger particles given the same energy

Activation Energy Ea

• a minimum amount of energy required for reaction to occur– bonds need to be broken first

• the molecules must posses sufficient energy to get over the activation energy barrier.

Collision theory

• in order for particles to react1. particles must collide2. must collide in the correct orientation/angle3. must collide with enough kinetic energy to

overcome the activation energy (E > Ea)

• if the previous conditions are “enough”, particles can overcome the activation energy and reaction will occur– meaning the bonds holding the reactants together

will be broken and the reaction can take place

http://phet.colorado.edu/en/simulation/reactions-and-rates

Factors That Affect Reaction Rates

• any factor that increases the frequency of collisions or increases the energy with which particles collide will make the reaction go faster:1. temperature2. pressure (gas only)3. surface area4. concentration5. catalysts

1. Temperature• increase temp

– increases number of collisions per unit time• reaction rate approximately doubles for each

10oC or K rise in temperature– increases energy of the collisions

• greater chance E > Ea

– will not “help with” geometry

2. Pressure

• only for gasses– reducing volume while keeping temp

constant increases the reaction rate• forcing them together will increase number of

collisions per unit time– will not “help with” geometry and energy

3. Surface area

• smaller particles have more surface area and will increase the reaction rate– more collisions per unit time– will not “help with” geometry and energy

http://www.google.com/url?sa=i&rct=j&q=surface+area+reaction+rate&source=images&cd=&cad=rja&docid=w-OtZmyZs-Cw_M&tbnid=TG-4MfHZPLc4MM:&ved=0CAUQjRw&url=http://www.jeron.je/anglia/learn/sec/science/patterns/page04a.htm&ei=fxo8Ud22MsG-2gXProGYCA&bvm=bv.43287494,d.b2I&psig=AFQjCNFeHaU4A2xGiqfqFOOCvmw30YanGQ&ust=1362979409532114

http://www.google.com/url?sa=i&rct=j&q=surface+area+reaction+rate&source=images&cd=&cad=rja&docid=w-OtZmyZs-Cw_M&tbnid=TG-4MfHZPLc4MM:&ved=0CAUQjRw&url=http://www.chemguide.co.uk/physical/basicrates/concentration.html&ei=Lho8UdmyOMeg2AXnooDQCg&bvm=bv.43287494,d.b2I&psig=AFQjCNFeHaU4A2xGiqfqFOOCvmw30YanGQ&ust=1362979409532114

4. Concentration• increasing concentration will increase the

reaction rate by increasing more collisions per unit time

• will not “help with” geometry and energy

http://4.bp.blogspot.com/_OdZZiyNKEoU/SYLuWB0braI/AAAAAAAAADY/Gl3ONEAHBgc/s1600/Slide1.GIF

• lowers the activation energy (Ea) for the reaction

• provide an alternate reaction (rxn) pathway

• increase the rate of a reaction• are not used up or chemically changed in

the reaction

5. Catalysts

Maxwell–Boltzman energy distribution curve

• another way to look at how many particles can react • area under the curve shows the number of gas

particles– the # of particles remains constant

• not all gas particles have the same energy– only some gas particles (blue area) have enough

energy to react

The affect of temperature

• the area under the curve remains the same because the number of particles doesn't change

• higher temps. shifts the curve to the right (therefore, the peek must be lower) resulting in an increase in collision frequency and thus more successful collisions

The affect of a catalyst

Never move the new activation energy to the left of the peak. Catalysts don’t help out that much!