Kinetics Topic 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 concentration is analogous to volume or mass changing over time

concentration of reactants decrease over time concentration of products increase over time (reactants)

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

Analyzing data from rate experiments (Concentration, Volume, and Mass) 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

Example C4H9Cl(aq) + H2O(l)  C4H9OH(aq) + HCl(aq) reaction slows down with time because the Concentration of the reactants decreases “rise over run” .040M/200s = .0002M/s .025M/400s = .000063M/s

the change in concentration of a reactant or product per unit of time [ ] refer to the concentration of the reactants or products

2NO2(g)  2NO(g) + O2(g) product [NO2] t [NO2] product t reactant

reaction is creating gas so Volume increases over time gas is being released in the reaction so Mass decreases 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 𝐾𝐸=1/2𝑚𝑣2

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 react particles must collide must collide in the correct orientation/angle 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: temperature pressure (gas only) surface area concentration 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

4. Concentration increasing concentration will increase the reaction rate by increasing more collisions per unit time will not “help with” geometry and energy

5. Catalysts 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

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