Kinetics

1. Kinetics Rates of Reaction

2. Faster only means faster • It doesn’t mean bigger • It doesn’t mean more of the product • It only means the rxn is completed faster, the reactants are turned into products in less time

3. Not a reaction but think about it • Come up with at least 3 ways you can make sugar dissolve faster in tea. • Then, thinking about collisions (even though these are not gases), explain why these things make the sugar dissolve faster?

4. Collision theory • Particles can’t react until they collide • Collisions must be successful • To be successful they must have enough energy and proper orientation • So, rate depends on: • Energy of particles • Orientation of particles when collide

5. Activation Energy • Ea = activation energy = minimum energy needed for reaction to occur • Rate of reaction depends on # particles with E ≥ Ea • How could you raise the energy of particles? • Temperature increases the energy of particles • This increases the # particles with E ≥ Ea • This is why rate usually increases with temperature!

6. Catalyst • A catalyst is something that makes a reaction go faster without being used in the reaction • Catalysts increase rate of rxn by lowering the Ea • So, more particles have E ≥ Ea • Ex: MnO2 and H2O2

7. Frequency of Collisions • Increasing the frequency of collisions increases the chances of correct orientation • Note: FREQUENCY of collisions, not number of collisions • Frequency of collisions can be increased by: • Increasing concentration of reactants – more particles to collide • For gases – increasing pressure increases frequency of collisions • Increase surface area (by decreasing size of particles)

8. Factors that affect the rate of a reaction • Temperature • Size of particles reacting – surface area • Concentration of particles reacting • Catalysts • For each of these, describe how and why they can increase the rate of rxn based on the collision theory

9. Compare reactions • How do the rates compare? • How do the amounts of product compare? dynamicscience.com.au

10. Representing effect of Temperature • Increased Temperature increases number of molecules with E >Ea • Can show this effect on a diagram • Temperature is a measure of average kinetic • This means some particles have more energy and some have less energy 

11. Maxwell – Boltzman curve • Area under curve = total # of particles, therefore area must remain constant • When temperature increases curve shifts to the right as more particles have greater energy 

12. Rate of Reaction • Kinetics is the study of the rates of a reaction • Reaction rates are determined experimentally • Rreactionrates are measured as changes in concentration over time

13. Baking soda and vinegar • There are 3 things you could measure – what are they? NaHCO3(s)+ CH3COOH(aq)  NaCH3COO (aq)+ CO2 (g)+ H2O (l)

14. Reaction Rates continued • Always positive value • Rate can be given as the rate of disappearance of reactant or appearance of product • Units of mol/Lx time or M/time • Rate usually decreases eventually as the reactants get used up

15. Average rate of a reaction • For reaction A → B • rate = - ∆[A] / ∆t or rate = ∆[B] / ∆t • Rate with regard to A is negative because reactant is disappearing • Rate with regard to B is positive because appearing • If reaction was A  2B then B would be forming twice as fast as A • But the rate of the overall rxn is the same!

16. Coefficients and reaction rates • The rate of a reaction is always the same, but it can be written using any reactant or product • Ex: aA + bB → cC + dD • rate = -1/a ( ∆[A] / ∆t ) = -1/b ( ∆[B] / ∆t ) = 1/c ( ∆[C] / ∆t ) = 1/d ( ∆[D] / ∆t )

17. Example #1 • H2 (g) + 2 BrCl (g) → Br2 (g) + 2HCl (g) • H2 (g) being used at ½ the rate of HCl being made • How do the rates of BrCl and Br2 compare? • Rate = -∆[ H2]=- ½ (∆[ BrCl] )=∆[ Br2] =½ (-∆[ HCl]) ∆t ∆t ∆t ∆t

18. Example #2 • For the equation 2N2O5 (g) → 4 NO2(g) + O2(g) • At a certain time in the rxn, the rate of formation of NO2 = 0.0072 M/sec • Find the rate of change of O2, rate of change of N2O5 and the rate of rxn.

19. Answer Ex. #2 • Given: Rate ∆NO2 = 0.0072 M/sec • From equation ¼ ∆[NO2] = ∆[O2] = - ½ ∆[N2O5] ∆t ∆t ∆t • Rate ∆[ O2] = ( ¼ )0.0072 M/sec = 0.0018 M/sec • Rate ∆[ N2O5] = (- ½ )0.0072 M/sec = -0.0036 M/sec • Rate rxn = 0.0018 M/sec • Can get rxn rate using any term

20. Experimentally Determining Reaction Rates • Can measure any property that is different between reactants and products – best if the change is large • Ex: • If gas is produced – measure change in pressure and relate to concentration using ideal gas law • If let gas escape, can use change in mass

21. Measuring reaction rates • Ways to measure a reaction rate include: • Changes in pH • Changes in color (Use a colorimeter) • Conductivity • Volume of a gas • Pressure of a gas • Changes in mass…

22. Determining Rate from Graphs • Time on x-axis • Mass, volume of gas, mass of gas lost, concentration on y-axis • Determine if graph showing reactants used up or products made • The rate usually indicated by slope of tangent to initial point on graph

23. Example simplechemconcepts.com

24. On graphs looking for: • How rate changes – steepness of curve • How much product is made or reactant is used – final point on graph academic.brooklyn.cuny.edu