6. Chemical Kinetics

Question 1

Collision Theory

Answer 1

• Particles must collide
• Particles must have enough kinetic energy to overcome electron cloud repulsion to react - activation energy
• particles must collide in the correct orientation - steric factor

Question 2

Factors Affecting Rates of Reaction

Answer 2

• Concentration
  
  • increase the rate at which particles collide
• Surface area
  
  • increase area of contact, increased collisions
• Temperature
  
  • increases the number of particles that will have sufficient active energy
• Catalysts
  
  • decrease the Activation energy required

Question 3

Maxwell-Boltzmann Curve - catalyst

Answer 3

Question 4

Maxwell-Boltzmann Curve - temperature

Answer 4

Question 5

Enthalpy level diagram

Answer 5

Question 6

Rate of Reaction equation

Answer 6

-d[R] / dt = d[P] / dt

_{mol dm^-3 s^-1}

Question 7

Ways to measure reaction rate

Answer 7

• Gas collection syringe
• Gas collection over water
• Change in mass
• Change in color
• Change in pH / conductivity
• Chang to a point (clock reactions)

Question 8

The Rate Equation

Answer 8

• Links to the rate of reactions to the concentration of reactants
• can only be found by doing actual experiments
• catalysts must be included

A + B –> C + D

r = k[A][B]² _k=constant

If [A] is doubled, the rate is doubled

If [B] is doubled, the rate is quadrupled

Question 9

Order reaction (Individual / overall)

Answer 9

Individual order - the power to which a concentration is raised in the rate equation

Overall order - the sum of all the individual orders in the rate equation

r = k[A][B]²

_{overall order = 3}

The value is 0 if the rate is unaffected by how much substance is present

Question 10

Units for the rate constant (k)

Answer 10

The units of k depend on the order of reaction

• divide the rate by as many concentrations in the equation

Question 11

Graphical determination of rate - Conc. and Rate

Answer 11

Question 12

Reaction Mechanism

Answer 12

• In a multistep process, one of the steps will be slower than all others
• the overall reaction cannot occur faster than this slowest, rate-determining step
• the order of the reaction for this step will be the same as the order of reaction for the entire reaction

Question 13

Molecularity of the Reaction Mechanism

Answer 13

Unimolecular steps - involve a single species as a reactant

• A –> Products

Bimolecular steps - involve collisions of 2 species

• A + B –> products

Question 14

Intermdiate vs Transition States

Answer 14

Intermediates

• Exists for a finite time
• occur at P.E. minimum
• Formed in one step of a reaction and consumed in a subsequent step

Transition States

• Have only a transient existance
• occur at P.E. maximum
• Exist part way through every step of a reaction

Question 15

Arrhenius Equation

Answer 15

k = Ae - Ea/RT

A = arrhenius constant, a number that represents the likelihood that collisions would occur with the proper orientation

Ea = activation energy

T = temp in K

R = universal thermodynamics constant

Question 16

Arrhenius Plot

Answer 16

ln(k) = -Ea/RT + lnA

when K is determined experimentally at several temperatures, Ea can be calculated from the slope of a plot of ln(k) vs 1/T

Question 17

Determining Ea

Answer 17

1. convert the rate constants (k) to natural logs ln(k)
2. convert temperatures to kelvin and then to 1/T
3. draw an arrhenius plot of ln(k) vs 1/T
4. measure the gradient of the line
5. gradient = -Ea/R, solve for Ea

* for most equations, an increase of 10c will double the rate of reaction