![REACTION MECHANISMS
If each elementary process occurs at different
rates, how fast is the overall reaction?
Step 1: HBr + O2 HOOBr
Step 2: HOOBr + HBr 2 HOBr
Step 3: (HOBr + HBr H2O + Br2) x2
slow
fast
fast
The overall reaction can only be as fast as the
SLOWEST elementary process. So the rate of the
reaction (r=k[HBr]x
[O2]y
) is the same as the rate of
the slowest step.
This step is called the rate-determining step](https://image.slidesharecdn.com/tang09-ratemechanisms-151102163152-lva1-app6891/85/Tang-09-rate-mechanisms-5-320.jpg)
![REACTION MECHANISMS
Rate Law for Elementary ProcessesRate Law for Elementary Processes
• ONLY FOR ELEMENTARY PROCESSES, the
exponents of the rate law is the same as the
coefficients of the balanced elementary
process equation
i.e. A + 2B AB2 r=k[A]1
[B]2
• if only given the overall equation, then rate
experiments must be conducted](https://image.slidesharecdn.com/tang09-ratemechanisms-151102163152-lva1-app6891/85/Tang-09-rate-mechanisms-10-320.jpg)
![REACTION MECHANISMS
Rate Law for Elementary ProcessesRate Law for Elementary Processes
EXAMPLE 1EXAMPLE 1
2 NO2Cl 2 NO2 + Cl2
i)NO2Cl NO2 + Cl
ii)NO2Cl + Cl NO2 + Cl2
rate = k[NO2Cl]
rate = k[NO2Cl][Cl]
Elementary
processes](https://image.slidesharecdn.com/tang09-ratemechanisms-151102163152-lva1-app6891/85/Tang-09-rate-mechanisms-11-320.jpg)
![REACTION MECHANISMS
Rate Law for Elementary ProcessesRate Law for Elementary Processes
EXAMPLE 2EXAMPLE 2
rate = k[NO2]2
rate = k[NO3]
Elementary
processes
2 NO2 2 NO + O2
i)2 NO2 NO3 + NO
ii)NO3 NO + O2](https://image.slidesharecdn.com/tang09-ratemechanisms-151102163152-lva1-app6891/85/Tang-09-rate-mechanisms-12-320.jpg)
![REACTION MECHANISMS
Predicting MechanismsPredicting Mechanisms
EXAMPLE 1bEXAMPLE 1b
2 NO2Cl 2 NO2 + Cl2
i)NO2Cl NO2 + Cl
ii)NO2Cl + Cl NO2 + Cl2
rate = k[NO2Cl]
Experimentally-determined
1st
order reaction
rate = k[NO2Cl]
(predicted)
Since this mechanism
yields a rate law that
matches the
experimental one, it
should be correct.
(slow)
(fast)](https://image.slidesharecdn.com/tang09-ratemechanisms-151102163152-lva1-app6891/85/Tang-09-rate-mechanisms-14-320.jpg)
![REACTION MECHANISMS
Predicting MechanismsPredicting Mechanisms
EXAMPLE 2bEXAMPLE 2b
rate = k[NO2]2
Experimentally-determined
2nd
order reaction
rate = k[NO2]2
(predicted)
Since this mechanism
yields a rate law that
matches the
experimental one, it
should be correct.
2 NO2 2 NO + O2
i)2 NO2 NO3 + NO
ii)NO3 NO + O2
(slow)
(fast)](https://image.slidesharecdn.com/tang09-ratemechanisms-151102163152-lva1-app6891/85/Tang-09-rate-mechanisms-15-320.jpg)
![REACTION MECHANISMS
Predicting MechanismsPredicting Mechanisms
The reaction of ozone, O3, with nitric oxide,
NO, forms nitrogen dioxide and oxygen gas.
It is believed to be a one step mechanism to
produce smog.
Determine the rate law.
NO + O3 NO2 + O2 rate = k[NO][O3]](https://image.slidesharecdn.com/tang09-ratemechanisms-151102163152-lva1-app6891/85/Tang-09-rate-mechanisms-17-320.jpg)
![REACTION MECHANISMS
Predicting MechanismsPredicting Mechanisms
2 N2O5(g) 2 N2O4(g) + O2(g)
a)Determine the rate equation if this
was a one step reaction.
b)Actual rate equation: rate = k[N2O5]
Propose the reactants for the rate
determining step.
rate = k[N2O5]2
N2O5 some reaction intermediates](https://image.slidesharecdn.com/tang09-ratemechanisms-151102163152-lva1-app6891/85/Tang-09-rate-mechanisms-18-320.jpg)
![REACTION MECHANISMS
Predicting MechanismsPredicting Mechanisms
X + 2 Y + 2 Z XY2Z2
i)doubling [X] = no effect
ii)doubling [Y] = 4 x rate
iii)doubling [Z] = 2 x rate
a)What is the rate law?
b)Propose the reactants of the rate
determining step?
rate = k[Y]2
[Z]
2Y + Z some product(s)](https://image.slidesharecdn.com/tang09-ratemechanisms-151102163152-lva1-app6891/85/Tang-09-rate-mechanisms-19-320.jpg)
Tang 09 rate mechanisms
- 2. REACTION MECHANISMS A + B + C ABC A + B AB Which reaction do you think will proceed faster? Why? Reactions that require a greater number of particles to collide at the same time will decrease the chances of a successful reaction to occur.
- 3. REACTION MECHANISMS Of all possible chemical reactions, how many would require three particles colliding? Many reactions take place through multiple two- collision steps at a time, especially when more than two reactants are involved. Overall: 4 HBr + O2 2 H2O + 2 Br2 Step 1: HBr + O2 HOOBr Step 2: HOOBr + HBr 2 HOBr Step 3: (HOBr + HBr H2O + Br2) x2
- 4. REACTION MECHANISMS Each step of the mechanism is known as an elementary process. The complete series of elementary processes is known as the reaction mechanism of a chemical reaction. You are not expected to come up with elementary processes!!
- 5. REACTION MECHANISMS If each elementary process occurs at different rates, how fast is the overall reaction? Step 1: HBr + O2 HOOBr Step 2: HOOBr + HBr 2 HOBr Step 3: (HOBr + HBr H2O + Br2) x2 slow fast fast The overall reaction can only be as fast as the SLOWEST elementary process. So the rate of the reaction (r=k[HBr]x [O2]y ) is the same as the rate of the slowest step. This step is called the rate-determining step
- 6. REACTION MECHANISMS 4 HBr + O2 2 H2O + 2 Br2 Potential energy diagram Potentialenergy Reaction coordinate Reactants Products Ea ΔH HBr + O2 HOOBr 2 HOBr 2H2O + 2Br2 3 peaks, therefore 3 steps in the reaction mechanism
- 7. REACTION MECHANISMS • catalysts increase reaction rate without being used up in the reaction • all catalysts reduce activation energy for a reaction • many catalysts also change the mechanism by which the reaction occurs
- 8. REACTION MECHANISMS CH2O2(aq) CO(g) + H2O(l) uncatalyzed catalyzed
- 10. REACTION MECHANISMS Rate Law for Elementary ProcessesRate Law for Elementary Processes • ONLY FOR ELEMENTARY PROCESSES, the exponents of the rate law is the same as the coefficients of the balanced elementary process equation i.e. A + 2B AB2 r=k[A]1 [B]2 • if only given the overall equation, then rate experiments must be conducted
- 11. REACTION MECHANISMS Rate Law for Elementary ProcessesRate Law for Elementary Processes EXAMPLE 1EXAMPLE 1 2 NO2Cl 2 NO2 + Cl2 i)NO2Cl NO2 + Cl ii)NO2Cl + Cl NO2 + Cl2 rate = k[NO2Cl] rate = k[NO2Cl][Cl] Elementary processes
- 12. REACTION MECHANISMS Rate Law for Elementary ProcessesRate Law for Elementary Processes EXAMPLE 2EXAMPLE 2 rate = k[NO2]2 rate = k[NO3] Elementary processes 2 NO2 2 NO + O2 i)2 NO2 NO3 + NO ii)NO3 NO + O2
- 13. REACTION MECHANISMS Predicting MechanismsPredicting Mechanisms 1. For a proposed mechanism to be acceptable, the rate law of the rate-determining step should be the same as the rate law for the overall reaction 2. overall reaction rate is determined by the rate determining step This means that the rate law for the rate- determining step is directly related to the rate law for the overall reaction.
- 14. REACTION MECHANISMS Predicting MechanismsPredicting Mechanisms EXAMPLE 1bEXAMPLE 1b 2 NO2Cl 2 NO2 + Cl2 i)NO2Cl NO2 + Cl ii)NO2Cl + Cl NO2 + Cl2 rate = k[NO2Cl] Experimentally-determined 1st order reaction rate = k[NO2Cl] (predicted) Since this mechanism yields a rate law that matches the experimental one, it should be correct. (slow) (fast)
- 15. REACTION MECHANISMS Predicting MechanismsPredicting Mechanisms EXAMPLE 2bEXAMPLE 2b rate = k[NO2]2 Experimentally-determined 2nd order reaction rate = k[NO2]2 (predicted) Since this mechanism yields a rate law that matches the experimental one, it should be correct. 2 NO2 2 NO + O2 i)2 NO2 NO3 + NO ii)NO3 NO + O2 (slow) (fast)
- 16. REACTION MECHANISMS Predicting MechanismsPredicting Mechanisms • proposed mechanisms that match experimental rate laws DO NOT PROVE the mechanism is correct –additional experimentation is required • other proposals may need to be examined
- 17. REACTION MECHANISMS Predicting MechanismsPredicting Mechanisms The reaction of ozone, O3, with nitric oxide, NO, forms nitrogen dioxide and oxygen gas. It is believed to be a one step mechanism to produce smog. Determine the rate law. NO + O3 NO2 + O2 rate = k[NO][O3]
- 18. REACTION MECHANISMS Predicting MechanismsPredicting Mechanisms 2 N2O5(g) 2 N2O4(g) + O2(g) a)Determine the rate equation if this was a one step reaction. b)Actual rate equation: rate = k[N2O5] Propose the reactants for the rate determining step. rate = k[N2O5]2 N2O5 some reaction intermediates
- 19. REACTION MECHANISMS Predicting MechanismsPredicting Mechanisms X + 2 Y + 2 Z XY2Z2 i)doubling [X] = no effect ii)doubling [Y] = 4 x rate iii)doubling [Z] = 2 x rate a)What is the rate law? b)Propose the reactants of the rate determining step? rate = k[Y]2 [Z] 2Y + Z some product(s)