SCH4U1
CHAPTER 6: CHEMICAL KINETICS(SUMMARY NOTES)
*chemical kinetics  the branch of chemistry concerned with chemical reaction __________(i.e.
how fast or slow reactions progress)
*rate of reaction  the speed at which a chemical change occurs, generally expressed as change in
_____________________ per unit ___________
average rate of reaction =
change in concentration
change in time
or
rate =
*Units: ________________ or _________________
GRAPHING REACTION RATES:
*average rate of reaction  the speed at which the reaction proceeds over a period of time (often
measured as change in concentration of a ______________or _____________ over time) **The
average rate of reaction over a time period is the absolute value of the slope of the
________________.
*instantaneous rate of reaction  the speed at which a reaction is proceeding at a particular point in
time **The instantaneous rate of reaction can be found by determining the slope of the
_______________ at a particular point in time.
*How would the graph for the production of a product versus time (e.g. HCl from the reaction above)
be different? Provide a rough sketch.
SOME PROCEDURES USED TO MEASURE REACTION RATES
1. Collection of an Evolved Gas/Increase in Gas Pressure
*A gas produced in a reaction can be collected by using a gas ____________ or in a
_________________ vessel over _________. The ________________ of gas collected at different
times can be recorded. This technique cannot be used if the gas evolved is ____________ in water. If
this is the case, measuring the change in gas ______________ could be used as an alternative.
This technique would be suitable for measuring the rate of reaction between ________ metal and
___________________ acid.
Zn(s) +
HCl(aq) 
Net Reaction:
2. Measurement of the Mass of the Reaction Mixture
*The total mass of a reaction mixture will only vary if a __________ is evolved. By measuring the
rate of mass ____________, the rate of reaction can be determined. This technique is especially
effective if the gas evolved has a high molar mass. Also, the gas should not be significantly
_____________in the solvent used. (Why is this important?)
This technique would be suitable for measuring the rate of reaction between _____________
________________ and hydrochloric acid.
CaCO3(s) +
Net Reaction:
HCl(aq) 
3. Light Absorption
If the reaction involves a _________________ reactant or product, then the _______________of the
colour can be used to monitor the concentration of that species.
Examples:
a) S2O32-(aq) + 2H+(aq)  H2O(l) + SO2(g) + S(s) *Coloured entity - ______________
b) CH3COCH3(aq) + *I2(aq)  CH3COCH2I(aq) + H+(aq) + I-(aq) *Coloured entity - _______
4. Electrical Conductivity
*A solution will conduct when _________ are present. Therefore, if the reaction is accompanied by a
change in the __________________of ions, the reaction rate can be determined by measuring the
change in _____________________.
Using this technique would be suitable for the following reaction. Note the presence of non_______________ reactants. *Why is this important?
PCl3(aq) + 3H2O(l)  H2PO4-(aq) + 4H+(aq) + 3Cl-(aq)
6.2 Factors Affecting Reaction Rate
1.______________________________
e.g The activity series of metals (e.g. Lithium and potassium react with water while silver and gold do
not.)
2. ___________________
 If the initial concentration of a reactant increases, the reaction rate generally increases. [e.g. The
rate of hydrogen gas production is greater when magnesium reacts with a more concentrated acid (e.g.
3 M HCl vs. 1 M HCl)]
3.___________________
 Higher temperatures generally accompany higher reaction rates and vice versa.
4. Presence of a _________________
 Catalysts lower the Activation Energy associated with a reaction, thereby increase the rate of
reaction.
5. _________________________
 Increasing the surface area of a reactant increases the reaction rate. (e.g. Kindling will burn much
faster than a log of wood.)
6.3 EXPLAINING REACTION RATES: COLLISION THEORY
 Collision theory is used to explain rates of reaction.
Recall:
*For a reaction to occur, the bonds of the reactant molecules must be_____________. The energy
required to accomplish this is provided by the _____________(i.e. movement) energy of the
molecules.
Concepts of Collision Theory
1.
The particles of a chemical system are in constant random motion. The higher the __________
__________ __________ of the particles, the higher the ________________.
2.
A chemical reaction must involve _________ of particles with eachother or the walls of the
container.
3.
The collision must be ______________. For a collision to be effective, molecules must:
(i)
collide with _______________ energy; and,
(ii)
collide with correct __________________.
http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/collis11.swf
4.
The rate of a given reaction depends on two factors:
(i)
the _________________ of collisions; and,
(ii)
the fraction of those collisions that are _______________.
*[rate = collision ________________ x fraction _____________]
*Factors that affect collision frequency:
1. ___________________
2. __________ ________
3. __________________
*Factors that affect effectiveness:
1. ________ __ ____________
2. _______________
3.__________________
*Activation Energy (Ea) the minimum increase in ___________ energy of a system required for
molecules to react (i.e. the minimum amount of energy with which particles must collide before they
can react).
 Common mistake: You do not “give the particles activation energy”. Activation energy is the
energy BARRIER. You do not give runners “hurdles”, you give them the energy to jump over the
hurdles.
http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/activa2.swf
*Activated Complex  an unstable chemical species containing ______________ broken and
_____________formed bonds representing the __________________potential energy point in the
change; also known as the _______________ state (The activated complex is an immeasurable,
transient entity.)
6.5 RATE LAW / RATE EXPRESSION (and Order of Reaction)
*The Rate Expression/Law  The rate will always be proportional to the product of the
____________ concentrations of the reactants, where these concentrations are raised to some
exponential value. The Rate Expression is always ______________________determined.
*This can be expressed  Rate 
*The rate law equation  Rate =
Where:
k = __________ constant,
[X] and [Y] = _____________ concentrations of reactants,
m is the order with respect to reactant _____,
n is the order with respect to reactant _____,
m + n is the _____________ order of reaction.
Example: rate = k[X]1[Y]2 [Z]0
*actually written 
*The Overall Order of Reaction =
(i.e. This equation represents a _______ order reaction)
Individual Orders:
*order of reaction with respect to X = ____
*order of reaction with respect to Y = ____
*order of reaction with respect to Z = ____
*Why?
*What do the exponents (i.e. individual orders) mean?
a) Exponent 0 (e.g. r α [Z]0):
 Doubling the initial concentration of Z, has ____ __________ on the rate. Tripling the initial
concentration of Z, has ___ _________ on the rate, and so on. (*i.e., changing the initial
concentration of the reactant has ____ _______ on the rate when the order is equal to ________.)
*This is why “[Z]0” does not appear in the rate equation.
b) Exponent 1 (e.g. r α [X]1):
 Doubling the initial concentration of X, __________ the rate. Tripling the initial concentration
of X, ___________ the rate, and so on. (*i.e., whatever happens to the initial concentration of X
______ happens to the rate when the order is equal to ______.)
c) Exponent 2 (e.g. r α [Y]2):
 Doubling the initial concentration of Y, _______________(22) the rate. Tripling the initial
concentration of Y, increases the rate by a factor of _____ (32), and so on. (*i.e., the rate change is
equal to whatever happens to the initial concentration of Y, __________… when the order is equal to
_______.)
Summary:
Order of Reaction
Concentration
0
1
2
3
Change
x1
x 2 (doubling)
x 3(tripling)
Determining Rate Laws  Example:
*When aqueous bromate and bisulfite ions react to produce bromine, the overall equation is:
2BrO3-(aq) + 5HSO3-(aq)  Br2(g) + 5SO42-(aq) + H2O(l) + 3H+(aq)
Consider the series of experiments recorded in the following table, in which initial reactant
concentrations are varied and rates are compared.
a) From the evidence provided, determine a rate equation.
Trial
Initial [BrO3-(aq)]
Initial [HSO3-(aq)]
-3
/ mmol dm
/ mmol dm-3
1
2
3
4.0
2.0
2.0
6.0
6.0
3.0
Initial rate of Br2(g)
production / mmol dm-3
s-1
1.60
8.0
2.0
Summary of Procedure:
*To determine x:
 Find 2 trials for which the [HSO3-(aq)] remains ____________. As a result, any rate changes that
occur during these two trials would be a result of changes to the ___________concentration. (Trials =
_____ & ______)
 During these two trials the [BrO3-(aq)] _____________ . As a result of this doubling in
concentration, the rate of reaction also _____________. Therefore, the order of reaction with respect
to BrO3-(aq) is ________.
*To determine y: repeat the same process. (y = _______);
*Therefore, Rate =
b) Determine the value of the rate constant, K, by substituting values from _________ trial into the
rate law equation. Also, determine the units of K.
6.6 REACTION MECHANISMS
*Quite often, chemical changes are too complicated to happen in one simple stage. Instead,
the reaction may involve a series of small changes one after the other.
*A ______________ ______________ is a sequence of elementary steps that make up an
overall reaction.
*Each step, called an ________________ reaction (or elementary step), involves one-,
two-, or three-particle collisions.
Example:
Consider the following overall reaction:
2NO(g) + O2(g)  2NO2(g)
Proposed two-step mechanism: (Each step is called an ________________ reaction.)
Step 1:
Step 2:
NO(g) + O2(g)  NO3(g)
NO3(g) + NO(g)  2NO2(g)
*The sum of the elementary reactions yields the overall reaction. Notice that NO3(g) is produced in the
first step and consumed in the second step. NO3(g) is an example of a reaction ________________.
*Reaction intermediate  a chemical entity formed in an elementary reaction and ____________ in
a subsequent elementary reaction (Reaction intermediates do not appear in the _____________
reaction; they are finite, measurable.)
The Molecularity of Elementary Reactions
*Molecularity – refers to the __________ of reactant particles that are involved in an elementary
reaction.
*Unimolecular elementary reaction  occurs when ______ molecule or ion reacts
e.g. Cl2(g)  2Cl(g)
e.g. O3(g)  O2(g) + O(g)
*Bimolecular elementary reaction  occurs when _______ particles collide and react
e.g. O3(g) + O(g)  2O2(g)
e.g. H2(g) + I2(g)  2HI(g)
*Termolecular elementary reaction  involves __________ particles colliding all at once(very
rare)Why?
Practice Problems:
1. The following two-step mechanism has been proposed for the gas-phase decomposition of nitrous
oxide (N2O)
Step 1: N2O(g)  N2(g) + O(g)
Step 2: N2O(g) + O(g)  N2(g) + O2(g)
(a) Determine the overall equation. 
(b) Identify the reaction intermediate(s). 
(c) What is the molecularity of each elementary step? Step 1____________ ; Step 2
____________
Rate Law Equations for Elementary Reactions
*The rate law for an elementary step follows directly from the molecularity of that step. (Exception to
follow.)
That is,
 unimolecular E.R’s are always _________ order
 bimolecular E.R’s are always _________ order
 termolecular E.R’s are always __________ order
*Mechanisms are _________________ rather than definitively stated. Proposed mechanisms must
satisfy the following criteria:
1.
2.
3.
The equations for the elementary steps must combine to give the equation for the _________
reaction.
The proposed mechanism must be reasonable.
The mechanism must support the _____________________ determined _________ _________.
The Rate-Determining Step (RDS) and the Rate Law:
*Elementary reactions in mechanisms all have different rates. The ________________ elementary
reaction, the ___________-determining step, determines the overall rate.
Practice Problem:
Propose the rate law for the following reaction: 2NO2(g) + Cl2(g)  2NO2Cl(g)
Proposed mechanism:
Step 1:
NO2(g) + Cl2(g)  NO2Cl(g) + Cl(g) (slow)
Step 2:
NO2(g) + Cl(g)  NO2Cl(g)
(fast)
*Rate-determining step = Step ________
*Rate law for Step 1 = proposed rate law for the overall reaction; that is: Rate1 = ________________
*The proposed rate law must be consistent with the ________________ determined rate law in order
for the proposed mechanism to be considered reasonable.
TEXT PRACTICE QUESTIONS
6.1: READ AND p. 350 #1 ; p. 352 #1 ; p. 356 #1
6.2: READ
6.3: READ AND p. 372 #1,2
6.5: READ AND p. 380 #1-5 *More Practice p. 382 #1-4
6.6: READ AND p. 386 #1-3 *More Practice p. 387 #1-9