Ch. 13: Chemical Kinetics
13.1-13.3
13.1 What is Chemical Kinetics?
Measure of how fast a reaction occurs
Reflects change in concentration of a reactant (or product) per unit of time
Measured in M/s
Why this matters?
Rates of chemical reactions, and especially the ability to control these rates, are
important in many phenomena in science and within human function
e.x. A successful rocket launch depends on the rate at which fuel burns -- too
quickly and the rocket could explode, too slowly and it will not leave the
State, Concentration, Temperature, Catalyst
physical state of the reactants
concentration of the reactants
temperature of the reaction
presence or absence of a catalyst
13.2 Reaction Rates
It is often determined by measuring the change in concentration of a reactant
or product with time
The speed of the chemical reaction is its reaction rate.
For a reaction A to B:
Average rate with respect to B= change in the concentration of B/change in time
•Here the change in the concentration of B is defined as:
Δ(concentration of B) = (concentration of B at final time) − (concentration
of B at initial time)
Let’s Use an Example!
Suppose A reacts to form B. Let us begin with 1.00 M A.
At t = 0 (time zero) there is 1.00 M A and no B present.
At t = 20 sec, there is 0.54 M A and 0.46 M B.
At t = 40 sec, there is 0.30 M A and 0.70 M B.
We can use this information to find the average rate with respect to B
Avg Rate = Δ (concentration B) / Δ (time) =
Change of Rate with Time
In most chemical reactions we will determine the reaction rate by monitoring a change in concentration (of a
reactant or product)
The most useful unit to use for rate is molarity
Since volume is constant, molarity and moles are directly proportional
Consider the following reaction: C4H9Cl(aq) + H2O(l) ---> C4H9OH(aq) + HCl(aq)
We can calculate the average rate in terms of the disappearance of C4H9Cl
The units for average rate are mol/(L•s) or M/s.
The average rate decreases with time.
Alternate Way to Measure Reactions
For the reaction A to B there are two ways of measuring rate:
the rate of appearance of product B (i.e., change in moles of B per unit time) and
the rate of disappearance of reactant A (i.e., the change in moles of A per unit time).
Average rate= - Δ [A]/ Δ t
Square brackets mean “the molar concentration of”. [A] = the concentration of A
in moles/L
Note the negative sign! This reminds us that rate is being expressed in terms of
the disappearance of a reactant.
Reaction Rates and Stoichiometry
For the reaction: C4H9Cl(aq) + H2O(l) --> C4H9OH(aq) + HCl(aq)
The rate of appearance of C4H9OH must equal the rate of disappearance of C4H9Cl.
Rate = -Δ[C4H9Cl] / Δt = Δ[C4H9OH] / delta t
What if the stoichiometric relationships are not one-to-one?
For the reaction: 2HI(g) ---> H2(g) + I2(g) the rate may be expressed as:
rate = -½ Δ[HI] /Δ t = Δ[H2] / Δt = Δ[I2] / Δt
We can generalize this equation a bit.
14.3 Concentration and Rate
In general, rates:
increase when reactant concentration is increased.
decrease as the concentration of reactants is reduced.
We often examine the effect of concentration on reaction rate by measuring the way in which reaction rate at the beginning of a reaction
depends on starting conditions.
Consider the reaction:
NH4 + (aq) + NO2 – (aq) --> N2(g) + 2H2O(l)
We measure initial reaction rates
The initial rate is the instantaneous rate at time t = 0.
NH4 + (aq) + NO2 – (aq) --> N2(g) + 2H2O(l)
Rate = k[NH4 + ][ NO2 – ]
The proportionality constant k is called the rate constant.
Once we have determined the rate law and the rate constant, we can use them to calculate
initial reaction rates under any set of initial concentrations.
Exponents in the Rate Law
For a general reaction with rate law:
Rate = k[reactant 1]m[reactant 2]n
The overall order of reaction is m + n + …
For the reaction: NH4 + (aq) + NO2 – (aq) N2(g) + 2H2O(l)
The reaction is said to be first order in [NH4 + ], first order in [NO2 – ], and second order overall.
Note that reaction orders must be determined experimentally. They do not necessarily
correspond to the stoichiometric coefficients in the balanced chemical equation!
Units of Rate Constants
Units of the rate constant depend on the overall reaction order.
For example, for a reaction that is second order overall:
Units of rate are:
Units of rate = (Units of rate constant) (Units of concentration )2
Thus the units of the rate constant are:
Units of rate constant = (Units of rate) / (units of concentration)2 = (M/s) / (M2) =
M -1 s -1
Using Initial Rates to Determine Rate Laws
To determine the rate law, we observe the effect of changing initial
concentrations.
If a reaction is zero order in a reactant, changing the initial concentration of that reactant will
have no effect on rate (as long as some reactant is present).
If a reaction is first order, doubling the concentration will cause the rate to double.
If a reaction is second order, doubling the concentration will result in a 22 increase in rate.
Similarly, tripling the concentration results in a 32 increase in rate.
A reaction is nth order if doubling the concentration causes a 2n increase in rate.
College Board Reqs.
Students apply, as well as engage and reason with, the following concepts in the performance expectations:
•
effective.
In order for reactions to occur, the reacting particles must collide in the appropriate orientation and with enough energy. Not all collisions are
•
Most reactions occur in solution or in the gas state because the reacting particles are free to move and can collide and interact with each
other. Reactions among solids are not as prevalent because a reaction can only occur at the surface of a solid.
The rate of reaction can be defined as the change in the amount of products or reactants per unit of time. •
The rates at which reactions occur are
affected by factors such as concentration, pressure, temperature and the addition of a catalyst. • All stable species require the input of energy to initiate a
reaction. The amount of energy required is called the activation energy barrier. • When the concentrations/pressures of the reactants are increased, the
probability of a molecular collision increases. Because a molecular collision may lead to a reaction, the rate of reaction increases as the probability of a
molecular collision increases. • When the kinetic energy of the reactants increases, indicated by a rise in temperature, the probability of a molecular collision
increases. When molecules/atoms collide with increased energy, they are more likely to react. •
The process of dissolving a solute in a solvent may be
considered a reaction, and the process is affected by many of the same factors (temperature, intermolecular forces and surface area) that affect reaction
rates. •
The addition of a catalyst provides an alternate pathway for reactions to occur, usually with a lower activation energy barrier. More molecules
therefore have enough energy to overcome the activation energy barrier, leading to an increased rate of reaction. • One of the functions of an enzyme is to hold
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