Ch. 14 Chemical Kinetics
Reactant Concentration
Larger concentration of the reactant increase the rate of the reaction, this increases the frequency of reactant molecule contact, concentration of cases depends on the partial pressure of the gas, higher pressure = higher concentration
Molecular Interpretation of Factors Affecting Rate-Catalysts
Lower energy of activation
What are the units of k for a zeroth order reaction?
M/s, rate = k[A]^0 > M/s = k x 1 = M/s
What are the units of k for a third order reaction?
M^-2 s^-1, rate = k[A]^3 > M/s = k M^3 M/M^3 s = M^-2 s^-1
Intermediates
Materials that are products in an early mechanism step, but then reactants in a later step.
Average Rate of Reaction
Measuring the change in concentration over specified period if time in the interval, average because the rate is not constant and can fluctuate.
Activation Energy
Minimal amount of energy required for a reaction to commence: kJ/mol, E(a) > 0
Powdered solids are more reactive than "blocks." Why?
More surface area for contact with other reactants.
Ions react faster than molecules. Why?
No bonds to be broken. For example, potassium is more reaction than sodium.
Predicting the rate at some future time is _______ possible from the balanced equations that determined the rate of change in concentration of any reactant/product at that point in time.
Not possible
Determine the overall order for a reaction with the following rate law: Rate = k[A]^2[B][C]^3
Sixth Order (2 + 1 + 3 = 6)
Factors Affecting Reaction Rate: Nature of the Reactants
Size of molecules, state, chemical formation
_____ molecules tend to react fast than ______ molecules.
Small; large
Catalyst and Rates
Substances that affect the rate of a reaction without being consumed. They work by providing an alternative mechanism for the reaction with a lower activation energy. May be consumed in an early mechanism step, and then made in a later step.
Factors Affecting Reaction Rate
Temperature, Catalysts, Reactant Concentration
Rate of a Reaction
The Change in Products/ Np x Change in Time = -The Change in Reactants/Nr x Change in Time Np = product coefficient, Nr = reactant coefficient
Half-Life
The amount when the initial concentration is halved, after 1 half life it is reduced by 50%, after 2 it is reduced by 25%, after 3 it is reduced by 12.5%
Activation Energy
The difference in energy between the reactants and the activated complex.
Why do reactants decrease?
They are consumed by the reaction.
Why do product concentrations increase with time?
They are formed in a reaction.
Validating a Mechanism
To validate, not prove, a mechanism-the elementary step must sum to the overall reaction, the rate law predicted by the mechanism must be consistent with the experimentally observed rate law.
Increasing the temperature of a reaction increases the kinetic energy of practically all molecules in the system.
True
Reaction Mechanisms
We describe chemical reaction with the chemical equation. The probability of more than three molecules colliding at the same instant with proper orientation and sufficient energy to overcome the energy barrier is negligible. Most reactions occur in a series of small reactions involving one, two, to at most three molecules. Describing the series of reaction is called a reaction mechanism. Knowing the rate law of the reaction helps us understand the sequence of reactions in mechanism.
Temperature
Increasing the temperature, raises the average kinetic energy of molecules. Increasing the temperature increases the number of molecules with sufficient kinetic energy to overcome activation energy.
What type of reaction rate is measured at time zero?
Initial Rate Reaction
How can you calculate activation energy?
It is the difference between energy levels of reactants and that of the local maximum.
The Effect of Temperature on Rate
k = A(e ^ -Ea/RT) T = temperature in kelvins, R = gas constant in energy units 8.314 J / mol K, A = frequency factor, the rate the reactant energy approaches the activation energy, Ea = activation energy, the extra energy need to restart the molecules reacting.
Higher frequency of collisions is
proportional to a faster reaction rate
A rate of change will always be negative for any _________, because they re consumed over time.
reactant
ln(k) =
-Ea/R (1/T) + ln(A)
How to Continuously Monitor the Reaction?
-Polarimetry: organic, measures optical activity exhibited by inorganic/organic compounds -Spectophotometry: color, absorb light/emit light -Total Pressure: gases, how pressure increases/decreased in a system with relation to molecules/particles
Properties for The Rate of a Reaction
-do not need coefficients when you are looking at the rate of a reactant/product on its own -disappearance = consuming = implies negative sign
Tips for Reading Activation Energy Diagrams
1) Activation energy is always a positive value and is the energy difference between the local maximum and the reactant(s)/intermediate(s). The most common units are kJ/mol. 2)The change in enthalpy is Hproducts - Hreactants. 3) For exothermic processes, Hproducts<Hreactants. (Change in H < 0) 4) For endothermic processes, Hproducts>Hreactants (Change in H > 0)
Concentration Curves
1) Slope is steepest near the start of the reaction (time zero) and the average rate is fastest there. 2) The reaction rate is seldom constant. If the reaction rate is proportional to the slope of the line, then a constant rate would give a horizontal line. 3) If enough time elapses, then the concentration curve approaches a horizontal line. The reaction slows down such that concentrations are held constant, defined in the region known as chemical equilibrium.
What are the two factors that determine when molecules collide they may or may not react with each other?
1. Whether the collision has enough energy to "break" the bonds holding reactant molecules together. 2. Whether the reacting molecules collide in the proper orientation for new bonds to form.
What is the average amount of collisions between molecules?
109 collisions/second
Suppose the activation energy is 100 kJ/mol and the change in enthalpy (H) us -30 kJ/mol. What would be the activation energy for the reverse reaction where the products went on to make reactants?
130 kJ/mol (100 - (-30)
A reaction rate is found to decrease by a factor of 1/4 when a reactant concentration is halved. What is the order with respect to the reactant?
2nd order, (1/2)^2
How many half lives are required for uranium to decay to 12.5% of its original concentration?
3
Suppose the activation energy is 100 kJ/mol and the change in enthalpy (H) us +30 kJ/mol. What would be the activation energy for the reverse reaction where the products went on to make reactants?
70 kJ/mol (100 - 30)
The order of a reaction can be determined only by experiment.
A common way to determine reaction order is the method of initial rates. In this method, the initial rate—the rate for a short period of time at the beginning of the reaction—is measured by running the reaction several times with different initial reactant concentrations to determine the effect of the concentration on the rate.
Collision Theory and the Frequency Factor of the Arrhenius Equation
A: the frequency factor: ln(k) = -Ea / RT +ln(A) Relate to the two factors that determine whether a collision will be effective. Collision frequency is the number of collisions per second. The max collisions there are, the more collisions can be effective.
Thermal Energy Distribution
As temperature increases, the fraction of molecules with enough energy to surmount the activation energy barrier also increases.
Enzyme
Biomolecules are large and complex, require a catalyst to proceed at a reasonable rate, work by absorbing the substrate reactant on to an active site that orients the substrate for reactions.
Activated Complex
Chemical species with partial bonds
Effective Collision
Collision between two reactants with the correct orientation and with sufficient energy to overcome the activation barrier, result of a chemical reaction.
How and why does a system react?
Collisions
Effective Collisions
Collisions with necessary minimum amount of energy and proper orientation. The higher the frequency of effective collisions, the faster the reaction rate.
Rate Laws for Elementary Steps
Each step in the mechanism is like its own little reaction with its own activation energy and own rate law. The rate law for an overall reaction must be determined experimentally. But the rate law of an elementary step can bet deduced from the equation of the step.
Catalysts
Facilitates reaction, reformed as product at the end. Affects the speed of the reaction without being consumed.
Increasing the temperature of a reaction always brings the kinetic energy of all molecules in the system past the activation energy.
False
Increasing the temperature of a reaction increases the activation energy.
False
There is a linear dependence of the rate constant k on the Kelvin temperature.
False.
What reaction order has a half-life independent of its initial concentration?
First Order Reactions
Collision Theory: Consider A + BC > AC + B
For a chemical reaction to occur: 1) molecules A and BC must physically collide with each other 2) the collision between A and BC must occur with sufficient energy such that the bond between B and C breaks 3) reactant molecule A must collide with the C side of the BC compound (collisions must occur with correct spatial orientation)
Collision Theory of Kinetics
For most reactions, for a reaction to take place, the reacting molecules must collide with each other. Once molecules collide they may react together or they may not depending on two factors.
Heterogeneous Catalysts
Hold one reactant molecule in proper orientation for reaction to occur when the collision takes place. (sometimes they also start breaking bonds)
What are the two forms of a catalyst?
Homogeneous, heterogeneous
Heterogeneous Catalyst
In different phase
Rate Determining Step
In most mechanisms, one step occurs slower than the other steps. The result is that product production cannot occur any faster than the slowest step; the step determined the rate of the overall reaction. We call the slowest step in the mechanism the rate determining step. Slowest step has the highest activation energy.
Homogeneous Catalyst
In same phase
Temperature: Reaction Rate
Increasing temperature increases the reaction rate. Generally, for each 10* C rise in temperature, the speed of the reaction doubles.
Speed = Reaction ____
Rate
Zero Order Reactions
Rate = k[A]^0 = k slope = -k, y-intercept = [A}initial [A] = -kt + [A]initial t 1/2 = [A initial] /2k Rate = M/s, k = M/s
Relationship between Order and Half Life: Zero Order
Rate = k[A]^0 = k, t 1/2 = [A initial] / 2k -The lower the initial concentration of the reactants, the shorter the half life.
First Order Reactions
Rate = k[A]^1 = k[A] slope = -k, y-intercept = ln[A initial] ln[A] = -kt + ln[A initial] t 1/2 = 0.693 / k (constant) = ln 2 / k t 1/3 = ln 3 / k , and so on rate = M/s, k = 1/s
Relationship between Order and Half Life: First Order
Rate = k[A]^1 = k[A], t 1/2 = ln (2) / k -the half life is independent of the concentration
Second Order Reactions
Rate = k[A]^2 slope = k, y-intercept = 1/ [A initial] 1/[A] = kt + 1/[A initial] t 1/2 = 1/(k[A initial]) Rate = M/s, k = 1/Ms
Relationship between Order and Half Life: Second Order
Rate = k[A]^2, t 1/2 = 1/(k[A initial]) -the half life is inversely proportional to the initial concentration, increasing the initial concentration shortens the half life.
The Rate Law
Rate = k[A]^n[B]^m, must be determined experimentally, aA +bB > products, n,m are called the orders for each reactant, add reagents (exponents) together mathematically to determine the order.
Concentration
Rate increases as the concentration of reactant molecules increases, except for zero order reactions. More molecules leads to more molecules with sufficient kinetic energy for effective collisions.
Homogeneous Catalysts
React with one of the reactant molecules to form a more stable activated complex with a lower activation energy.
What factor affect the rate of a reaction?
Reactant Nature, Temperature, Concentration
Collisions alone are not sufficient. Why?
Reactants must possess enough kinetic energy during the collision, energy is transferred between reactant molecules and is mainly used to break existing chemical bonds.
Reactant Nature
Reactions are faster in solution than in pure substances. -mixing gives more particle contact -particles are separated, allowing more effective collisions per second -Forming some substances breaks bonds that need to be broken
Chromatography
Separate samples, observe concentrations over time.
Activation Frequency
The number of molecules that begin to form the activated complex in a given period of time.
Molecularity
The number of reactant particles in an elementary step is called its molarity. A unimolecular step involves one particle, a bimolecular step involves two particles, a trimolcular step involves three particles.
Zero Order Reaction
The of the reaction is always the same. Doubling [A] will have no effect on the reaction rate.
Second Order Reaction
The rate is directly proportion to the square of the reactant concentration. Doubling [A] will quadruple the rate of the reaction, exponential effect on the concentration.
First Order Reaction
The rate is directly proportional to the reactant concentration, it has a 1:1 ratio. Doubling [A] will double the rate of the reaction.
Defining Reaction Rate
The rate of a chemical reaction is generally measured in concentration by time. Rate = - Change in M / Change in Time
Why does the average rate decrease as the reaction progresses?
The reaction slows down as it proceeds.
Elementary Steps
The reactions cannot be broken down into simpler steps and that the molecules actually interact directly in the same manner without any other steps.
Chemical Kinetics
The study of the factors that affect the rates of chemical reactions, such as temperature.
Overall Order
The sum of the exponents (m + n).
Transition State/Activated Complex
Theoretical model that shows all bond-breaking and bond-making during a reaction.
Change in M (molarity concentration) / Change in t (time, seconds) is known as the
average rate of the reaction.
Average rate of change is ________ after enough time has elapsed.
constant
Half-Life For a First Order Reaction
constant and independent of concentration
Average rate of change is ________, near the start of an experiment.
fastest
______ tend to react faster than _____, which react faster than ______. Why?
gases; liquids; solids -particles are more spread out/less compact in this decreasing order
Arrhenius Equation: Two Point Form
ln (k2/k1) = Ea/R (1/T1 - 1/T2)
x = y^z
log(x) = z log (y)
Reactant concentrations descreas as a reaction proceeds, the change in concentration of a reactant is ________, the _________ sign thus makes the overall rate _________.
negative; negative; postive
The rate determining step in a reaction is the ________.
slowest step
A first order reaction has a half life of 26.4 seconds. How long does it take for the concentration of the reactant of the reaction to fall to 1/8 of its initial value?
t 1/2 = 26.4 seconds = ln 2 / k k = ln 2 / 26.4 seconds = 2.63 x 10^2/s t 1/8 = ln 8 / 2.63 x 10^2/s = 79.2 s
The larger a reactant's or product's stoichiometric coefficient,
the greater the rate of change of its concentration.
