Unit 9- Chemical Kinetics Flashcards
Define Kinetics
the study of rates
Define Reaction Rates
Change in concentration of a reactant or a product with time. (M/s)
Over the course of a reaction, reactants are _____ and products are _____.
consumed-produced
We can study the rate, consequently, by looking at a _____ in concentration of reactants or an _____ in concentration of products.
decrease-increase
For the reaction: A—>B, Rate=
-Delta[A]/Delta t= Delta[B]/Delta t (Concentration of A decreases with time)
What are the factors affecting rate?
Temperature- [] or Pressure of Gas- Catalysts- Surface Area- Nature of Reactants TCPCSN
Remember that the _____ can be found by finding the slope of a line drawn tangent to the curve at a specific point.
Derivative
The rate always has to be _____.
Positive
Define Derivative
Can be found by using the slope of the tangent line drawn on the curve at a specific point.
Define Average Rate
Reaction speed over a period of time.
For a reaction aA+bB—> cC+dD, Rate=
-Delta[A]/aDeltat= -Delta[B]/bDeltat= Delta[C]/cDeltat= Delta[D]/dDeltat
The orders of the given reactants are _____ necessarily the same as the coefficient of the reactants.
Not
The reaction tells us how _____ a substance is on the rate of the reaction.
Influencing
If something is of 0 order, it means it _____ influence the rate.
Doesn’t
In the Method of Initial Rates, Rate=
k[A]^x[B]^y
Define the Method of Initial Rates
Using initial concentrations and initial rates (instantaneous rate at time= 0 sec) to find the reaction order.
What are “x” and “y” in the rate equation for the method of initial rates?
Orders of the reactants.
What is the method for getting the values of the orders using the method of initial rates?
Look at the change in initial rate and concentrations of the reactants.
The Differential Rate Law describes rate as a function of _____
Time
Remember to determine the order with respect to each reactant, we consider the reaction at a time when the reverse reaction is not a problem, at time=0. This is the method of _____.
Initial Rates
What is the integrated rate law for a zero order reaction?
[A]= -kt+[A]0
What is the integrated rate law for a first order reaction?
ln[A]= -kt+ln[A]0
What is the integrated rate law for a second order reaction?
1/[A]= kt+1/[A]0
The zero and first order reactions have a _____ slope while the second reaction has a _____ slope.
Negative- Positive
What are the units of k in a zero order reaction?
M/units of time
What are the units of k in a first order reaction?
1/units of time
What are the units of k in a second order reaction?
1/M.units of time
Reaction mechanisms show a _____ series of steps for a reaction
Possible
Each reaction step is called an _____
Elementary Step
The overall balanced equation adds each step and eliminates _____
Intermediates and Catalysts
Define Intermediate
Species that is produced then consumed in a reaction mechanism (first right-then left)
Define Catalyst
Species that is consumed then produced in a reaction mechanism (first left-then right)
What do you do to get the overall balanced equation in a reaction mechanism?
Add all of the elementary steps and cancel out the things that are the same on both sides.
The slow step is the _____. We use this step to write a possible rate law. The orders in the rate law will correspond to the mole coefficients in the rate determining step.
Rate determining step
Mechanisms can never be absolutely proven. Likely, mechanisms:
Add to the overall reaction- The rate determining step gives a rate law that matches experimental results.
Define Unimolecular
1 reactant in an elementary step
Define Bimolecular
2 reactants in an elementary step
Define Termolecular
3 reactants in an elementary step (very unlikely)
Whenever you’re dealing with fast equilibrium, _____
Set the concentrations of the substances equal to one another.
Define Activation Energy
Threshold energy that must be overcome for a reaction to occur; difference in potential energy of reactants to transition state (activated complex)
Collision theory states that molecules must collide with enough energy to overcome _____ barrier in order for reaction to occur.
Activation Energy (Ea)
What are the components of the collision theory?
Particles must collide with at least the activation energy to produce a reaction- particles must be oriented correctly to allow the formation of new bonds.
How do you increase the reaction rate?
Often, you increase the temperature which increases the kinetic energy
Increase in _____ —> increase in reaction rate —> there is a two-fold reason.
Speed
An increase in temperature increases _____. But, it also increases the _____ of collisions so that the percentage of _____ collisions increases.
Collision energy- Number- Effective
Rule of Thumb: A 10 *C increase in temperature corresponds to a _____ of the reaction rate.
2x increase; however, this is not always true. Some reactions slow down- some reactions speed up exponentially (HCl Production)
Molecules must collide with the appropriate _____
Orientation
What is the Arrhenius Equation?
k= zpe^ (-Ea/RT); k= rate constant; z= collision frequency; p= steric factor that reflects the fraction of collisions with effective orientation; e^(-Ea/RT): fraction of collisions with sufficient energy to produce reaction
What is the two-point form of the arrhenius equation?
ln (k1/k2)= (1/T2-1/T1)Ea/R
What is another form of the arrhenius equation?
lnk1= -(Ea/RT)+ lnA ; plot of lnk vs. 1/T, slope= -Ea/R
What is important about e^(-Ea/RT)?
It gives us the fraction of collisions with required energy; can be useful to find the fraction of molecules at a specified energy value (plug in every value as Ea).
What are the steps for solving the two-point form of the arrhenius equation?
1) Do the right side- 2) the known “k” will be the coefficient behind “e” and the answer to the right side will be the power of “e”- 3) Solve
What is the unit for Ea?
Joules
Explain why increasing the temperature increases the reaction rate.
Increasing the temperature increases the energy of collisions making it more likely for the reaction to overcome its threshold energy (Ea). The frequency of collisions also increases which increases the chance of a correctly oriented collision.
Explain why increasing the reaction concentration increases the reaction rate.
More reactants increases collision frequency making it more likely for reactants to be oriented correctly.
Explain why using an enzyme increases the reaction rate
Enzymes are catalysts which lower the activation energy making lower energy collisions more likely to produce a fruitful reaction.
In terms of this chapter, explain why diamond is not converted to graphite even though the process has a large equilibrium constant
Diamond bonds are strong and room temperature does not have enough energy to overcome this barrier (Ea).
In terms of this chapter, explain why the rate constant for a zero order reaction equals the reaction rate.
A zero order reaction does not depend on the reactant concentration and is only influenced by reaction type and temperature (these change the k value)
In terms of this chapter, explain why the slow step in a reaction determines the overall rate of reaction
A reaction can only be as fast as the slowest step. The slowest step is the rate-determining step.
