General Chemistry- Chemical Kinetics Flashcards
Reactions can be __________ or ________.
Spontaneous
Nonspontaneous
What determines whether a reaction will occur by itself or with outside help?
The change in Gibbs free energy (/\G)
If a reaction is spontaneous, does it mean it will run quickly?
No
Every biochemical reaction that enables life proceeds, how?
So slowly that, without the aid of enzymes and other catalysts, measurable reaction progress might not actually occur over the course of an average human lifetime.
Enzymes like other catalyzed reactions, ca be what and experience what?
Can be saturated and experience a maximal turnover rate.
What is the mechanism of a reaction.
Many reactions proceed by more than one step, the series of which is known as the mechanism of a reaction.
The sum of the mechanism of a reaction gives what?
The overall reaction
Knowing the accepted mechanism of a reaction may help explain what?
Reaction’s rate,
Position of equilibrium
Thermodynamic characteristics
What is an intermediate?
The molecule A, B, which does not appear in the overall reaction
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Why might intermediates be difficult to detect?
They may be consumed almost immediately after they are formed
What is the rate-determining step?
The slowest step in any proposed mechanism
Why is it called the rate determining step?
It acts like a kinetic bottleneck, preventing the overall reaction from proceeding any faster than that slowest step.
What does the collision theory of chemical kinetics state?
The rate of a reaction is proportional to the number of collisions per second between the reacting molecules
What does the collision theory of chemical kineticcs suggest?
Not all collisions result in a chemical reaction
What is an effective collision according to the collision theory of chemical kinetics?
The molecules collide with each other in the correct orientation and with sufficient energy to break their existing bonds and form new ones.
What is the activation energy?
The minimum energy of collision necessary for a reaction to take place.
What is another name for activation energy?
The energy barrier
Do all colliding particles have enought kinetic energy to exceed the activation energy?
Only a fraction of the colliding particles have enough energy to exceed the activation energy
Are all collisions effective, according to the collision theory of chemical kinetics?
Only a fraction
What equation can express the rate of a reaction?
Rate= Zxf
What do the letters represent in the equation for rate of a reaction?
Rate=Zxf
Z: the total number of collisions occuring per second
f: The fraction of collisions that are effective
What is the arrhenius equation?
k=Ae^(-(Ea)/RT)
What do the letters represent in the Arrhenius equation?
k=Ae^(-(Ea)/RT) k: The rate constant of a reaction A: The frequency factor Ea: The activation energy of the reaction R: the ideal gas constant T: Temperature in Kelvins
What is the frequency factor?
A measure of how often molecules in a certain reaction collide, with the unit S^-1
What is another name for the frequency factor?
The attempt frequency
What is important when studying the Arrhenius equation?
Not the calculation, but the relationships between the variable and the exponent rules that govern equations.
In the Arrhenius equation, as the frequency factor increases, what happens?
The rate constant of the reaction also increases in a direct relationship
In the arrhenius equation, if the temperature of a chemical system were to increase to infinity, what would happen?
The value of the exponent would have a magnitude less than 1
Does the rate constant decrease as a result of the temperature of a chemical system increasing to infinity in the Arrhenius equation?
No, as the magnitude of the exponent get smaller, it actually moves from a more negative value towards zero
The rate of reaction increase with what?
Temperature
The frequency factor, in the Arrhenius equation, can be increased how? Why?
By increasing the number of molecules in a vessel. When there are more molecules, the opportunities for collision are increased
What happens when molecules collide with energy equal to or greater than the activation energy, in regards to the transition state theory
They form a transition state in which the old bonds are weakend and the new bonds begin to form
What happens after the transition state, in regards to the transition state theory?
The transition state dissociates into products, fully forming the new bonds
What is the reaction coordinate?
The progress which traces the reaction from reactants to products
What’s another name for transition state?
Activated complex
The transition state has greater ___________ compared to __________, in regards to the transition state theory.
Greater energy than both the reactanst and the products
What symbol is used for the transition state?
++ (but up and down)
What is the activation energy?
The energy required to reach this transition state
What happens once an activated complex is formed, in the transition state theory?
It can either dissociate into the products or revert to reactants without any additional energy input.
What distinguishes transition states from reaction intermediates?
Transition states are theoretical constructs that exist at the point of maximum energy, rather than distinct identities with finite lifetimes
What is a free energy diagram?
A diagram that illustrates the relationship between the activation energy, the free energy of the reaction, and the free energy of the system
What are the most important feactures in the free energy diagrams?
The relative energies of all the products and reactants
What is the free energy change of the reaction?
(/(Grxn))The difference between the free energy of the products and the free energy of the reactants.
What is an exergonic reaction?
A negative free energy change
or Energy is given off
What is an endergonic reaction?
Positive free energy change
or energy is absorbed
Where does the transition state exist in a free energy diagram?
At the peak of the energy diagram
What is the differenc in free energy between the transition state and the reactansts?
The activation energy of the forward reaction
What is the difference in free energy between the transition state and the products?
The activation energy of the reverse reaction
What are the 4 factors that affect reaction rate?
Reaction concentrations
Temperature
Medium
Catalysts
The greater the concentration of the reactanst, the greater _____________.
The number of effective collisions per unit time.
The greater the number of effective collisions per unit of time, leads to what?
An increase in the frequency factor (A) of the Arrhenius equation
The greater the concentration of reactants will cause what?
The reaction rate will increase for all but zero-order reactions
How do you meausure the reaction concentration for reactions occuring in the gaseous state?
The partial pressures of the gas reactants serve as a measure of concentration
For nearly all reactions, the reaction rate will increase with what?
The reaction rate will increase as the temperature increase
Why will an increase in temperature cause an increase in the reaction rate?
The temperature of a substance is a measure of the particles’ average inetic energy, increasing the temperature increase the average kinetic energy of the molecules. Consequently, the proportion of reactants gaining energy energy to supass (Ea) and thus capbable of undergoing reaction , increases with higher temperature.
All reactions have an optimal what?
Temperature for activity
Raising the temperature by _____ will result in an approximate _______ of the reaction rate.
Raising the temperature by 10 degrees C will result in an approximate doubling of the reaction rate.
Is the 10 degree C temperature raise always true?
It is generally true for biological systems, if the temperature gets too high, a catalyst may denature- and then the reaction rate plummets.
What are examples of some molecules being more likely to react in solvents that are non-aqueous?
Dimethyl Sulfoxide (DMSO) Ethanol
Generally, what type of solvents are preferred to cause faster reactions. Why?
Polar solvents because their molecules dipole tends to polarize the bonds of the reactants, thereby lengthening and weakening them, permitting the reaction to occur faster.
What are catalysts?
Substances that increase reaction rate without themselves being consumed in the reactions.
How do catalysts interact with the reactants?
Absorption or
Formation of intermediates.
What do catalysts do to the reactants?
Stabilize them so as to reduce the activation energy necessary for the reaction to proceed.
Many catalysts, __________, ___________ interact with the reactants.
Including all enzymes
Chemically interact with the reactants
What happens after catalysts chemically interact with the reactants?
They return to their original chemical state upon formation of the products.
What might catalysts do?
May increase the frequency of collisions between the reactants
Change the relative orientation of the reactants, making a higher percentage of the collisions effective
Donate electron density to the reactants
Reduce intramolecular bonding within reactant molecules
What is a homogeneous catalysis?
The catalyst is in the same phase (solid liquid, gas) as the reactant
What is a heterogeneous catalysis?
The catalysis is in a distinct and different phase
What is the only effect of the catalyst?
Is the decrease in the energies of activation, Ea, for both the forward and reverse reactions or, more specifically, the catalysts only change the rate of reactions
The catalysts have no impact on what?
The free energies of the reactants or products
the difference between them
Catalysts change what by the same factor?
The forward rate and the reverse rate
Catalysts will not do what? But will do what?
Not transform a nonspontaneous reaction into a spontaneous one.
Only make spontaneous reactions move more quickly toward equilibrium
Why do you put a negative sign in front of the reactants in the rate expression?
Because the reactants are being consumed in the process of formation of the products
What would the rate of the reaction for “A” be, if using the equation (2A + B –> C)?What about B + C?
A: -/[A]//\t /=Delta or Change in
B: -/[B]//\t
C: +/[C]//\t
How do you show a standard rate of reaction in which the rates, with respect to all reaction species, are equal?
The rate of concentration change of each species should be divided by the species’ stochiometric coefficients.
Rate= -/[A]/a/\t = -/[B]/b/\t = /[C]/c/\t
What units are used to express rate?
Units of moles per liters per second (mol/Ls)
Or Molarity per second (M/s)
What is the first thing you should look for when a question asks to determine the Rate Law for a reaction?
Look for the experimental data
What is the rate proportional to?
The concentration of the reactants, with each concentration raised to some experimentally determined exponent
what equation is used to determine rate based on the general reaction aA+ bB –> cC + dD?
Rate=k[A]^x[B]^y
What do the letters represent for the rate equation based on the general reaction aA+ bB –> cC + dD?
Rate=k[A]^x[B]^y
k: reaction rate coefficient or rate constant
x and y: the orders of the reaction.
What is the rate law?
Rate=k[A]^x[B]^y
Rate is always measured how?
In units of concentration over time: Molarity per second (M/s)
The exponents in the rate law equation can state what?
x, y, and z (if there are 3 reactants) can state the order of the reaction with respect to each reactant or overall
The x exponent in the rate law is the order in respects to what? What about the y exponent?
x is the order with respect to reactant A
y is the order with respect to reactant B
How do you calculate the overall order of the reaction in regards to rate law?
The overall order of the reaction is the sum of x and y.
What may the exponents be in the rate law equation?
Integers or fractions
They must be determined experimentally
What might the MCAT focus on in regards to rate law and the exponents used in the equation?
Zero- First- Second- and third-order reactions In most cases, the exponents will be integers.
what are the common traps for chemical kinetics?
First, Most common, the assumption that the orders of a reaction are the same as the stoichiometric coefficients in the balanced overall equation.
Second, Mistaking the equilibrium constant expression (Law of mass action) for the rate law.
Third, assuming k is constant. k changes based on the specific chemical reaction and the activation energy for that reaction and the temperature at which the reaction takes place
Fourth, The notion and principles of equilibirum apply to the system only at the end of the reaction, that is, after the system has already reached equilibrium.
what are almost never the same in regards to the rate law?
Values of x and y are almost never the same as the stoichiometric coefficients. The orders of a reaction must be determined experimentally.
What are the two cases in which the stoichiometric coefficients match the orders of the reaction?
When the reaction mechanism is a single step and the balanced overall reaction is reflective of the entire chemical process
When the complete reaction mechanism is given and the rate-determining step is indicated. The stoichiometric coefficients on the reactant side of the rate-determining step are equal to the orders of the reaction.
What law must be used when the rate-determining step involves an intermediate an intermediate as a reactant?
One must derive the intermediate molecule’s concentration by the law of mass action (That is, the equilibrium constant expression) for the step that produced the intermediate.
What is the difference between the equilibrium constant expression and the rate law expression?
The expression for equilibrium includes the concentrations of all the species in the reaction, both reactants and products. The expression for chemical kinetics- the rate law expression- includes only the reactants. (Keq) indicates where the reaction’s equilibrium position lies. The rate indicates how quickly the reaction will get there.
when is the rate constant, k, constant?
For a specific reaction, at a specific temperature, the rate constant is indeed a constant
What happens to the rate constant, k, for a reversible reaction?
Keq is equal to the ratio of the rate constant for the forward reaction, k, divided by the rate constant for the reverse reaction, k-1.
The reaction rate can theoretically be measured when?
At any time. Usually measured at or near the beginning of the reaction to minimize the effects of the reverse reaction.
On the MCAT, how is experimental data for determining rate order usually provided?
As a chart that includes the initial concentrations of the reactants and the initial rates of product formation as a function of the reactant concentrations. Often the data for three or four trials are included in this chart.
How can you use the data given to determine rate order?
Identify a pair of trials in which the concentration of one of the reactants is changed while the concentrations of all other reactants remains constant. under these conditions, any change in the rate of product formation from one trial to the other is fully attributable to the change in concentration of that one reactant.
Using the general equation A + B –> C and you notice in two trials the concentration of A remains constant while the concentration of B doubles. If the rate of the formation of product C quadruples, then what is B’s exponent? Why?
The exponent of B must be two.
Looking at the rate law: rate=k[A]^x{B]^y, then doubling [B] has resulted in a quadrupling of the rate, so to determine the order of the reaction, y, with respect to b, I need to cacluate the exponent to which the number 2 must be raised to equal 4, because 2^y=4, so y=2.
what is the second step for determining the order in the rate law?
Repeat the process for the other reactant, using data from a different pair of trials, making sure that the concentration of only the reactant we are trying to analyse is changed from one trial to the other while the concentrations of all other reactants remain the same. Once the orders of the reaction have been determined with respect to each reactant, we can write the complete rate law, replacing the exponents x and y with actual numbers.
How do you determine the constant k when determining rate order?
Plug in actual values from any one of the trials; pick whichever trial has the most arithmetically convenient numbers.
How do you classify chemical reactions?
Chemical reactions are zero-order, first-order, second-order, higher-order or mixed-order on the basis of kinetics.
what is a zero-order reaction?
Reaction which the rate of formation of product C is independent of changes in concentration of any of the reactants, A and B.
Zero-order reactions have a constant what?
Reaction rate equal to the rate constant (rate coefficient), k.
What is the rate law for a zero-order reaction?
rate=k[A]^0[B]^0=k
What are the units for k in the rate law equation for a zero-order reaction?
M/s
Rate constant is dependent on what, in rate law?
Temperature
Is it possible to change the rate for a zero-order reaction?
Yes, only by changing the temperature, and adding a catalyst.
The addition of a catalyst in a zero-order reaction does what, causing what?
Lowers the activation energy thereby increasing the value of k.
What are the axes when plotting a zero-order reacton?
Concentration vs. Time
What results when plotting a zero-order reaction on a graph?
A linear graph
What does the zero-order graph demonstrate?
The line shows that the rate of formation of product is independent of the concentration of reactant. The slope of such a line is the opposite of the rate constant, k.
What is a First-order reaction?
A reaction with a rate that is directly proportional to only one reactant, such that doubling the concentration of that reactant results in a doubling of the rate of formation of the product.
What is the rate law for a first-order reaction?
rate=k[A]^1 or k[B]^1
what are the units of k for a first-order reaction rate law?
s^-1
What is a classic example of a first-order reaction?
The process of radioactive decay. From the rate law, in which the rate of decrease of the amount of a radioactive isotope A is proportional to the amount of A
rate= -/[A]//\t = k[A]
What is the equation for concentration of a radioactive substance A at any time “t” for a first-order reaction?
([A]t)= ([A]0)e^-kt
What do the letters represent in the equation for concentration of radioactive substance A of a first-order reaction?
([A]t)= ([A]0)e^-kt ([A]t): The concentration of A at time t. ([A]0): Initial concentration of A k: the rate constant t: time
What is important to recognize in regards to a first-order rate law with a single reactant?
A first-order rate law with a single reactant suggest that the reaction begins when the molecule undergoes a chemical change all by itself, without a chemical interaction, and usually without a physical interaction with any other molecule.
What are the axes when plotting a first-order reaction on a graph?
Concentration vs. time
What results when plotting a first-order reaction on a graph?
A nonlinear graph
What does a first-order reaction graph show/represent?
The curve shows that the rate of formation of product is dependent on the concentration of reactant. Plotting ln [A] vs. time reveals a straight line; the slope of such a line is the opposite of the rate constant, k.
What is a second-order reaction?
Second-order reaction has a rate that is proportional to either the concentrations of two reactants or to the square of the concentration of a single reactant.
The rate law reflects a second-order reaction?
rate= k[A]^1[B]^1 rate= k[A]^2 rate= k[B]^2
What are the units of k for a second-order reaction?
M^-1 s^-1
What is important to recognize for a second-order rate law?
Often suggests a physical collision between two reactant molecules, especially if the rate law is first-order with respect to each of the two reactants.
What are the axes when plotting a second-order graph?
concentration vs. time
How do you plot a second-order reaction?
Plotting a reaction with respect to a single reactant
what is the resulting graph when plotting a second-order reaction?
Nonlinear graph
what does the graph show when plotting a second-order reaction?
The curve shows the rate of formation of product is dependent on the concentration of reactant. Plotting 1/[A] vs. time reveals a linear curve; the slope of such a curve is equal to the rate constant k.
There are very few reactions in which a single reaction step involves what?
In which a single reaction step involves a termolecular process; in other words, there are few processes with third-order rates.
Why are there so few reactions which involve termolecular process/third-order rates?
It is far more rare for three particles to collide simultaneously with the correct orientation and sufficient energy to undergo a reaction.
What are mixed-order reactions?
Sometimes refer to non-integer order (fractions) and in other cases to reaction with rate order that vary over the course of the reaction.
What are broken-order reactions?
Fractions are more specifically described as broken-order.
What is the more modern definition of mixed-order reactions?
Reactions that change order over time.
What is an example of a mixed-order rate law?
rate=((k1)[C][A]^2)/ ((k2) + (k3)[A])
What do the letters represent in the rate law equation for mixed order reactions?
A: a single reactant
C: a catalyst
What are you responsible for knowing on the MCAT in regards to mixed-order rate law?
The MCAT will not ask you to derive a rate expression for a mixed-order reaction, you are responsible for being able to recognize how the rate order changes as the reactant concentration changes.
What should be noted when looking/using the mixed-order reaction rate law?
The result of the large value for [A] at the beginning of the reaction is that (k3)[A] > (k2), and the reaction will appear to be first-order with respect to A. At the end of the reaction, when [A] is low, (k2) > (k3)[A], making the reaction appear second-order with respect to A.
