Chapter 5: Chemical Kinetics Flashcards
What is Gibbs free energy? What does Gibbs free energy determine? Are kinetics and thermodynamics the same thing?
Gibbs free energy is the difference in the free energy of a reaction. It helps us determine if a reaction is spontaneous or non-spontaneous.
+deltaG is endergonic, energy is absorbed, and therefore non-spontaneous.
-deltaG is exergonic, energy is released, and is therefore spontaneous.
Kinetics and thermodynamics are not the same thing and should be considered separately.
If a reaction is spontaneous, does that mean it will run quickly?
No. Just because a reaction is spontaneous does not mean that it will run quickly. In fact, nearly every biochemical reaction that enables life to exist, while perhaps spontaneous, proceeds so slowly that without the usage of enzymes and other catalysts, measurable reaction progress might not actually occur over the course of an average human lifetime.
What is the mechanism of reaction?
Many reactions proceeded by more than one step, the series of which is known as the mechanism of a reaction, and the sum of which gives the overall reaction.
What is a reaction intermediate and where does it occur in a free energy diagram? How are reaction intermediates experimentally determined?
The reaction intermediate is a molecule which does not appear in the overall reaction, but is theoretically formed between the reactants and the products of a chemical reaction. Reaction intermediate can be found at the peak of the energy curve on a free energy diagram.
Reaction intermediates are experimentally determined through kinetic experiments.
What is the rate determining step?
The rate determining step is the slowest step in any proposed mechanism because it acts like a kinetic bottleneck, preventing the overall reaction from proceeding any faster than that slowest step.
What are the two theories that have been proposed to explain the events that are taking place at the atomic level through the process of a reaction?
The collision theory of chemical kinetics and the transitional state theory.
What is the collision theory of chemical kinetics? What is the Arrhenius equation (uh-ren-ee-us) and what do I need to know about it (important variables in relationship to the rate of a reaction)?
What is activation energy (Ea)?
Activation energy, Ea, or energy barrier, is the minimum energy of collision necessary for a reaction to take place.
Only a fraction of colliding particles have enough kinetic energy to exceed the activation energy. This means that only a fraction of all collisions are effective. The rate of a reaction can therefore be expressed as:
What is transition state theory? What’s the funny sign in the picture that looks sort of like a +?
Transition state theory proposes a transition state in which old bonds are weakened and new bonds begin to form. The transition state then dissociates into products, fully forming the new bonds.
What is a positive delta G mean, what is a negative delta G mean?
What is free energy diagram? Explain how to determine Delta G and what that means, what is Ea and what is it associated with?
A free energy diagram illustrates the relationship between the activation energy, the free energy of the reaction, and the free energy of the system.
Delta G is the change in energy in the free energy diagram. Positive,Delta G is endergonic, absorbs energy, and non-spontaneous. Negative Delta G is exergonic, releases energy, and is spontaneous.
Ea is energy of activation. Intermediate reactants or theorized to form at the peak of energy of activation.
What factors affect reaction rate and how?
The greater the concentration of the reactants, the greater number of effective collisions per time, which leads to an increase in frequency factor A of the Arrhenius equation.
The greater the temperature, the greater the reaction rate because the temperature of a substance is a measure of the particles average kinetic energy, increasing the temperature increases the average kinetic energy of the molecules. Consequently, the proportion of reactants having enough energy to surpass the energy of activation increases with higher temperature .
The rate at which a reaction takes place may also be affected by the medium in which it takes place. Some reactants react better in aqueous environments, some more likely to react in non-aqueous solvent. The physical state of the medium (solid, liquid, gas) can also have a significant effect.
Catalyst are substances that increase reaction rate without themselves being consumed in the reaction. Catalyst interact with the reactants, and stabilize them so as to reduce the activation energy necessary for the reaction to proceed.
What does a catalyst do for a reaction? What does a catalyst not do for a reaction? What can a catalyst not do for reaction (think delta G)?
The only effect of the catalyst is the decrease in the energy of activation for both the forward and reverse reactions. The presence of the catalyst has no impact on the free energies of the reactants or the products or the difference between them. Consequently, they have no impact whatsoever on the equilibrium position or the measurement of Keq. Catalyst will not transform a non-spontaneous reaction into a spontaneous one, they only make spontaneous reactions move more quickly toward equilibrium.
Do catalyst work on non-spontaneous reactions? Explain.
Catalyst only work on spontaneous reactions, catalyst do not work on non-spontaneous reactions. This is because catalysts have no impact on the change of free energy of a reaction, they only reduce the energy of activation and therefore cannot make a non-spontaneous reaction a spontaneous reaction.
In other words, if a reaction is nonspontaneous, addition of a catalyst would not make it proceed.
What is the definition of rate? What are the units of rate? What would the rate equation be if the rates of all the reaction species are equal? Why is the sign for changing concentrations and time negative?
Rate of a reaction either the disappearance of reactants over time or the appearance of products overtime. The units of rate are
mol/Lxs or molarity/s (as molarity is mol/L).
The sign for change in concentration and time in the image is negative because the reactants are being consumed.
Given the following data, find the rate law for the following reaction at 300K.
How to solve this problem:
Find two trials (because we are solving for two variables) in which the concentration of all but one is the substances are held constant.
Evaluate the consumption of reactants and products to set up a rate law equation.
Solve for rate laws.
What is zero order reaction? Is it possible to change the rate of a zero order reaction? What are the units of k?
What is an example of a zero order reaction?
Zero order reaction is one in which the rate of formation of product is independent of changes in concentration of any of the reactants.
It is possible to change the rate of a zero order reaction by the addition of a catalyst, or increasing the temperature.
Zero order reactions often occur when the reaction is limited by the availability of a catalyst, such as decomposition of N2O on a platinum plate where the rate of reaction is not dependent on the concentration of the reactant, but the availability or surface area of the catalyst. Photochemical reactions are another example where the rate is dependent on light intensity and not concentration of reactants.
What is the kinetics of a zero order reaction (essentially describing the graph of concentration of a per time, including slope of the graph and relation relation to k)?
Plotting a zero order reaction on a concentration v time curve results in a linear graph. This 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? What are the units of k?
A first order reaction has a rate that is directly proportional to only one reactant, such the doubling the concentration of that reactant results in a doubling of the rate of formation of the product.
What is a primary example of a first order reaction?
Recall that it is important to recognize that 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 reaction, and usually without a physical interaction with any other molecule.
Knowing that, the primary example of a first order reaction is the process of radioactive decay.
What does the graph look like when you plot first order reactions on a concentration versus time curve, what does the graph when plotting the natural log of the concentration versus time reveal?
Plotting a first order reaction on a concentration versus time curve results in a nonlinear graph, showing that the rate of formation of products is dependent on the concentration of reactants.
The natural log of the concentration of a reactant versus time reveals a straight line. The slope of such a line is the opposite of the rate constant, k.
What is the second order reaction? What are the units of k? What is an example of a second order reaction?
The second order reaction has a rate that is proportional to either the concentrations of the two reactants, or to the square of the concentration of a single reactant.
Dimerization is an example of a second order reaction, where two identical molecules combine to form a larger molecule.
What is a mixed order reaction?
What is an example of a mixed order rate law equation?
Mixed reactions sometimes refer to non-integer orders (fractions) and other cases to reactions with rate orders that vary over the course of a reaction. Fractions are more specifically described as broken order, the term mixed order has come to refer solely to reactions that change order over time.
The example equation demonstrates a mixed reaction where a high concentration of A would appear to be first order with respect to a. At the end of the reaction, when concentration of A is low, makes the reaction appear second order with respect to A.
Describe the effects the following conditions would have on initial rate of reaction. Explain.
Determine the rate law and rate order for the following:
These questions are all about proportionality.
First. Determine if there is any zero order reactions (no change in rate with change in concentration)
Second. Select trials where one reactant changes while the others remain constant and the rate changes.
The numbers entered are all about proportionality and changes in concentration.
What is the equation for radioactive decay, what order reaction is radioactive decay?
Radioactive decay is a classic first order reaction (rate that is directly proportional to only one reactant, such that doubling the concentration of that reactant results in a doubling of the rate to formation of the product).
