chapter 13 Flashcards
Chapter 13: Kinetics
What does the study of chemical kinetics deal with?
The rate of chemical reactions
On a microscopic level, what factors are important in chemical kinetics?
Collision energy and collision orientation
Describe the rate equations for a reaction aA + bB ā> cC + dD
rate = (-1/a) x (deltaA/deltaT)
= (-1/b) x (deltaB/deltaT)
= (-1/c) x (deltaC/deltaT)
= (-1/d) x (deltaD/deltaT)
For a concentration vs. time plot, what do the slope and y-intercept represent?
Slope represents rate, and y intercept represents initial concentration
What are the two parts of rate law?
Order of reaction and rate constant
What is the unit of the rate constant for a first order reaction?
1/s
What is the unit of the rate constant for a second order reaction?
1/(M*s)
What is the unit of the rate constant for a third order reaction?
1/(MĖ2*s)
What is the unit of the rate constant for a 0 order reaction?
M/s
For linear plots, what indicates reaction order (what does a straight line tell us, and on what type of graph)?
A concentration vs. time graph will give a straight line for a zero order reaction.
A ln[concentration] vs. time graph will give a straight line for a first order reaction.
A 1/[concentration] vs. time will give a straight line for a second order reaction.
What does each part of the Arrhenius equation mean?
(k = Ae^(-Ea/RT))
k: rate constant
A: frequency factor
Ea: activation energy
R: R constant (8.314)
T: Temperature (kelvin)
What is the physical meaning of activation energy?
A threshold of collision energy energy, above which reaction can happen.
Speed of molecules is correlated with _______
Temperature
As temperature increases, (statistical) distribution of molecules moves to the ______. What does this mean in practice?
Right; more molecules have higher energy
What is the physical meaning of frequency factor?
Frequency factor is related to orientation of molecules during collision
What is transition state theory?
The concept of transition state being a species partway between reactant and products.
What do energy diagrams plot?
Potential energy against reaction coordinate (or reaction progress)
What is a catalyst?
A substance that increases the rate of reaction without being consumed in the reaction.
A catalyst that can speed up the forward reaction can also _________
speed up the backward reaction
What are the two possible ways of catalysts increasing the rate of chemical reactions?
By stabilizing the transition state and thus lowering the activation energy, or by providing reaction pathway with a lower activation energy
What does the rate of reaction represent?
Amount of products appearing per unit time // amount of reactants disappearing per unit time
What are some factors that influence reaction rate?
- Particles must collide
- Higher concentration of reactant lead to higher reaction rate (allows for more collisions)
- The physical state of reactants influence rxn rate (substances must mix in order for collision)
- Higher temperature = higher rxn rate (high temperature = more energy = more collisions)
- Presence of a catalyst will increase rxn rate
Can rate be negative?
No. Though concentration of reactant can/will be decreasing, the overall rate will be positive.
How to determine the individual order with respect to a reactant?
Double the concentration of the reactant and see what happens to the rate. If the rate doubles, the rxn is 1st order wrt reactant. If it quadruples, the rxn is 2nd order wrt reactant. If it does not change, the rxn is 0 order wrt reactant.
Can you determine rxn order from balanced equations?
No! Must be determined from experimental data. With that said, reaction order can be determined from elementary reactions.
What does a smaller rate constant mean about reaction speed?
Smaller k means slower reaction
What is half life for a reaction?
The time taken for the concentration of a reactant to drop to half its initial value
Half life does not depend on starting concentration for a _____ order reaction (meaning it is constant)
First
For a second order reaction, how does half life relate to initial concentration
It is inversely proportional to the initial concentration
For a zero order reaction, how is half life related to initial concentration
It is directly proportional to the initial concentration
As T (temp) increases, what happens to the k value
It increases exponentially
What is the transition state on the energy diagram?
The highest energy point along the reaction coordinate
Do transition states have reactants or products?
Neither. They have transitional species partway between the two
At the transition state, does the reaction have to move forward?
No, by probability the system can move forward or backward at the transition state
What is a reaction mechanism?
the sequence of single reaction steps that make up the overall equation
What are the individual steps of a reaction mechanism called?
Elementary steps
How do you determine molecularity?
The molecularity of a step equals the total number of reactant particles
What are the three criteria for a reaction mechanism?
Elementary steps must add up to overall balanced equation, elementary steps must be reasonable, and mechanism must correlate with observed rate law.
In a reaction mechanism, which step is rate determing?
The slower step
Can a reaction have more than one transition state?
No! Each step in a multi step reaction has its own transition state, which occurs at the energy maximum for that step
What is the difference between a homogeneous and heterogeneous catalyst?
A homogenous catalyst is in the same phase as the reaction mixture, while a heterogeneous catalyst is in a different phase
Do catalysts affect overall yield (if so how)? Do they affect deltaH (If so how)?
No for both
