Chapter 14 - Chemical Kinetics Flashcards
What is studied in chemical kinetics?
the rate (or speed) at which a chemical process occurs
Define reaction mechanism.
a molecular-level view of the path from reactants to products
List four factors that affect reaction rates.
- Physical state of the reactants.
- Reactant concentrations.
- Reaction temperature.
- Presence of a catalyst.
The more readily the reactants collide, the more (rapidly/slowly) they react.
rapidly
Homogeneous reactions are often (faster/slower).
faster
Heterogeneous reactions that involve solids are faster if the surface area is (increased/decreased).
increased
Increases reactant concentration generally (increases/decreases) reaction rate.
increases
Why does increasing reaction concentration generally increase reaction rate?
Since there are more molecules, more collisions occur.
Reaction rate generally (increases/decreases) with increased temperature.
increases
_____ energy of molecules is related to temperature.
kinetic
At (higher/lower) temperatures, molecules move more quickly, (increasing/decreasing) numbers of collisions and the energy of the molecules possesses during the collisions.
higher, increasing
Catalysts affect rate (while/without) being in the overall balanced equation.
without
Catalysts affect the __________, changing the individual reactions that are part of the pathway from reactants to products.
kinds of collisions
Define rate.
a change in concentration over a time period Δ[ ]/Δt
Define Δ in the statement Δ[ ]/Δt.
change in
Define [ ] in the statement Δ[ ]/Δt.
molar concentration
Define t in the statement Δ[ ]/Δt.
time
List three types of rate measured.
- Average rate.
- Instantaneous rate.
- Initial rate.
How is the rate of a reaction measured?
using the concentration of a reactant or a product over time
How is a plot graph of reaction rate data useful?
it gives more information about the rate
What does the slope of the curve at one point in time on a rate data graph tell us?
the instantaneous rate
Define initial rate.
the instantaneous rate at time zero on a rate data graph
How do we determine what effect the concentration of each reactant has on the rate of the reaction?
We keep every concentration constant except for one reactant and see what happens to the rate. Then we change a different reactant. We do this until we have seen each reactant has affected the rate.
Define rate law as stated by the equation rate= k[A]^x[B]^y.
a statement of the relationship between rate and concentration for all reactions
Define k in the statement rate=k[A]^x[B]^y.
the rate constant
What is the rate constant dependent on?
temperature
Define the exponents in the statement rate=k[A]^x[B]^y.
the order of the reaction with respect to each reactant
How is the overall reaction’s order determined?
by adding up all of the reactants’ orders
Define [A] and [B] in the statement rate=k[A]^x[B]^y.
the reaction rates of reactants [A] and [B]
Define first order reactions.
reaction rates that depend only on one reactant to the first power
What is the rate law for a first order reaction?
rate=k[A]
How is k related to [A] in a first order reaction?
rate = k [A] rate = −Δ [A] / Δt So: k [A] = −Δ [A] / Δt Rearrange to: Δ [A] / [A] = − k Δt Integrate: ln ([A] / [A]o) = − k t Rearrange: ln [A] = − k t + ln [A]o
How does a plot of ln [A] vs t appear?
linear
How can a rate constant be found from a plot of ln [A] vs. t?
When the integrated rate law, ln [A] = − k t + ln [A]o, is plotted as a line, its slope will equal -k.
Define half life.
the amount of time it takes for one half of a reactant to be used up in a chemical reaction
How is half-life calculated in a first order reaction?
ln [A] = − k t + ln [A]o ln ([A]o/2) = − k t½ + ln [A]o − ln ([A]o/2) + ln [A]o = k t½ ln ([A]o / [A]o/2) = k t½ ln 2 = k t½ or t½ = 0.693/k
Define second order reaction.
a reaction rate that depends only on a reactant to the second power
What is the rate law for a second order reaction?
rate=k[A]^2
How is k related to [A] for a second order reaction?
rate = k [A]^2 rate = − Δ [A] / Δ t So, k [A]2 = − Δ [A] / Δ t Rearranging: Δ [A] / [A]2 = − k Δ t Using calculus: 1/[A] = 1/[A]o + k t
How is half-life derived in a second order reaction?
Using the integrated rate law, we can see how half-life is derived: 1/[A] = 1/[A]o + k t 1/([A]o/2) = 1/[A]o + k t½ 2/[A]o −1/[A]o = k t½ t½ = 1 / (k [A]o)
Half-life is a ____________ dependent quantity for second order reactions.
concentration
Define zero order reaction.
a reaction in which the reaction rate is independent of the concentration of the reactant
Zero order reactions are _________ in concentration.
linear
The rate constant is temperature dependent; it (increases/decreases) as temperature increases.
increases
Rate constant _____ (approximately) with every 10 C temperature increase.
doubles
The collision model is based on the _________ theory.
kinetic molecular
Molecules must _____ to react.
collide
If there are more ______, more ______ can occur. If there are _____ molecules, the __________ is faster.
collisions, reactions
more, reaction rate
If the temperature is higher, molecules move _______, causing more collisions and a ____________ of reaction.
faster, higher rate
What occurs in a chemical reaction?
bonds are broken and new bonds are formed
Molecules can only react ___________.
if they collide with each other
Can molecules collide without forming products?
yes
Define activation energy.
the minimum energy needed for a reaction to take place
What must be overcome for a reaction to take place?
an energy barrier
Reactants gain ______ as the reaction proceeds until the particles reach the _______________.
energy, maximum energy state
Define transition state.
the organization of the atoms at the maximum energy state
Name another term for transition state.
activated complex
What is the energy needed to form a transition state?
activation energy
Reactions can be _____ or _____ after the transition state is formed.
endothermic, exothermic
Gases have an average temperature, but each individual molecule has ________.
its own energy
At __________, more molecules possess the energy needed for the reaction to occur.
higher energies
Define the Arrhenius equation.
the relationship between activation energy as temperature stated as k = Ae−Ea/RT
How can activated energy be determined graphically using the Arrhenius equation?
by reorganizing the equation ln k = −Ea/RT + ln A
Define reaction mechanism.
a series of stepwise reactions that show how reactants become products
What are the individual steps of a reaction mechanism called?
an elementary reaction or elementary process
Define molecularity.
tells how many molecules are involved in that step of the mechanism
Nearly all reaction mechanisms use only ___________.
unimolecular or bimolecular reactions
Termolecular reactions are ________.
rare, if they occur at all
Define rate-determining state.
the slowest reaction in the reaction mechanism
What is required of a plausible reaction mechanism?
The rate law must be able to be devised from the rate-determining step.
The stoichiometry must be obtained when all steps are added up.
Each step must balance, like any equation.
All intermediates are made and used up.
Any catalyst is used and regenerated.
Define catalyst?
a component which increases the rate of a reaction by decreasing the activation energy of the reaction
Catalysts _________.
change the mechanism by which the process occurs
List three types of catalysts.
- Homogeneous catalysts
- Heterogeneous catalysts
- Enzymes
Define homogeneous catalyst.
a catalyst in the same phase as the reactant
How are many homogeneous catalysts seen?
dissolved in the same solvent as the reactant
Define heterogeneous catalyst.
a catalyst in a different phase then the reactant
What is frequently the rate determining step in reactions involving heterogeneous catalysts?
the adsorption of the reactants
Define enzymes.
biological catalysts
Define active site.
a region on an enzyme where the reactants attach
Define substrates.
the reactants affected by an enzyme
Define the enzyme-substrate model.
in the enzyme-substrate model the substrate fits into the active site of an enzyme much like a key fits into a lock
