Flashcards in Block 3: Kinetics Deck (50)
What is chemical kinetics?
The study of the rates of chemical reactions and the factors that affect them
What is the rate of a chemical reaction?
The change in concentration per unit time
What are the units of a rate?
What is the formula for the rate of reaction:
aA + bB --> cC + dD
Rate = -(1/a) x (change in [A])/(change in t)
This is equal to +(1/c) x (change in [C])/(change in t)
What is the rate proportional to, and give an example to illustrate this.
Rate is proportional to the moles of the chemical.
Eg. in 3O2 --> 2O3, the rate is 1/3 the disappearance of O2, or 1/2 the appearance of O3.
What does the rate measure?
The initial rate of reaction, or average rate of the overall reaction. The initial rate is faster than the average rate.
How do we make the average rate as close as possible to the initial rate?
Use a shorter timespan
What factors affect the rate of reaction?
- Nature of the reaction itself
- Concentrations of the reactants
- State of the reactants
- Presence of a catalyst
- Action of light
What is a rate law?
The relationship between the reaction rate and the reagents' concentrations.
What are the 4 different orders, and what do they mean?
1st order: The rate id directly proportional to the conc.
2nd order: The rate is proportional to the conc. squared
3rd order: The rate is proportional to the conc. cubed
Zero order: The rate is not related to the conc.
What happens if a reaction has more than one reactant affecting rate, and what is the resultant order (ie not a reactive intermediate)? +Example
The rate is proportional to the concentrations of all reactants.
Eg. CH3Br + OH- --> CH3OH + Br-.
The rate is proportional to [CH3Br] and [OH-].
Rate is proportional to [CH3Br][OH-]
It is 1st order for each reactant, but 2nd order overall.
What is k?
The rate constant for these reactions. It differs depending on the reaction, and only changes within a single reaction with temperature.
Now we can phrase rate constants as
rate = k[A][B].
What are the units of k?
They depend on the order of the reactions.
As k = rate/[A][B], the units are also molL-1s-1/[units for A][units for B].
For example, if both A and B were first order, molL-1 x molL-1 = mol2L-1, and the equation simplifies to mol-1Ls-1.
(the way to work this out is to subtract the bottom line from the top line: minus 2 = -1, -1 minus -2 = 1 etc)
How do we determine the order of a reaction experimentally?
If the reaction rate doubles when one reactant is doubled (and other(s) constant), that reagent is 1st order. If reaction rate quadruples/octuples/no change, it is 2nd/3rd/0 order.
The overall rate is determined by simply adding the reactants' orders.
How do we determine the order of a reaction using graphs of reaction rate?
When we graph concentration vs. time:
We want a straight line, where the gradient = k. This is because k, the gradient, must be constant throughout the reaction.
However, we need to know what form of graph to plot, necessitating the use of the integrated rate law.
What is the integrated rate law for a 1st order reaction?
[R] = [R]o e^-kt. This simplifies to ln[R] = ln[R]o - kt
What is the function of the integrated rate law?
This allows us to work out k, if we know the concentration of R at 2 points in time.
What is the integrated rate law for a 2nd order reaction?
1/[R] = 1/[R]o + kt
What is the integrated rate law for a 3rd order reaction?
1/[R]^2 = 1/[R]o^2 + kt
What is the integrated rate law for a 0 order reaction?
How does the k value tell us the speed of the reaction?
A larger k value denotes a faster reaction.
What is a half life?
The time in seconds for the reactant's concentration to halve.
What is the symbol of a half life?
What is the formula for half life in a 1st order reaction, and what does the formula tell us?
t1/2 = ln2/k.
It is not dependant on the concentration of the reactant, so its half life will be constant regardless of the concentration.
What is the formula for half life in a 2nd order reaction, and what does the formula tell us?
t1/2 = 1/ ([R]o x k)
The rate depends on the initial concentration of the reactant, so its half life will increase exponentially as concentration falls.
What does the Arrhenius equation describe?
The relationship between k and temperature
What is the Arrhenius equation?
k = Ae ^ (Ea/RT)
Where: R = gas constant
A= pre exponential factor. Both Ea and A are different in different reactions. The units of A are the same as for k.
What does the equation rearrange to to find Ea?
Rearrange so that lnk = lnA - Ea/RT.
Now, the slope = -Ea/R, and Ea = -slope x R
What is collision theory?
In order for two molecules to react, they must first collide at the correct energy, in the correct orientation.