Reaction Kinetics - Week 21 Flashcards
How is instantaneous rate calculated?
Instantaneous rate, d[A]/dt = y/x determined by drawing a tangent to the curve
What are the 5 factors that affect rate of reaction?
- nature of chemical species involved
- physical state of substances involved
- concentration of species or pressure for
gases - temperature
- presence of a catalyst
What is the rate expression?
rate = k[A]x[B]y
What is k in the rate expression?
k is the rate constant (equal to the rate when there are unit concentrations of A and B)
What is x in the rate expression?
x is the order of the reaction with respect to substance A
What is y in the rate expression?
y is the order of the reaction with respect to substance B
What is x+y in the rate expression?
x+y is the overall order of the reaction
What will the rate law be if the reaction takes place in a single step?
rate = k [A][B]
What is rate of reaction usually measured in?
mol dm-3 s-1
What are the units for the rate constant, k?
Units for k can be deduced from the rate equation.
rate has units of mol dm-3 and concentration has units of mol dm-3
What is the rate-determining step or rate-limiting step in a reaction?
The slowest step in the reaction, controlling the overall rate of formation of products.
How is rate data obtained?
if A + B –> products
[A] can be measured at different times during the reaction, and rates obtained. These rates can be related to concentrations of A.
To measure change in [A] on its own, [B] must be kept constant. Can be done by carrying out reaction with excess B.
How are initial rates measured?
Often measured by timing how long it takes the reaction to reach an identifiable point and then measuring 1/ time.
Measured by 1/t1 and 1/t2 etc.
What does it mean if a reaction is zero order with respect to a particular chemical?
means that rate = k[A]0 (=k) for the chemical
The rate does not depend on the concentration of the chemical. It is not involved in the rate - determining step of the mechanism.
How do you find half life in a first order reaction?
If the rate equation is [A] = [A]0 e-kt
When [A] has fallen to half its original value ([A] = [A]0/2) after substituting in the rate equation we get
t 1/2 = ln2/k t1/2 = the half life of the reaction
What is another way to demonstrate that a reaction is first order?
To show that it has a constant half-life.
What 3 factors does collision theory predict?
- Rate depends on concentration
- Rate depends of Activation Energy
- Rate depends of molecular orientation
What is the Arrhenius equation?
k = Ae ^ -Ea/RT
What do each of the letters in the Arrhenius equation stand for? (K, A, Ea, R, T)
K = rate constant
A = pre-exponential factor
Ea = activation energy
R = gas constant
T = temperature
What does the Arrhenius equation describe?
Describes the collision theory model, A relating to the number of collisions and e^-Ea/RT being the factor restricting the number of collisions to those that are effective.
What other way can the Arrhenius equation be expressed? (when a graph is present)
Lnk = -Ea/R(1/T) + LnA
If k is known at two temperatures, T1 and T2. Eliminating LnA in the Arrhenius equation allows Ea to be obtained from:
Ln(k2/K1) = -Ea/R(1/T2 - 1/T1)
