Kinetics Flashcards
Define rate
The change in the concentration of a reactant or product per unit time (mol L-1 s-1)
Which component do we usually base conversion on?
The rate limiting component (key component)
What is a rate law?
An expression which relatres the rate of a reaction to the rate constant and the concentrations of the reactants.
How can reaction order for a reaction be calculated?
By adding the exponents of the reactants in the rate law.
Which equation shows that the rate constant depends on temperature? State this equation.
The Arrhenius equation
What does Ea represent in the Arrhenuis equation?
The activation energy of the reaction
What does A represent in the Arrhenius equaton and what is it related to?
A is the pre-exponential factor and is related to the frequency of collision between reaction molecules, so is a measure of the rate at which collisions occur, irrespective of their energy.
What is the general formula for the units of k for a reaction of nth order.
Give the Arrhenius equation in it’s logarithmic form.
With the Arrhenius equation in it’s logarithmic form, which variables would you plot on the y and x axis?
y = ln(k)
x = 1/T
From a plot of the logarithmic Arrhenius equation, how would you find the pre-exponential factor and the activation energy?
lnA is given by the value of the y-intercept and -Ea/R is given by the gradient of the line.
If given time instead of rate, what would you plot on the y-axis of a logarithmic Arrhenius plot?
ln(1/time), which is proportional to ln(k)
What is the definition of activation energy?
The minimum kinetic energy that reactants must have in order to form products.
What does the rate constant represent in terms of kinetics?
The rate of successful collisions between reactants.
What does the exponential factor represent in the Arrhenius equation in terms of kinetics?
The fraction of collisions that have enough kinetic energy to lead to a reaction.
Draw the graph of potential energy of a reaction against the progress of the reaction (exothermic).
How does reaction rate change with temperature?
As temperature increases, rate will increase because the overall kinetic energy of the reactants will increase, resulting in a greater rate of successful collisions and hence products forming.
What rule can be used to estimate how reaction rate will change with temperature?
Reaction rate will double each time the temperature is increased by 10 degrees.
What is the limit on increasing the temperature of a reaction?
The temperature of a reaction may be increased until the temperature is a the point at which the reactants will degrade, at which the point the rate of the reaction will decrease.
What equation can be used to compare the Arrhenius equation and two different temperatures?
Define a catalyst.
A class of molecules that lower the activation energy of a reaction, but are not consumed in the reaction themselves. The assist in the progress of the reaction and therefore increase the rate.
What is the most common type of catalyst?
An enzyme.
How does an enzyme decrease the activation energy?
By ensuring that the reactants collide in perfect orientation, which increases the likelihood that the collision is successful.
State what the 3 different rates (r, r’ and r’’) are based on.
r - a unit volume of fluid
r’ - the weight of the catalyst
r’’ - the surface area of the catalyst
Write the rate equation based on the unit volume of a fluid.
Write the rate equation based on the weight of a catalyst.
Write the rate equation based on the surface area of a catalyst.
For a homogeneous reaction, what is the rate based on?
A unit volume of the fluid.
For a heterogeneous reaction, what is the rate based on?
The surface area of the species.
What is an elementary reaction?
When the rate law of the reaction reflects the stoichiometry of the reaction.
If the rate law or a reaction reflects the stoichiometry, is the reaction elementary?
It may be, or it may just be coincidence.
What is a non-elementary reaction?
When the rate law does not reflect the stoichiometry of the reaction.
State the three applications of rate laws.
- Determining k to predtict the rate of reaction from a mixture of components.
- To predict mixture compositions at different stages of the reaction
- To determine the reaction mechanism
Explain the isolation method to determine the rate law.
This is a way of determining the rate law by having one of the reactants present in a large excess so it will not have an effect on the reaction rate (it can be assumed to be constant). The affects of changing the concentration of the isolated component can then be observed.
In the isolation method, how can the rate law be described when one reactant is present in a much greater concentration than the other?
The rate law can be said to be psuedo first-order.
Explain the initial rate method to determine the rate law of a reaction.
The rate is measured at the start of the reaction for several initial concentration combinations. The initial rate law is compared for each combination to find the order of the reaction with respect to each reactant.
If there is only one reactant and one data set, the order of the reaction can be found by plotting the logarithmic curve of the initial rate equation.
How is the integrated rate law found?
Separate the differential equation and integrate with respect to t and CA.
How would you find k from an integrated rate law for a first order reaction?
Plot ln(CA/CA0) against t. The gradient is -k.
How do you find the equation for half-life of a substance?
Integrate the rate equation with the final concentration equalling half of the initial concentration.
How can k be graphically determined from the equation for half life of a second order reaction?
Plotting 1/CA against t, the slope will have a gradient of k.
What is the equation for amount of material remaining using the half-life of the material?
starting material x 0.5n = ending material, where n is the number of half lives
How can the rate equation for a bimolecular reaction (first order wrt A and B) be manipulated into an equation with one variable?
By letting m = CA0-CA = CB0-CB, therefore dCA=dm. m = 0 when t = 0. The integration can then be carried out by separating the fraction, then replacing m back with the original variables.
How do you derive the equation for the equilibrium constant?
For a reversible reaction A to B, integrate the rate equation for CA. Equilibrium occurs when t tends to infinity. This can be used to find CAeq and CBeq, and K is equal to the latter divided by the former.
Or it can be derived by equalling the dCA/dt to 0, as there is no change in concentration at equilibrium. Then rearrange for CAeq and follow the same method.
