Rates and rate constant Flashcards
What is rate defined as?
A rate is a change in concentration with time
Rate = Gradient
What does it mean to rate when time change is 0?
f there is no change in time, then there is no change in concentration, and therefore the rate is undefined
What does it suggest to rate if time change is really small?
When the time change approaches zero, the calculated rate is known as the instantaneous rate
How would you calculate rate from a graph?
(Change in concentration)/(change in time)
What do reaction rates depend on? (5)
Temperature
Pressure
pH
Ionic strength
Reagent concentration
How does ionic strength influence the reaaction rate?
When two ions with the same charge (both positive or both negative) react, increasing the ionic strength of the solution can accelerate the reaction
This is because the added ions shield the charges of the reacting ions, reducing electrostatic repulsion and making it easier for them to come together and react
What is an irreversible unimolecular reaction?
A single molecular undergoes a transformation into one or more products, and this process proceeds in only one direction.
How does an irreversible unimolecular relate to first order kinetics?
The rate of reaction is directly proportional to the concentration of the reactant
Why is there a negative sign in the rate formula?
The negative sign in the reaction rate formula is used to ensure that the rate of reaction is always a positive value, even though the concentration of reactants decreases over time
We want the rate of reaction to be a positive value, as it represents the speed at which the reaction is occurring
What are the units of the rate constant, k?
Rate =k[A]
- Rate = mM min^-1
- K = min^-1
- [A] = mM
What is the difference between first order and unimolecular
- First order: In a first-order reaction, the rate is directly proportional to the concentration of one reactant raised to the power of one
-Refers to the number of molecules involved in the rate-determining step of a reaction mechanism
The system P -> Q is first order with a rate constant k = 5 min-1. Predict the rate of reaction at [P] = 5 mM.
Rate = (5 min⁻¹) * (5 mM)
Rate = 25 mM/min
Therefore, the rate of reaction at [P] = 5 mM is 25 mM/min.
A system obeys X -> Y with a rate constant k. A cuvette containing 40 mM X is monitored in a spectrophotometer allowing [X] to be determined every second.
When [X] is 40 mM, [X] drops by 2 mM in one second. What is K?
When [X] = 40 mM, the rate of decrease is 2 mM/s.
So: 2 mM/s = k * 40 mM k = 0.05 s⁻¹
Therefore, the rate constant k is 0.05 s⁻
Rate = k[X]
In a system obeying R -> P, with a rate constant k, calculate the rate of formation of product when:
[R] = 20 mM and k = 3 s-1.
[R] = 3 µM and k = 1 min-1.
[R] = 10 mM and k = 0.03 s-1.
- [R] = 20 mM and k = 3 s⁻¹
Rate = (3 s⁻¹)(20 mM) = 60 mM/s - [R] = 3 µM and k = 1 min⁻¹
First, we need to convert the units of k to s⁻¹. There are 60 seconds in a minute, so k = 1/60 s⁻¹.
Rate = (1/60 s⁻¹)(3 µM) = 0.05 µM/s - [R] = 10 mM and k = 0.03 s⁻¹
Rate = (0.03 s⁻¹)(10 mM) = 0.3 mM/s
What does the answer of this equation ‘k1[A] - k2[B]’ mean?
Finding the difference between the two terms represents the net rate of change in the concentration of species A or B
