Lecture Twenty One - Reaction kinetics II Flashcards
What are first order reactions?
The reaction rate is directly proportional to the concentration of one of the reactants.
Have a general form A -> Product(s).
First order reactions are very common.
If the [A] is doubled, the rate of the reaction is doubled.
If the [A] is increased by a factor of 10, the rate of the reaction is increased by a factor of 10.
The units of a first order rate constant K are S^-1. (where s is seconds).
These units are found by:
Rate = K[A]
K = Rate/[A]
K = (mole/L/second) / (mole / L)
Which cancel to give 1/seconds
First order reactions are exponential when graphed.
What are second order reactions?
The reaction rate is directly proportional to the square of the concentration of the reactant A.
If the concentration of A doubles, the rate of the reaction increases by a factor of four.
Units of second order rate constant K are L/mol/second.
K = rate/[A]^2
= (mol/L/second) / (mol/L * mol/L)
= L/mol/second.
How would you work out a second order reaction?
Can you tell the difference between first and second order reactions graphically?
No, they look very similar.
What are zero order reactions?
A zero order reaction is independent of concentrations.
Units of rate constant K are mol/L/second.
The rate of this retain does not vary with time.
Graph of the [reactant] as a function of time is a straight line with a slope of -K (consternation decreases with time).
Graph of [product] as a function of time is a straight line with a slope of +K.
How would you work out the gradient of the zero order reaction graph?
How do reactions with more than one reactant work, with regards to their reaction order.
A + B -> products.
Rate = K[A]^n [B]^m.
The reaction order is n + m.
How can rate order be determined?
The methods that is used to determine the rate law depends on what data is available.
Method of initial rates:
This method can be used when you have data about:
- The initial concentration of the reactant(s).
- The initial rate of the reaction.
- If you have preferment the reaction several times, using a variation of concentrations.
- In this case we can use the rate law.
Using this technique, the rate of the reaction is measured as close to t=0 as possible.
This initial rate is measured again several times using different concentrations of reactant.
Advantages - only need to follow reaction for short period of time (several data points).
Disadvantages - Drawing the tangent to the curve may not be accurate (especially if uncertainty in concentration due to very rapid changes).
Give an example of finding the rate order using the method of initial rates.
