Rate Equations Flashcards
What does zero order mean
As the concentration changes the rate stays the same
What does first order mean
As concentration changes the rate changes by the same factor
What does second order mean
As the concentration changes by a factor, the rate changes by the same factor Squared
What does a
a) zero order
b) first order
c) second order
Graph look like
a) straight horizontal line
b) straight diagonal line
c) exponential curve
Define rate of reaction
Change in concentration
————————————-
Time
Define order of reaction
The Power of concentration term in the rate equation
How to work out overall order
The sum of all the orders shown in the rate equation
Give a generic example of the rate equation
Rate= k[A]^m[B]^n
Where m=order with respect to [A]
And n=order with respect to [B]
When determining orders with tricky experimental data, what equation would you use
[X]^x x [Y]^y= change in rate
What reactants will appear in rate equation
Reactants upto and including the rate determining step will appear
When do intermediates appear in the rate equation
Never
Some reactions take place over several stages. What are these collectively referred to as?
The mechanism
How do we determine the rate of the overall mechanism
Consider the rate of the slowest of all stages (the rate determining step)
The rate equation only includes species that have an effect on the rate of the reaction. The RDS is the slowest step so will contain the same reactants as those in the rate equation
If a species appears in the rate equation but no in the overall equation, what must it be?
A catalyst
Zero order species always appear where?
After the rate determining step
True or false?
In the mechanism the steps up to and including the RDS contain the same species as those shown in the rate equation, the correct equation
True
How and why does k vary with temperature?
Concentrations don’t change with temperature but the rate still changes so k must change (as it is the only value that can change)
How does k change as temperature increases
K Increases with temperature
What is the Arrhenius equation
lnk = lnA - Ea/RT
What are the units for the Arrhenius constant
Same units as k (depend on Oder of reaction)
Relate the Arrhenius equation to y=mx+c
lnk= -Ea/R (1/T) +lnA
How do you calculate Ea from the gradient of a 1/T vs lnk graph
m= -Ea/R -mR= Ea
What temperature units must be used for a 1/T vs lnk graph?
Kelvin (K)
How to determine initial rate of reaction from a concentration vs time graph
m at t=0
How to work out order of reaction from a concentration vs time graph
Work out initial rate
Work out rate at half the concentration of the beginning of the reaction and compare
What does a zero order concentration-time graph look like
Straight line - as concentration changes gradient stays The same
When given a graph in an exam what should you always do?
Calculate gradient
What are the two graphs you need to know?
Rate vs conc
Conc. vs time
What can a rate axis be replaced with?
What reactions are these graphs used for
1/T
Disappearing X or iodine clock
Why can rate = 1/T
Change in conc. is a fixed amount
What can a conc axis in conc vs time be replaced with?
Volume of gas
Mass
What does an Arrhenius graph look like
1/T on top x axis
lnK on y going down
Template for “why is this the rate determining step”
By this step all the species in rate equation have appeared in correct ratio
