rate equations Flashcards
what are the orders of rate in respect to A
-doubling conc of A then rate is directly prop to A so the rate does not change - order is 1
-doubling A = quadruples rate
(A squared) and order is 2
-doubling A had no effect on rate so order is zero
what is the rate equation
rate = k[A]^m[B]^n
how do scientists find the order of the reactants
through experiments - DO NOT USE BALANCE NUMBERS
what is the rate constant dependent on
temp
how to find overall order
add all orders together
how to work out the order of a catalyst
same method as reactants
what are initial rate experiments
conc changed one at a time but everything else remains the same
how to measure rate immediately at the start eg use disappearing cross method
how to find orders using table
work out K
put the values in formula/ rearrange to find which one u are looking for
how to rearrange rate equation to find K
k = rate/reactants
what are the units for K
mol^-1 dm^3 s-1
the mol and dm will change
work out using same method as for kc and kp
what does the conc-time graph of a zero order reactant look like
In a zero-order the concentration of the reactant is inversely proportional to time
This means that the concentration of the reactant decreases with increasing time
The graph is a straight line going down
what does the graph for a conc-time graph for first order look like
In a first-order reaction the concentration of the reactant decreases with time
The graph is a curve going downwards and eventually plateau
what does the graph for a conc-time graph for second order look like
In a second-order reaction the concentration of the reactant decreases more steeply with time
The concentration of reactant decreases more with increasing time compared to in a first-order reaction
The graph is a steeper curve going downwards
what is the rate -conc graph for zero order
In a zero-order reaction the rate doesn’t depend on the concentration of the reactant
The rate of the reaction therefore remains constant throughout the reaction
The graph is a horizontal line
The rate equation for this one reactant is rate = k
to find k - to find the gradient - work out from conc-time graph
how to plot a rate-conc graph
The progress of the reaction can be followed by measuring the initial rates of the reaction using various initial concentrations of each reactant
These rates can then be plotted against time in a rate-time graph
what is the rate -conc graph for first order
In a first-order reaction the rate is directly proportional to the concentration of a reactant
This means that if you doubled the concentration of the reactant, the rate would also double
If you increased the concentration of the reactant by a factor of 3, the rate would increase by this factor as well
The graph is a straight line
The rate equation for this one reactant is rate = k [A]
find k to find the gradient- use rate - conc graph
change in y over change in x
what is the rate -conc graph for second order
In a second-order reaction, the rate is directly proportional to the square of concentration of a reactant
The graph is a curved line
The rate equation for this one reactant is rate = k [A]2
k = gradient of rate-conc^2
how do u draw rate-conc from conc-time for first and second order
draw tangents at specific conc and calculate the gradients
use these to plot rate-conc
with gradients on x-axis
what is the rate determining step
the slowest step in the reaction
how to determine which reactants are in the rate equation
If a reactant appears in the rate-determining step, then the concentration of that reactant will also appear in the rate equation
what is the overall rate of the reaction dependent on
the slowest step thus the RDS
how can u predict the RDS
The overall reaction equation and rate equation can be used to predict a possible reaction mechanism of a reaction
This shows the individual reaction steps which are taking place
-deduce which reactants are which order
-then this is how many molecules are in the RDS
-compare to mechanism given
-pick which one is RDS by seeing if it matches rate equation and overall
example of RDS prediction
For example, nitrogen dioxide (NO2) and carbon monoxide (CO) react to form nitrogen monoxide (NO) and carbon dioxide (CO2)
The overall reaction equation is:
NO2 (g) + CO (g) → NO (g) + CO2 (g)
The rate equation is:
Rate = k [NO2]2
From the rate equation it can be concluded that the reaction is zero order with respect to CO (g) and second order with respect to NO2 (g)
This means that there are two molecules of NO2 (g) involved in the rate-determining step and zero molecules of CO (g)
A possible reaction mechanism could therefore be:
Step 1:
2NO2 (g) → NO (g) + NO3 (g) slow (rate-determining step)
Step 2:
NO3 (g) + CO (g) → NO2 (g) + CO2 (g) fast
Overall:
2NO2 (g) + NO3 (g) + CO (g) → NO (g) + NO3 (g) + NO2 (g) + CO2 (g)
= NO2 (g) + CO (g) → NO (g) + CO2 (g)
are species that appear after the RDS included in the rate equation
no