Rate and Kp Flashcards
What is the rate equation?
Change in conc/ change in time
What are the units of rate?
mol dm-3 s-1
What are orders of reaction related to?
After carrying out multiple experiments, rate can be proportional to the conc of certain reactants [A]
How would you create a rate expression?
rate= k[A][B]^2
add a constant (k)
add the orders
How do you find the overall order?
Just add up the orders
e.g first and second order makes 3
What order is a reactant thats conc has no effect on rate?
zero order
-not included in the rate equation
If a catalyst is used, do you include it in the rate expression with an order?
Yes
It always has an effect on rate so will never be a zero order
[H+]
How would you calculate the constant ‘k’?
k= rate/ concs of products
e.g rate/ [A] [B]2
How do you find rate on a graph?
Draw a tangent to the point you want on the curve, and do rise/ run to find the gradient which is equal to rate
What does a 1st order reactant look like on a graph?
Straight line through the origin
/
What does a 2nd order reactant look like on a graph?
Curved
)
What does a zero order look like on a graph?
Horizontal line
__________
What are elementary steps?
Basic steps in a reaction that cannot be broken down any further.
-Can take place at different rates
-Multiple steps to a reaction
What are the 3 types of elementary steps and explain them?
1- Decomposition: one particle breaks into two
Cl2 —-> Cl+Cl
2- Substitution: particles collide and rearrange to form 2 completely different molecules.
Cl + CHCl3 —-> HCl + CCl3
3- Synthesis: particles collide and join as a single particle.
CCl3 + Cl —-> CCl4
Together these steps make a REACTION MECHANISM
What is it called where a reaction only has one elementary step?
An elementary reaction
The reaction mechanism is just the overall equation.
What is an intermediate?
Produced in one step and used up in another.
They are not shown in the overall equation.
How do you make an overall equation from elementary steps?
- Write out all equations
- Cross out any intermediates
- Add up duplicates of each other e.g. A+A. 2A
How would you make a rate expression for a multi-step reaction?
-Do the same as a rate equation by crossing out intermediates.
-Take out any reactant that comes after the slowest step.
Only use reactants form before and during the slowest step.
What is the rate-determining step?
The slowest step
What is the rate expression useful for?
-Can work out what order the reactants are
-Use it along with the mechanism to determine the rate determining step.
-Use it to determine a correct mechanism
What is the Arrhenius Equation?
K= Ae ^ -Ea/RT
A= arrhenius constant
Used to quantify a relationship between rate constant and temp
Where is ‘e’ on a calculator? (arrhenius equation)
e and a square box
need to press shift
What would help you find Ea form the Arrhenius equation?
ln (log)
ln(e^36)= 36
How would you rearrange the Arrhenius equation?
Use ln of A and k
ln(k) = ln(A) -Ea/RT
Rearrange from there
e.g. Ea= RT( ln(A) - ln(k) )
How would you find Ea on a graph using the Arrhenius equation?
- Rearrange the equation in y=mx+c format —> ln(k) = -Ea/R. x 1/T. + ln(A)
- On a graph y axis= ln(k) and x axis= 1/T
- In y=mx+c, ‘m’ is the gradient therefore -Ea/R is the gradient
- Ea = R x (-gradient). in KJ mol-1
What is the Kp equation?
Kp= pressure of products/ pressure of reactants
What is partial pressure?
You have a mixture of A and B in a container…
Partial pressure is if A were in that same container on its own
What is partial pressure represented as?
Capital P with tiny compound
e.g. Po2
(P is big)
(O2 is small)
What is the mole fraction equation?
mol fraction of gas A= n.o of mol gas A/. total n.o of mol in mixture
How do you calculate partial pressure?
Partial pressure of A= total pressure x mol fraction of A
How do you express Kp?
pressure in round brackets
Kp= pressure products/ pressure reactants
e.g. Kp= (Po2)^2 (Po2). /. (Po3)
How would you answer a Kp question?
- Write out expression
- Find mol of each at equilibrium using an ICE table
- Find mole fraction of each gas
- Find partial pressures
- Plug in values to Kp expression
What effects Kp?
Only temperature
