18 Kinetics + Kp Flashcards
Define reaction rate
change in concentration of reactant or product per unit time
What is zero order?
the concentration has no effect on the rate
What is first order?
the concentration is directly proportional to the rate
eg. if the conc doubles, the rate also doubles
What is second order?
the concentration squared is proportional to the rate
eg. if the conc doubles, the rate is x4
Define the overall order
the sum of orders of each reactant
Rearrange the rate equation to find k:
rate = k [A][B]
k = rate/ [A][B]
What are the units for rate?
mol dm-3 s-1
What happens to the rate constant (k) if you increase the temperature?
k also increases - more frequent collisions, increasing the energy of each collision. The conc and orders remain the same, so k must increase for the rate equation to balance.
Why do small changes in temperature produce large changes in reaction rates?
As temp increases, there are more successful collision, and a greater proportion of molecules have enough energy to react. Particles only react if collisions have sufficient energy to start bond breaking.
What is the rate determining step?
The slowest step in a multi-step reaction. If a reactant doesn’t appear in the rate equation, it won’t be involved in the rate determining step.
What does the Arrhenius Equation show?
Shows how rate constant varies with temperature and activation energy.
What is the Arrhenius Equation?
k = Ae^-Ea/RT
What is the simplified form of the Arrhenius Equation?
lnk = lnA - Ea/RT
(natural log = opposite of exponential)
What happens to k if you increase Ea?
Decreases - less particles with sufficient energy for a collision to occur.
What happens to k if you increase T?
Increases - particles have more kinetic energy, so higher chance of successful collisions.
Rearrange the Arrhenius Equation to make Ea the subject.
Ea = RT x ( LnA - Lnk)
Rearrange the Arrhenius equation to make T the subject.
T = Ea/R x (LnA - Lnk)
In the Arrhenius equation, what do k, Ea, T and R represent? (with units)
k = rate constant
Ea = activation energy (J/mol)
T = temperature (K)
R = gas constant (8.31 JK-1mol-1)
What is the equation for the gradient?
gradient = change in y
/ change in x
Describe how you would find Ea from a graph of Ln k against 1/T.
- Find the gradient by drawing a tangent
- Multiply the gradient by the gas constant 8.31.
Describe how you would find ln A from a graph of ln k against 1/T.
Substitute the gradient and coordinates of any point on the line into ln k = -Ea/RT + ln A, and rearrange.
Define the total pressure of a gas.
The sun of all partial pressures of the individual gases.
What is a mole fraction?
The proportion of a gas mixture that is made up of a particular gas.
Equation to calculate mole fraction.
moles of substance A / total moles of all substances
*sum of all mole fractions = 1
Equation to calculate partial pressures.
mole fraction of A x total pressure
*sum of partial pressures = total pressure
What is Kp?
The equilibrium constant for a reversible reaction where all reactants and products are gases.
Write a Kp expression for the equilibrium:
aA + bB <===> dD + eE
Kp = (pD)d(pE)e / (pA)a(pB)b
What happens to the equilibrium if you increase / decrease temperature?
Increase: shifts in the endothermic direction (positive H)
Decrease: shifts in the exothermic direction (negative H)
Is Kp affected by temperature?
Yes - changing temp changes how much product is formed at equilib. This changes the mole fractions of gases present, which changes their partial pressures.
Is Kp affected by pressure?
No - the equilibrium will shift to keep it the same.
Is Kp affected by a catalyst?
No - it just gets the system to equilibrium more quickly.
