Topic 6 & 16 Kinetics Flashcards
outline some of the benefits and drawbacks to using mass loss methods
- use a cotton wool bung to prevent solution from escaping due to spitting whilst still allowing gas to escape
- not suitable for H2 as mass loss is not significant enough, so it is only suitable for heavier gases
which methods can be used to measure rate of reaction
- gas collection (i.e. gas syringe)
- mass loss methods
- measuring pH
- colorimetry
- conductivity
- titration
when using a colorimeter, what will the absorbances be for a darker and lighter color
darker color will have high absorbance, lighter color will have lower absorbance
how and why does rate change over time
rate decreases because the limiting reactant is used up, causing its concentration in solution to decrease which means the frequency of collisions decreases, and hence so does the rate
what does the gradient of the tangent at t = 0 tell you
gives the initial rate
how would you calculate average rate
quantity being measured / time
how would you calculate the instantaneous rate
draw a tangent line then calculate the gradient
outline the collision theory
for reactions to take place, reactants must collide with one another with 1. proper orientation and with 2. sufficient energy to react (i.e. energy must be at or greater than Ea)
how does concentration and pressure affect the rate
increases the number of particles in a certain volume, so the frequency of collisions between particles increases, so the rate of rxn inc as well (**Note: pressure only applies to gases)
how does surface area affect the rate
increase SA means inc # particles exposed at the surface, increases the frequency of collision between other reactant particles and particles on surface of solid, so rate of rxn increases.
*powdered reactants react the fastest
how does temperature affect the rate
particles have greater average kinetic energy, more frequency collisions between particles, greater proportion of particles with energy greater than or equal to the Ea, so ROR increases
describe some features of Maxwell Boltzmann for two curves whereby T2 > T1
the curve at temp 2 will have its peak at a higher energy, but the peak will be lower than the graph for T1 because the area under the two curves must be equal.
what does the area under the curve of a maxwell Boltzmann graph tell you
area is proportional to the total number of molecules
catalyst
substance that increases the rate of a chemical reaction by providing an alternative pathway of lower activation energy without being used up
how does lower Ea affect the rate
more particles have energy at or greater than the Ea, so a greater proportion of collisions will be successful
which factors would affect the leveling off point for a reaction
if the concentration of the limiting reactant is changed, if the volume or mass of the limiting reactant is changed (because the moles of the reactant are being changed)
on a conc v time graph, outline what happens to the half lives for 0,1,2 order
zero order: half-life decreases
1st order: half life remains constant
2nd order: half life increases
what is the half-life
time taken for concentration of reactant to half
what should you always make sure to draw in on the graph of rate vs conc for first and second order
the point (0,0)
outline the variables for the arrhenius equation
k = Ae^(-Ea/RT)
where k is the rate constant
A is the frequency factor
Ea is the activation energy
R is the gas constant (8.31 in data booklet too)
T is temperature
which two factors is the rate constant affected by
T and Ea
how does an increase in temperature affect the rate constant
as T increases, -Ea/RT decreases in magnitude, so e^(-Ea/RT) becomes bigger/more positive, so Ae(-Ea/RT) increases, so k also increases and hence the rate increases
how does adding a catalyst affect the rate (explain using the Arrhenius equation)
Ea decreases, so Ea/RT decreases in magnitude, so -Ea/RT is less negative, so k increases
what would you plot on the x and y axis to find the activation energy using arrhenius
1/T on the x-axis and lnk on the y-axis
and then find the gradient of the line (-Ea/R) and then solve for Ea using gas constant
what sign should the activation energy always end up being
positive
what does lnA act as on the graph of lnk against 1/T
y-intercept
what does the “A” in the arrhenius equation tell you
pre-exponential/frequency factor; takes into account the frequency of collisions with proper orientation and is constant as temp changes
why is the frequency factor smaller for more complicated molecules
the molecules can collide in many different ways, and only a small proportion of the collisions will be successful
which reaction takes place in a single step with a transition state
SN2
which reaction is a multi-step process with an intermediate
SN1
what is the rate equation
rate = k[A]^x[B]^y
where k is the rate constant and the overall order of the reaction is x + y
x is the order with respect to A
y is the order with respect to B
what is the unit of rate
moldm^-3 s^-1
outline how titrations could be used to measure the rate of reaction
for a neutralization reaction, take aliquots of the solution and then quench the reaction by putting them in the freezer to reduce KE and slow down the reaction so the concentrations reflect the concs of when it was aliquoted. the titre volume of the NaOH will decrease over time because the concentration of the acid should also be decreasing over time
