18) Rates of reactions Flashcards
Define order
the power to which the concentration of a reactant is raised in the rate equation
For zero order reactants, change in concentration _?
doesn’t influence rate
For first order reactants, change in concentration _?
results in the same change in rate
For second order reactants, change in concentration _?
results in the change^2 in rate
Give a generalised rate equation
k [A]^m [B]^n
Overall order = ?
sum of orders with respect to each reactant
Define initial rate
the instantaneous rate at the beginning of an experiment when t = 0
Give 3 methods of continuous monitoring of rate
by gas collection
by mass loss
with a colorimeter
How does a colorimeter work?
the wavelength of light passing through a coloured solution is controlled using a filter, and the amount of light absorbed by a solution is measured
Give an 8 step method for continuous monitoring of rate with a colorimeter
- prep standard sols. of known conc. of the coloured chemical
- select filter w/ complementary colour
- zero w/ water
- measure absorbance readings for each conc.
- plot calibration curve of absorbance against conc.
- carry out the reaction with the coloured chemical + take absorbance readings of the reacting mixture at regular / measured time intervals
- use the calibration curve to measure concentration at each absorbance reading
- plot a conc.-time graph to determine order of reaction with respect to the coloured chemical e.g. iodine
Define rate constant k
the constant that links the rate of reaction with the concentrations of the reactants raised to the powers of their orders in the rate equation; proportionality constant
For concentration-time graphs rate of reaction = ?
gradient
drawing a tangent at a specific conc. enables rate at that conc. to be calclulated
For concentration-time graphs, a zero order reactant is shown as ? k = ?
a straight line
gradient = k
For concentration-time graphs, a first order reactant is shown as ?
a curve with a constant half life (exponential decay)
Define half-life
the time taken for the concentration of a reactant to decrease by half
Give 2 ways which k can be determined for a first order reaction
- calculate rate constant from rate (gradient) - rearrange the rate equation and sub. in values of rate (gradient of tangent) and conc. at the position where the tangent has been drawn
- calculate rate constant from half-life
k = ln2 / t 1/2
For rate-concentration graphs, a zero order reactant is shown as ? k = ?
horizontal, straight line
y-intercept = k
For rate-concentration graphs, a first order reactant is shown as ? k = ?
straight, diagonal line up from origin
gradient = k
For rate-concentration graphs, a second order reactant is shown as ?
curved line up from origin
How can k for a second order reactant on a rate-concentration graph be calculated?
plot a 2nd graph of rate against conc.^2
the gradient of this graph = k
How can initial rate be found?
measuring the gradient of a tangent drawn at t = 0 on a conc.-time graph
Describe a clock reaction (5)
- obtains initial rate by taking a single measurement
- time from start is measured for a visual change (colour or ppt.)
- provided there is no sig. change in rate it can be assumed average rate = initial rate (proportional to 1/t)
- the reaction is repeated with dif. conc. and values are calculated for each
- graph of 1/t against conc. plotted and order with respect to each reactant determined
Describe an iodine-clock procedure (5)
- relies on the formation of iodine (usually orange-brown but starch is added)
- separate experiments are carried out using dif. conc. of one of the reactants whilst all other conc. are kept constant
- the solution begins colourless and time measured for the blue-black colour of the starch-iodine complex to appear
- colour change is delayed by a small amount of aqueous sodium thiosulfate which removes iodine as it forms (once it is used up, the colour appears)
- further series of experiments are then carried out in which the conc. of one of the other reactants is changed e.g. hydrogen peroxide
Give the equation for the reaction of iodine with thiosulfate ions
2 S2O3 2- (aq) + I2 (aq) -> S4O6 2- (aq) + 2I- (aq)
The initial rate measured during a clock reaction is ?
an approximation
reasonably accurate provided less than 15% of the reaction has taken place
Define reaction mechanism
the sequence of bond-breaking and bond-forming steps that shows the path taken by e- during a reaction
Define rate-determining step
the slowest step in the reaction mechanism of a multi-step reaction
The rate equation only includes _?
reacting species involved in the rate-determining step
The orders in the rate equation match _?
the no. of species involved in the rate-determining step
As temp. increases and rate increases - what will also increase?
the value of k
The increased frequency of collisions is _ compared with the increase in the proportion of molecules that exceed Ea from the _, therefore rate is mainly determined by _?
comparatively small
shift in the Boltzmann distribution
Ea
Give the Arrhenius equation
k = A e ^(-Ea/RT)
What is the pre-exponential term / frequency factor in the Arrhenius equation?
A
frequency of collisions with the correct orientation; constant over a small temp. range
What is the exponential factor in the Arrhenius equation?
e ^(-Ea/RT)
proportion of molecules that exceed Ea + have sufficient energy for a reaction to take place
Give the logarithmic arrangement of the Arrhenius equation
ln k = - Ea/RT + lnA
ln k = -Ea/RT x 1/T + lnA
Describe a graph of the logarithmic form of the Arrhenius equation
y axis = lnk
x axis = 1/T / k^-1
gradient = - Ea/R
y-intercept = lnA
How can Ea and A be calculated from the logarithmic form of the Arrhenius equation?
- calc. values of lnk and 1/T from data
- plot graph
- calc. Ea from gradient
Ea = R x -gradient (in Jmol^-1 so x10^-3) - calc lnA from y-intercept
- A = e^lnA (exponential value on calculator)
