Reaction Kinetics Flashcards
What does kinetics mean?
things related to the rate of reaction
What is the rate of reaction?
the rate of change of concentration of a reactant or product with time
What are the units for rate?
moldm-3 s-1
What are the 2 ways that rate can be measured?
- samples taken at interval time points
- continuous monitoring of the reaction
How does a colorimeter work?
uses a filter to select light in a wavelength range where there is a big difference in strength of absorptio by the products compared to the reactants
What is the method of monitoring a reaction using colorimetry?
- select filter for an appropriate wavelength
- calibrate the colorimeter by measuring absorbance rate of standard solutions with known concs of the coloured substance
- plot a calibration curve of absorbance v conc
- start reaction in the sample cell of the colorimeter, measure absorbance rate continuosly
- use calibation curve to turn absorbance readings into conc readings
What is the definition of the rate equation?
the rate equation for a reaction shows how the rate depends on the concentrations of reactants
What is the generic rate equation?
Rate = K[A]^m [B]^n
What is K in the rate equation?
the rate constant
What is m/n in the generic rate equation?
the order of reaction with respect to the reactant
How do you calculate the overall order of reaction (generic)?
m + n
the sum of the separate orders
What is the definition of the rate constant?
a measure of the probability that a reaction will happen when the particles collide
What happens to K as the temp increases? and why?
K increases as the temp increases
because the proportion of reactant molecules (or ions) with energy greater than the activation energy increases
How does concentration and pressure affect K?
it doesnt have any affect on K
What is half life?
the time taken for the concentration of a reactant to fall to half its original value
What is the abbreviation for half life?
t 1/2
What is the rate equation for 0 order?
rate = K
What happens to the t1/2 of a 0 order reaction?
sucessive half-lives decrease
What are the units for K for 0 order?
the same as rate - moldm-3 s-1
What is the rate equation for a first order reaction?
Rate = K[A]
What happens to the t1/2 of a 1st order reaction?
successive half-lives remain constant
How can you calculate the value of K from the t1/2 (t1/2 equation)?
K = ln 2 / t1/2
What is the relationship between the [A] and the rate of a 1st order reaction?
they are directly proportional
How would you find the value of K from a 1st order rate v conc graph?
the gradient = K
What is the rate equation of a 2nd order reaction?
rate = K[A]^2
What happens to the t1/2 in a 2nd order reaction?
successive half-lives increase
What is the units of k for a 1st order reaction?
s-1
What are the units of K in a 2nd order reaction?
mol-1 dm3 s-1
What are the 2 ways rate can be found?
- measuring the change in a reactant mass/ conc over time
- measuring the change in a product mass/conc over time
How can titrations be sed to find the rate of reaction?
small samples of reaction mixture can be removed at regular intervals
samples can be titrated to determine the conc of a product/reactant
How can colorimetry be used to determine rate of reaction?
formation or depletion of a coloured species
measures the amount of light absorbed by solution = proportional to the conc of the coloured species
What is the rate determining step?
the slowest step in a reaction pathway
How do you find the order of A from using initial rates?
change the conc of A in the reaction A + B —> C
keep conc of B constant
if 1st order than proportion of increase in conc to the initial rate is equal
if 2nd order then the proportion of the initial rate is squared = doubling the conc of A will cause the inital rate to x4
What effect does 0 order have on rate while changing the conc of an reactant?
has no effect on rate
What effect does 1st order have on rate while changing the conc of an reactant?
increasing the conc of reactant by a factor of x increases the rate by the same factor of x
What effect does 2nd order have on rate while changing the conc of an reactant?
increasing the conc of reactant by a factor of x increases the rate by a factor of x squared
What do clock reactions find?
find an approximate value for the intitial rate of reaction by measuring the time it takes for a certain amount of product to form
What is the general method for analysis of clock reactions?
times for colour change are measured for a range of different initial reactant concs = T
1/T is calculated
What does the number of moles represent in a rate equation?
the number of moles of a substance is equal to the order of reaction for that substance
How do you identify a catalyst from a reaction mechanism?
heterogneous catalyst = it will be used up and regenerated in a later step
What happens to the rate of reaction when temp increases? (boltzmann distribution)
the proportion of molecules with an energy higher than the Ea
successful collisions occurr more frequently
What happens to the rate constant (k) when temperature increases?
increases = K is related to the probability of successful collisions
What is the arrhenius equation?
K = Ae (-Ea/RT)
A = pre-exponential factor
R = gas constant
e = e on calc
What is the pre-exponential factor (A) ?
relates to the conentrations of reactants to the frequency of collisions
units are the same for rat constant
What is the exponential term mean - e^ -(Ea/RT) ?
reflects the probability of a collision being successful = very temp dependent
it has no units
How can you rearragnge the arrhenius equation to find the Ea from a graph?
ln K = (-Ea/R) x 1/T + ln A
—–> y = mx + c
gradient = -Ea/R
y intercept = ln A
