Rate Equations Flashcards
define the term rate of reaction
change in concentration per unit time
at a given instant how could you calculate rate of reaction
rate of reaction = change in conc. / change in time
how could you measure the rate of reaction experimentally
- use a colorimeter at a suitable intervals if there is a colour change
- if gas is evolved, use a gas syringe to collect volume of gas evolved, use a gas syringe to collect volume of gas evolved, or measure the change in mass of the reaction mixture
how would you measure reaction rate for really fast reactions
use a flash of light to break bonds, use probe flashes to record amount of light absorbed by a species; this can show its concentration. first used for ClO2 –> ClO* + O*
can now monitor reactions that occur in times as fast as 10-12 seconds
how can you determine the rate constant and rate expression for a reaction
only experimentally
what affects the value of the rate constant for a given reaction
temperature, nothing else
write a generic rate expression and state what each term means
Rate = k [X]^x [Y]^y
k = rate constant for the reaction
[X] and [Y] are concentrations of species X and Y respectively x and y are the orders of reaction with respect to X and Y
do species need to be in the chemical equation to be in the rate expression
no - species in the chemical equation may be excluded and species may not in the chemical equation e.g. catalysts, may be included
define the term order of reaction with respect to a given product
the power to which a species’ concentration is raised in the rate equation
define the term overall order of reaction
sum of the orders of reaction of all species in the rate expression e.g. total order = x+y
how would you draw a rate concentration graph
plot [A] against time, draw tangents at different values –> draw a secondary graph of rate against [A]
draw a rate concentration graph for a zero order reactant
straight horizontal line
draw a rate concentration graph for a first order reactant
directly proportional
how could you find the rate expression using the initial rate method
- do a series of experiments, during which you vary concentrations, so the concentration of just one reactant changes each time
- plot a graph of concentration against time for each reactant and use a tangent at t=0 to find the initial rate of reaction
- compare rates and concentrations between each experiment to find order of reactants and overall rate equation
when does the starch turn a blue-black colour in an iodine clock reaction and why
when all of the Na2S2O3 has been used up and so I2 is produced, which reacts with starch leading to a blue black colour
how can you calculate the rate of reaction from the data from an iodine clock reaction
record time taken for colour change to occur
use rate = 1/t
effectively the initial rate
what is the effect of a 10K temperature increase on the rate of reaction, roughly
doubles rate of reaction
what is true of the half life of a first order reactant
half life is constant
what is the Arrhenius equation
k = Ae -Ea/RT
k = rate constant for reaction
A = pre-exponential factor
e = mathematical quantity
R = gas constant
T = temperature in kelvin
Ea = activation energy for reaction in Joules
how can you convert the Arrhenius equation into a useful form for plotting a graph
ln k = -Ea/RT + ln A
graph of lnk against 1/T is a straight line:
gradient = -Ea/R and y intercept is lnA
what is the rate determining step
slowest step in a reaction mechanism, which determines the overall rate of reaction
