Rate Equations Flashcards
Define the term rate of reaction
Change in concentration (of any reactant or product) per unit time.
At a given instant, how could you calculate the rate of reaction?
Rate of reaction = change in concentration / change in time
How could you measure the rate of reaction experimentally (different methods)?
Use a colorimeter at suitable intervals if there is a colour change.
If gas is 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 not in the chemical equation e.g. catalysts, may be included
Define the term order of a 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.
The sum of the orders of reaction of all species in
the rate expression e.g. (from earlier), total order
= x + y
How would you calculate the units of the rate constant?
Units of rate are moldm-3s-1 and units of concentration are mol dm-3
Rearrange rate equation to get k=
Sub in units and cancel them out
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]
How could you confirm that a reactant was second order?
Graph of rate against
concentration squared would be a straight line through the origin
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
What must you add to react with the I2 as it is produced for an iodine clock reaction?
Known moles of sodium thiosulfate and a little starch; Reacts with I2 in 1:2 ratio
I2 + 2S2O32- → S4O62- + 2I
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. This is 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
(concentration against time graph)?
Half life is constant
What is the Arrhenius
equation?
k= Ae x -EA/RT
What does each term in the Arrhenius equation mean?
k= rate constant A = pre-exponential factor e = mathematical quantity R= gas constant T= temperature in Kelvin EA = activation energy 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?
The slowest step in a reaction mechanism, which
determines the overall rate of reaction
How does the rate determining step link to the
species involved in the rate expression?
Any species involved in the rate determining step appear in the rate expression.
Species only involved after the rate determining step do not appear in the rate expression
