1.9 Rate equations Flashcards
Define the term rate of reaction
Change in concentration per unit time. State what is being monitored (usually production of a product)
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
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 species; this can show its concentration
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 value
Temperature, nothing else
Write a generic rate expression and state what each term meand
Rate = k[A]^m[B]^n
k=rate constant for the reaction
[A] and [B] are concentrations of species A and B
m and n are the orders of reaction with respect to A and B
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 eg 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 a reaction
The sum of the orders of reaction of all species in the rate expression, total order = m + n
How would you calculate the units of the rate constant
Units of rate are moldm^-3s^-1 and units of concentration are moldm^-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]
Draw a rate conc graph for a zero order reactant
Draw a rate conc graph for a first order reactant
Draw a rate conc graph for a second order reactant
How could you find the rate expression using the initial rate method
Do a series of experiments, during which you vary concs, so the conc 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 concs between each experiment to find order of reactants and overall rate equation
What must you add to react the I2 as it is produced for an iodine clock reaction (equation)
Known moles of sodium thiosulfate and a little starch. Reacts with I2 in 1:2 ratio
I2 + 2S2O3^2- –> S4O6^2- +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 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 (conc against time graph)
Half life is constant
What is the Arrhenius equation
What does each term mean
k = Ae -EA/RT
k = rate constant for reaction
A= pre-exponential factor (number of collisions between reactant molecules)
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
lnk= -EA/RT + lnA
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
