Kinetics II Flashcards
What is meant by the term rate of reaction ?
The rate of change of concentration of a reactant or product with time.
Units - mol dm^-3 s^-1
What is the rate equation?
r = k[A]m[B]n
What is meant by the term order with respect to substance in a rate equation?
The power to which the concentrations of that substance is raised to in the rate equation
Overall order of reaction?
The sum of the powers to which the concentrations of the reactants are raised in the experimentally determined rate equation
What is meant by the term rate constant?
k is the constant of proportionality that connects the rate of reaction with the concentration of the reactants.
How can the orders be worked out?
They are worked out experimentally , they have nothing to do with the stoichiometric coefficients in the balanced equations
A + 2B + 3C -> products.
It was found to be first order in the A and B and second in C.
Write the rate equation for this reaction.
Rate = k [A] [B] [C]^2
Name methods of measuring rate of reaction
Measuring the volume of gas evolved
Measuring the change in mass of a reaction mixture
Monitoring the change in intensity of colour of a reaction mixture (calorimetry)
Measuring the change in concentration of a reactant or product using titration
Measuring the change in pH of a solution
Measuring the change in electrical conductivity of a reaction mixture
How would the reaction between calcium carbonate and dilute hydrochloric acid to measure the ROR ?
CaCO3 (s) + 2HCl (aq) -> CaCl2 (aq) + H20 (l) + CO2 (g)
measuring the volume of gas given off at regular intervals
By measuring the change in mass of the reaction with time
How would the reaction between propanone and iodine in aqueous solution measure the ROR? CH3COCH3(aq) + I2 (aq)-> CH3COCH2I (aq) + H+ (aq) + I- (aq)
to follow the reaction by monitoring the decrease in intensity of colour of reaction mixture as I 2 is. the only coloured species
What are the two most common techniques for collecting and measuring the volume of gas evolved during a reaction?
Collection over water into a measuring cylinder
Collection using a gas syringe
Chosen one will depend partly on the level of precision required. Greater degree of precision in gas syringe but if a larger volume of being measured not as precise
Measuring the change in mass of a reaction mixture
reaction flask and contents are placed on a digital balance and the decrease in mass is measured as the reaction proceeds.
More precise when the gas given off has a relatively high density
calorimetry
monitoring the colour change if the reactant or product is coloured
The reactants are mixed and a clock is started. Change is measured in set intervals
How is Titration used to find the ROR
Measure out samples of the reactants with known concentrations
Mix them together start a clock and stir the mixture thoroughly
At regular time intervals withdraw samples using a pipette and quench the reaction. Note the time when half the contents of the pipette is placed
The quenched solution is then titrated against a suitable standard solution
When can titration be used to find the ROR?
When an acid, alkali or iodine is a reactant or product.
Acid titrated with standard alkali
Alkali titrated with standard acid
Iodine titrated with sodium thiosulfate
measuring the electrical conductivity
measuring change via conductivity meter
In a reaction A —> B+C, the value of [A] fell from 0.12 mol dm -3 to 0.012 mol dm -3 in the 60s. Explain why it is inaccurate to state that the rate of reaction is 0.0018 Moldms-1?
For this to be accurate the concentration must change by 10% or less. Here it changes by 90%
What is the unit for zero order?
mol dm-3 s-1
What is the unit for first order?
s-1
What is the unit for second order?
dm^3 mol-1 s-1
What is the unit for third order?
dm^6 mol-2 s-1
The slowest step in a reaction determines
the overall ROR
It is known as the rate determining step
How can we determine the order of reaction with respect to each reactant?
continuous method
initial rate method
What is a continuous method?
This is data from one experiment where the concentration of one substance is followed throughout the experiment.
In order for the continuous method to be used,
the concentrations of the reactants not being followed must be in large excess in the experiment so their concentrations stay virtually constant and do not affect rate
How is the data processed in the continuous method?
By drawing a concentration-time graph then calculating the successive half lives
If the half lives are constant …
it is 1st order
If the half lives rapidly increase
it is 2nd order
If the rate stays constant as the reactants is used up
it is zero order
Concentration has no effect on rate
What is meant by the term half life?
Time taken for the concentration of the reactants to halve
From a conc-time graph, if it is a horizontal line
the reaction is not taking place = 0
From a conc-time graph, if a straight line downwards is obtained
the rate is constant - zero order
From a conc-time graph, if a downward curve is obtained
the rate is decreasing as concentration falls hence first order or greater
How is the rate calculated at any particular concentration?
by drawing a tangent at the wanted concentration then using gradient
Graphs of initial rate against concentration show what?
different orders
How is the initial rate calculated using a graph?
The initial rate may have been calculated from taking gradients from concentration /time graphs
In an initial rate graph , zero order means
For zero order: the concentration of A has no effect on the rate of reaction
r = k[A]0 = k
In an initial rate graph first order
For first order: the rate of reaction is directly proportional to the concentration of
A r = k[A]1
In an initial rate graph second order is
For second order: the rate of reaction is proportional to the concentration of
A squared r = k[A]2
When using data from experiments within table format to figure out the order,
at least one of the concentrations must be the same, when identifying a pattern ethene the concentration of the other reactant and initial rate.
Often experimental values need to be rounded up to the whole number when getting a pattern.
Determining zero order from rate-conc graphs
horizontal line 1/t = k[A]0
Determining first order from rate-conc graphs
straight line 1/t = k[A]1
Determining second order from rate-conc graphs
curved line. 1/t = k[A]2
However not able to eternise directly hence draw another graph 1/t against [A]^2. F straight its 2nd
For a rate concentration graph to show the order of a particular reactant….
For a rate concentration graph to show the order of a particular reactant the concentration of that reactant must be varied whilst the concentrations of the other reactants should be kept constant.
Propanone reacts with iodine in acidic solution (the acid is a catalyst) . Give the chemical equation
CH3COCH3(aq) + I2(aq) →
CH3COCH2I(aq) + H+(aq) + I–(aq)
Investigating the rate of reaction between iodine and propanone
This reaction can be followed by removing small samples from the reaction mixture with a volumetric pipette. The sample is then quenched by adding excess sodium hydrogencarbonate to neutralize acid catalyst which stops the reaction. Then the sample can be titrated with sodium thiosulfate using a starch catalyst
Give the orders for rate of reaction between iodine and propanone
This reaction is zero order with respect to I2 but 1st order with respect to the propanone and acid catalyst
Give the rate equation for rate of reaction between iodine and propanone
The rate equation for the reaction is.
Rate = k[CH3COCH3(aq)][H+(aq)]
RDS for the reaction between iodine and propanone
The rate determining step of this reaction must therefore contain one propanone molecule and one H+ ion forming an intermediate. The iodine will be involved in a subsequent faster step.
What is meant by the term activation energy?
the minimum amount of energy required to initiate a reaction
What is meant by the term heterogeneous catalyst?
Is in a different phase to that of the reactants
What is meant by the term homogeneous catalyst?
Is in the same phase as the reactants
Why does an increase in temperature increase the rate of reaction?
an increase in the fraction of molecules with energy equal or greater to the Ea for the reaction
Overall increase in the frequency of collisions between the reactions molecules
State the Arrhenius equation
k = Ae^ (-EA/RT)
Increasing temperature increases the rate constant k.
A is the Arrhenius constant, R is the gas constant, and EA is activation energy.
Units in Arrhenius Equation
Temperature uses the unit K
R = 8.31 J mol-1K-1 ActivationenergywillneedtobeinJmol-1 tomatchtheunits of R
The unit of the Arrhenius constant A will be the same as the unit of the rate constant k
Taking the natural log of an Arrhenius Equation
lnk = (-Ea/R . 1/T) + lnA
If a graph of lnk is plotted against 1/T
A straight line is obtained
The gradient = -Ea/R
The intercept at y axis is lnA0
