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Further Kinetics
Reaction rate – The change in the amount of reactants or products per unit time
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Continuous monitoring methods of following the rate of reaction:
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Gas Volume- If gas is given off, collect it (syringe) and record it at regular time intervals
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of the reactant
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If small volume is collected syringe is better
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Loss of mass- If gas is given off system loses mass
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Mole
calculations to see how much gas was lost, and thus how many moles of reactants are left
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(CO2)

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Colour Change- Using colorimeter
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More conc
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To do this:

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plot a calibration curve- known conc
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o

During experiment take small samples at regular intervals and read the absorbance
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Change in pH- If H+ is used or produced pH will change
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of H+ ions
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Quench them
(stop the reaction by adding something or ice bath) and then titrate them using a standard
solution
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over time
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Other physical properties that give a change- change in volume, chirality and refractive
index
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The rate at point is gradient at
that point
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Negative – reactants
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Order of reaction with respect to reactant tells how reactant’s conc
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The power to which
the concentration of the species is raised in the rate equation
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The overall order of reaction is the sum of the orders of all reactants
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of particular reactant
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If rate is proportional to X- order is 1
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The continuous method of finding the order of reaction:
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One reaction mixture is made
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The reaction is stopped by quenching
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of reactants it then determined
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time graph
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Shape of rate-conc
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The graph can be drawn from conc
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If it’s 1/t = rate
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To work it out plot conc
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Then
draw line from y- axis where the conc
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They are useful for identifying a first order reaction
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Initial rate of reaction – The rate right at the start of the reaction
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They have to be done by:
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Carrying out separate experiments using different initial conc
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Change only
one conc
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See how the change in initial conc
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Could be done using continuous monitoring method and drawing conc
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Calculating the gradient at t=0 gives initial rate
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You measure how the time taken for a set amount of product to form changes as you vary
the conc
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There will be a sudden increase in the conc
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Usually there is easily observable end point – colour change – to tell when the desired
amount of product has formed
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Assumptions made:
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The conc
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The temp
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When end point is seen the reaction has not proceeded too far
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2−
−
Sodium thiosulfate reacts instantaneously with iodine: 2S2 O2−
3 + I2 → 2I + S4 O6

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At first all iodine is used up in 2nd reaction
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This indicates end of
clock reaction
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Will give different times for colour change
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Of reactants will allow to calculate the initial
rate with respect to iodide or hydrogen peroxide
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Transfer small amount of sulfuric acid of known conc
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(reaction vessel)

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Add distilled water to the beaker
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Measure known amount of KI solution of known conc
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Add sodium thiosulfate solution and swirl the vessel to mix them
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Continue to stir
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Record time with quantities of sulfuric
acid, water, KI, sodium thiosulfate
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Comparing the initial rates will allow to find the order with respect to KI
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Stop the reaction using
sodium hydrogencarbonate to neutralise the acid
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of Iodine
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Rate= k[CH3 COCH3 ][ H + ]

Alkaline hydrolysis of primary halogenoalkanes:
Alkaline hydrolysis of 1-chlorobutane:

CH3CH2CH2CH2Cl + OH- → CH3CH2CH2CH2OH + ClThe rate equation is:
Rate= k[CH3CH2CH2CH2Cl][ OH-]
We can assume that mole particle of each reactant is present in rate-determining step:
This suggest Sn2 mechanism
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It’s a continuous
single step reaction and the complex is a transition state
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Sn1 mechanism, and the rate determining step is unimolecular since there is only one reactant
present
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– will show if works for more than 1 conc
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NMR can be used
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Rate equation
An equation expressing the mathematical relationship between the rate of reaction and the concentration of the
reactants
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Bigger = faster reaction
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The overall order of reaction is m + n
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It’s always the
same for a certain reaction at particular temperature
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The overall rate is decided by the step with the slowest rate – the rate determining step
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The rate determining step doesn’t have to be the first step in the reaction
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The order of a reaction with respect to a reactant shows the no of molecules of that reactant
which are involved in or before the rate-determining step
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31 J K-1 mol-1), A- another constant
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As the activation energy, Ea, gets bigger, k decreases
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As temp increases, k increases
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This gives gradient of Ea/R
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Catalyst
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Increases the rate of reaction by providing alternative pathway with lower activation
energy
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They are classified into 2 different types:
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Homogeneous- same state as reactant
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Usually a mesh or fine
powder to increase surface area
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They can be easily separated from the products and reactants
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It’s used in three-way catalytic converters

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