Kinetics Flashcards
What effect do the following have on the rate of reaction?
a) temperature
b) pressure
a) increase in temperature:
Greater average kinetic energy
Greater proportion of successful collisions
b) increase in pressure:
Collision frequency increases
Same speed & average kinetic energy
Describe how to carry out an iodine-clock titration
Measure samples of reactants of know concentration
Mix them together, start the clock and stir
At regular intervals withdraw samples and quench with sodium hydrogencarbonate or ice cold water (quenches & neutralises acid)
Titrate iodine with sodium thiosulfate
How can colorimetry be used to determine the rate of reaction?
Use a colorimeter to measure the absorbance or transmittance of light through a sample.
Then use a calibration curve to convert absorbance/transmittance into concentration.
Define the order of reaction
The sum of the powers to which the concentrations of the reactants are raised in the rate equation.
What is the rate determining step?
The slowest step in a multi-step reaction
What effect does a catalyst have on the rate of reaction?
What is autocatalysis?
A catalyst speeds up the rate of reaction by providing an alternative reaction route of lower activation energy. A greater proportion of molecules have energy >_ the activation energy.
Autocatalysis is when a reaction product is the catalyst.
What is the collision theory?
That reactant molecules must collide with correct orientation and kinetic energy >_ activation energy.
Why do tertiary halogenoalkanes react via SN1, but primary halogenoalkanes react via SN2?
Tertiary: favour SN1 because they have electron donating methyl groups that stabilise the charge on the carbocation
Primary: favour SN2 because they do not form stable carbocation intermediates and have less steric hinderance from methyl groups.
What is the purpose of sodium thiosulfate in the titration?
To reduce the iodine to iodide ions and prevent and colour change until a fixed amount of reaction has occurred (helps measure time taken for iodine to react)
Explain why the initial concentration of a reagent is much higher than that of the reagent being measured?
To eliminate the effect of change in concentration on the rate. (So it’s concentration remains unchanged and so the concentration of the reactant being measured is the only variable.)
