kinetics + rate equations Flashcards
what is activation energy
it is the minimum energy which particles need to collide to start a reaction
what happens to the rate of reaction when you increase the temperature.
decreases the activation energy, so more successful collisions because of the higher kinetic energy
what happens when you increase the concentration
at higher conc, there are more particles per unit volume so the particles collide with a greater frequency and there will be a higher frequency of effective collisions.
- the activation energy remains the same
- but there are more molecules with energy
what happens when you increase the pressure
at higher conc, there are more particles per unit volume so the particles collide with a greater frequency and there will be a higher frequency of effective collisions.
- same activation energy
but more molecules with energy
the gas phase reaction between hydrogen and chlorine is very slow at room temp.
give one reason why the reaction is very slow at room temp
- the activation energy is too high
- so fewer particles have the activation energy
explain how the graph shows that the reaction is zero 0 order with respect to iodine in the reaction between propanone and iodine.
(the graph has sodium thiosulphate solution on one side and time on the x axis. there is a very linear graph which is a straight line)
- the graph is straight and it has a constant gradient
- therefore the rate of the reaction does not change
what must particles do in order to react
collide with sufficient energy (activation energy) and the correct orientation
do most collisions result in a reaction
no
what is a catalyst
a substance which increases the rate of reaction but it is not used up in the reaction
how do catalysts work and how do they increase the rate of reaction
they provide an alternative reaction pathway (with lower activation energy)
lowers activation energy so more particles have energy greater than activation energy, so more frequent successful collisons, so increased rate of reaction
what is the rate of reaction
change in conc per time
how could you measure the rate of reaciton experimentally (different methods)
- use colorimeter at suitable intervals if there is a colour change
if gas is evolved use 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 flash of light to break bonds
- probe flashes to record amount of light absorbed by a species –> shows its conc
- first used for ClO2 –> ClO. + O. (dot means free radical)
- can now monitor reactions that occur in times as fast as 10-12 seconds
what affects the value of the rate constant
only temp
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 equations eg. catalysts, may be included
what must you add to react with the I2 as it is produced for an iodine clock reaction? (equation)
known moles of sodium thiosulphate and a little starch ; reacts with I2 in 1:2 ratio
I2 + 2S2O32- –> S4O62- + 2I-
when does the starch turn blue blac colour in iodine clock reaciton and why
when all if the Na2S2O3 has been used up anad so I2 is produced which reacts with starch, lreading to a blue black colour
how to calculate rate of reaction from data
rate = 1/t (t is time)
what is the rate determining step
the slowest step in a reaction mechanism, which determins 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 stp do not appear in the rate expression
