Rates Flashcards
what is rate of reaction ?
change in amount of product / reactant per time
Name 5 ways of measuring rate of reaction
- measure gas volume formed
- measure loss of mass due to gas
- complimentary to measure colour change
- measure pH change
- time for cross to disappear
how to find reaction rate on Conc-time graph
find gradient (tangent)
what are clock reactions for ?
calculating initial rate of a reaction
when 1st order …
rate increases proportionally to concentration
when 0 order …
rate is not affected by concentration
when 2nd order …
rate increases by a x^2 to concentration increase
how do you find the overall order of a reaction ?
add all of the orders of the reactants
what does [A] show ?
concentration of A
in rates experiments, why have the other reactants in excess ?
so that their concentration won’t change much, so change in rate is only due to reactant you are investigating
the bigger the rate constant (k) is …
the faster the reaction
on a 1st order rate-conc graph, k =
the gradient (graph is linear)
what is the half-life a reaction ?
time taken for half of the reactant to be used up
describe the half life for 1st order reactions
each half life will be the same length
what are the units for k from half life ?
s^-1
what is the equation for finding k from half life ?
k = ln2 / time of half life
what is the rate determining step ?
the slowest step in a multi-step reaction
what is the overall rate of a reaction decided by ?
rate determining step
what are the two rules for working out mechanism of chemical reaction from rate equation ?
- any reactant in rate equation affects the rate, so must be (or something derived from) in rate determining step
- if reactant isn’t in rate equation, then not in rate determining
the order of a reactant shows …
the number of molecules of that in the rate determining step
the rate constant will change at different …
temperatures
what is A in the Arrhenius equation ?
the pre-exponential factor (a constant)
what does R stand for in Arrhenius equation ?
gas constant (8.31)
what are the units for temperature in Arrhenius equation ?
kelvin
for the Arrhenius equation, how is k affected when activation energy increases ?
as Ea gets bigger, k gets smaller
ln k =
-Ea/RT + ln A
what is an Arrhenius plot ?
graph with ln k against 1/T
for the Arrhenius plot, what is the gradient ?
-Ea/R
for the Arrhenius plot, what is the y-intercept ?
A
give an example of a catalyst in a reaction
iron is used in Haber process to make ammonia
what is a heterogeneous catalyst ?
catalyst that is in a different phase from the reactants
what is a homogeneous catalyst ?
catalyst in the same state as the reactants
how to increase rate of reaction by changing heterogeneous catalyst ?
increase SA of catalyst as reaction occurs on surface of catalyst
how does a homogeneous catalyst work ?
forms an intermediate species
why is catalyst good for Haber process ?
temp would have to be very high without, so saves money and energy
how is poly(ethene) different when made with/without a catalyst ?
more dense, more rigid and higher melting point when made with catalyst
Give an example of homogeneous catalyst
Catalysis of Ozone breakdown
Heterogeneous catalyst example
Catalytic converter
Any proposed mechanism must be consistent with :
Overall rate equation (correct reactants in rate determining step)
Overall equation for reactant
How collisions are there in a single two of a reaction ?
No more that 2 usually
A in arrhenius is related to what ?
The number of collisions per second
What is e^-Ea/RT related to ?
The proportion of collision which are successful
