how fast? Flashcards
define the rate of reaction
a change in concentration of a reactant or product per unit of time
at a given instant, how could you calculate the rate of reaction?
change in concentration of reactant or product / time
if the order is zero with respect to a reactant, what does that mean?
changing the concentration of the reactant has no effect on the rate
what is the effect on rate in a first-order reaction?
rate is directionally proportional to the concentration
what is the effect on rate in a second-order reaction?
change in rate = change in concentration squared
what is a generic rate equation
Rate of reaction = k[A]^m[B]
what do square brackets mean in chemistry?
concentration
do zero-order reactants appear on the rate equation? why?
no, because they don’t have an effect on the rate
how is overall order of a reaction calculated?
sum of individual orders
how would you calculate the units of the rate constant?
units of rate are mol dm^-3 s^-1
units of concentration are mol dm^-3
k = initial rate / initial conc
sub in units and cancel them out
when the overall order is 3, what would be the units for the rate constant?
dm^6 mol^-2 s^-1
what are the different ways you could measure the rate of reaction experimentally?
- use a colourimeter at suitable intervals if there is a colour change
- if gas is evolved, use a gas syringe to collect the volume of gas evolved
- measure the change in mass of the reaction mixture or the mass of precipitate formed
- measure change in pH
define the rate-determining step
the slowest multi-step in a mechanism that determines what order a reaction is
what does half-life mean?
the time taken for concentration of a reactant to decrease by half
what is the symbol for half life?
t 1/2
what is the relationship between first-order reactions and half-life?
first-order reactants have constant half lives
what is the equation that is used to determine rate constant using half-life in a first-order reaction?
k = ln2 / t 1/2
how would you draw a rate concentration graph?
- plot [A] against time
- draw tangents at different values
what is the relationship between rate and time?
rate is proportional to 1/t
how do you determine the rate constant from a rate concentration graph of first order?
k = rate / concentration
how does the rate-determining step link to the species involved in the rate equation
any species involved in the rds appear in the rate equation
species only involved after rds do not appear in rate equation
for a reactant in the rate equation, what indicates how many molecules of that reactant are involved in the rate-determining step?
the order of the reactant
if the rate equation shows that [A] is second order, how many molecules of A will be present in the rate-determining equation?
2
what affects the value of the rate constant for a given reaction?
temperature, nothing else
what is the effect of a 10C temperature increase on the rate of reaction, roughly?
doubles the rate of reaction
what is the Arrhenius equation? what does each term mean?
k = Ae^ - Ea/RT
k - rate constant
A - pre-exponential factor (number of collisions between reactant molecules)
e - mathematical quantity
R - gas constant
T - the temperature in Kelvin
Ea - activation energy for reaction in Joules
how can you convert the Arrhenius equation into a useful form for plotting a graph?
ln k = -Ea/RT + ln A
