3.1.9 rate equations Flashcards
what is a rate eq
the mathematical relationship between ROR and [reactants]
rate of reaction
change in [r]/[p] over time
general rate eq
units of conc and rate
rate of reaction = k[A]^x[B]^y
rate is units moldm^-3^s-1
k is the rate constant
x and y are orders of reaction with respect to that species
conc in moldm^-3
what is an order of reaction
what types are there
- the power to the concentration term
- how will the conc of a reactant affect the ROR
TYPES
0 - conc reactant has no effect
1 - conc reactant is directly proportional to rate
2 - rate is directly proportional to (conc reactant)^2. (y=x^2 where y is rate and x is conc)
how do you determine the order of reaction
MUST be determined experimentally. cannot be determined by looking at equation.
initial rates method
measuring the rate at beginning of reaction. conc reactants are known and unaffected
you can figure out the initial rate of any experiment with a conc-time graph by taking a tangent at t=0.
how can experimental results give insight onto the order of reaction with respect to specific species (table)
theyll give you a table of repeats of the same exp with reactant concs changed and the rate for each exp. this is a presentation of initial rates data where concs differ
if a is doubled and rate doubles then a is 1st (same for halved)
if a is doubled and rate increases by x4, this is the same as x2^2 therefore a is 2nd order
if a changes at all and rate is unaffected then 0 order
graphs of experimental results can also show us the order of reaction with respect for that thing being changed
how can a conc-rate graph of experimental results (where the conc was changed for same reaction (x) and plotted against rate)
CONC(x) RATE(y)
the graph line
0 - horizontal, remains at constant rate when conc changes. therefore no effect
1 - diagonal straight line through origin. as conc increases rate increases in a linear relationship
2 - quadratic graph, y=x^2, conc reactant cause rate to increase exponentially as it increases
graphs of experimental results can also show us the order of reaction with respect for that thing being changed
how can a conc-time graph of experimental results (where the conc was determined as changed for same experiment (y) and plotted against time (x))
CONC(y) TIME (x)
by the shape of the graph also, but will be different to conc rate graphs
0 order - diagonal line going down. conc decreases at a constant rate over time
- rate=k for this because if [a]^0 thats just 1
- rate is also conc/time therefore rate=gradient=k
1 order - conc decreases over time in a curve. conc decreases over time so rate decreases over time. because conc affects the rate
2 order - ditto but effect is more acute. steeper shallow curve, can be inferred that rate slows quicker and stops as shown by the time (x)
whats special about the gradient in the conc time graph for 0 order
(0 order - diagonal line going down. conc decreases at a constant rate over time)
- rate=k for this because if [a]^0 thats just 1
- rate is also conc/time therefore rate=gradient=k
in the generic equation what is k
the rate constant - it remains fixed at a constant temp and so is independent of conc or time.
it has variable units which have to be calculated like Kp of Kc.
however it is affect by temp. it will increase as temp increases
what is the eq that exemplfiies how k changes with temp?
the arrhenius equation. it shows the relationship between k, Ea and temp
k= A e ^(Ea/RT)
OR
(ln)k = (ln)A - (Ea/RT)
k - rate constant, units to be determined
A - arrhenius constant, given, same units as k
e - button on calc
Ea - activation energy, Jmol^-1
R - gas constant 8.31 Jmol^-1K^-1 (J/mol/K)
T - temp, Kelvin
how can you use arrhenius to do a graph
what can the graph show you
DO AN EXP WHERE TEMP IS IND VARIABLE. THEREFORE RATE WILL BE DEP VARIABLE. HAVE THE DIFFERENT TEMPS DID IT AT CORRESPONDING TO DIFF RATES GOTTEN. DO lnRATE BY GETTING 1/time taken for reaction to occur)
lnk on y and 1/T (so K^-1) on x
- intercept is lnA
- gradient (will be linear because rate is affected by temperature in that way ig) is equal to -Ea/R
- so Ea = -gradient x R
OR CAN HAVE
lnrate on y and 1/T
- rate is proportional to k because ror increases with temp and so does k
- this will always be -ve slope (?)
why does temperature affect rate constant?
when temp increases
- more frequent collision due to increased kinetic energy so more successful collisions
- more molecules have activation energy so more successful collisions
therefore increased rate of reaction
so rate constant must increase because the conc reactants don’t change, and if you look at gen eq: rate=k[][] and rate is going up, for it to remain equal k must go up
arrhenius eq shows that the rate constant increases exponentially w temp with the term: e ^(Ea/RT)
what is a reaction mechanism
series of steps through which reaction progresses, forming intermediate compounds
what is the rate determining step
each step in mechanism has diff ror
slowest step determines rate so therefore rds
with the rate eq of a reaction and the mechanism of it, how can you determine the RDS
the molecularity of the slow step will correspond to reactants in rate eq. eg if a is 2nd order there will be two molecules of a in RDS (or before that step????)
species in ratio/proportion as in the rate eq
explain:
- by the end of [chosen step]
- [order] [species] and [order] [species] have been used up
basc to say that by the time RDS is gotten to, the molar quantities in the chem eq has been used up. the molar quantities of chem eq of RDS is the same as orders of reaction in rate eq
what is continuous monitoring
following the change in conc over time.
exs of continuous rate method;
- gas produced
- recording mass lost
the graph will be;
- conc on y, time on x. whether it goes up or down depends on if following conc r (down) or p (up)
- can take a tangent at certain time to determine rate at that time
how is the iodine clock an ex of an inital rates method
takes a certain time to react at a fixed conc (conc remaining of excess I2???)
