1.8 rate equations and KP Flashcards
Practicals in this topic
7a- iodine clock
7b- measuring the progress of a reaction
what is continuous monitioring?
following a reaction all the way through by recording the amount of product or reactant that you have at regular time intervals
examples of continuous monitoring methods
gas volume
changes in pH
colour change
loss of mass
clock reactions (eg iodine clock)
how to find rate at a specific point on a curve
draw a tangent
what is the rate equation?
units?
rate= K [A]^m [B]^n
[ ] means concentration of
m and n are the orders of reactions
k is the rate constant
units are usually moldm-3s-1
what does the rate equation show?
how the rate is affected by the concentrations of reactants
if a reactant is in the rate eq, then it must affect the rate
what is 0 order?
if conc of reactant changes and rate stays the same, the order of reaction with respect to that reactant is 0
what is 1st order?
if rate is proportional to conc of a reactant then it is 1st order w.r.t that reactant
eg [A] doubles, rate doubles
[A] triples, rate triples
what is 2nd order?
if rate is proportional to conc of reactant ^2, then it is 2nd order w.r.t that reactant
eg [A] doubles, rate quadruples
[A] triples, rates x9
what is the overall order of reaction?
the orders of each reactant added up
(m+n)
what is k?
the rate constant- a constant that links conc of reactants to ror
the bigger it is, the faster the rate
when does k change?
why?
increase temp, k rises because increased temp means increased ror
conc of reactants hasnt changed so increased ror must be due to increased k
how do you calculate k?
using the rate equation (tells you orders) and experimental data
also need to calculate units (cancel out)
what is initial rate?
how do you find it?
rate at start of a reaction
using a conc-time graph made by using continuous monitoring method and plotting results
draw tangent at time=0 and find gradient
how can you use initial rate method to find order of reaction for each reactant?
- carry out reaction and monitor progress (dont have to monitor to completion as only working out initial rate)
- draw conc-time graph
- repeat using diff initial conc of one of reactants but keep other concs and conditions same
- draw another conc-time graph
- calculate initial rate
- look at how initial conc effects initial rate and use this to work out order
- repeat for each reactant
how to work out order when more than one concentration changes
example on page 56
and practice
how do you work out reaction orders from graphs?
make a rate-conc graph using a conc-time graph:
find grad at various points- plot this on new graph and join points
0 order= horizontal line
1st order= straight line through origin
2nd order= curved line
what is the rate determining step?
the step of a mechanism that decides the overall rate
how to work out which is the rate determining step
write out what the rate eq would be for each step (m and n are the number of molecules)
the one that matches up with the actual rate eq is the rate determining step
how can you work out the reaction mechanism?
by knowing which reactants are in the rate determining step (see page 63)
what does the arhenious equation show?
how the rate constant varies with temperature and activation energy
what is the arrhenius equation? (equation)
K= Ae ^ -Ea/RT
k= rate constant
Ea= activation energy (Jmol-1)
T= temperature
R= gas constant (8.31 JK-1mol-1)
A= arrhenius constant
e= button on calc
relationship between k and Ea
Ea gets bigger, k gets smaller
relationship between k and temp
temp rises, k increases
what form of arrhenius eq do you use to find K or A?
K= Ae ^ -Ea/RT
(should it say Ae?)
what form of arrhenius eq do you use to find T or Ea? (should it say Ae?)
Ln(K) = -Ea/RT + Ln(A)
(should it say Ae?)
what is an arrhenius plot?
A graph of K against 1/T
logarithmic form of arrhenius eq in form of y=mx+c
ln(k) = -Ea/R 1/T + ln(A)
ln(k)= y axis
-Ea/R= gradient
1/T= x axis
ln(A)= y intercept
(should it say Ae?)
how to gather data for an arrhenius plot
carry out same experiment a few times at diff temps and work out rate at same point for each one
what is partial pressure?
in a mixture of gases, each individual gas exerts its own pressure
what is the total pressure of a gas mixture?
sum of all partial pressures (all diff unless present in equal quantities)
what is a mole fraction?
the proportion of a gas mixture that is made up of a particular gas
what are the 2 formula involving mole fractions?
mole fraction of a gas in a mixture= no of moles of gas / total moles of gas in mixture
partial pressure of a gas in a mixture = mole fraction of gas x total pressure of the mixture
what is Kp?
equilibrium constant for a reversible reaction where some or all of the reactants and products are gases
what is the expression for Kp?
(for aA(g) + bB(g) -> dD(g) + eE(g))
Kp= (PD)d (PE)e / (PA)a (PB)b
what is le chatalier’s principle?
if a reversible reaction at equilibrium is subject to change, the equilibrium shifts to counteract the change
effect of temp on Kp
if shift in eq to counteract temp change causes more product to be formed, Kp increases
vice versa
effect of pressure on Kp
doesnt affect Kp
partial pressures of reactants and products at new eq position keep Kp constant
effect of catalyst on Kp
no effect
increases ror equally in both directions
why are samples of a reaction mixture added to NaHCO3 before they are titrated?
To stop the reaction
(NaHCO3 neutralises the acid)
why does doubling temp have a greater effect on rate that doubling conc?
Reaction occurs when molecules have E>Ea
Doubling T by 10 °C causes many more molecules to have this E
Whereas doubling [E] only doubles the number with this E
what is a reaction order
power to which a concentration is raised by in the rate equation
why does an excess of a reactant mean that it has 0 order/ can be ignored?
their concentration remains essentially constant/ any change is negligible so rate only depends on other reactant
how can a graph of vol against time show that a reactant has 0 order?
constant gradient
so ror doesnt change as conc changes
