Ch 18 rates Flashcards
what is the unit for rate in chemistry?
moldm-3s-1
overal order is…
sum of the of orders with respect to each reactant
rate equation:
rate=k[A]^0,1,2 [B]
define order and the differnce between 0,1,2
The order with respect to a reactant is the power to which the concentration of a
reactant is raised in the rate equation.
zero order has no effect on rate
first order any change in conc has same change to rate
second order rate is proportional to concentration to power of two
unit for the rate constant is
calculated when you make k the subject and then sub in all the units to there resepctive orders and simplify by canceling down.
how and why do we undergo continuous monitoring
- monitoring gas collection
- monitoring mass loss
why: concentration time graphs can be plotted from continuous measurments taken
what are the methods to obtain order of a reaction from a concentration time graph
shape
zero:downwards striaght line (no change in rate)
first: the shape of the bottom half of a t (t for time) decreasing gradient
or by half life
to verify first order you can measure sucssesive half lifes if they are the same its first order with respect to the reactant, expoential deacay
how do we detrmin k from conc time graph?
if zero order: gradient =k
for first order:
* tangent to detrmin the gradient wich is equal to the rate then sub in the conc and rearagne the rate equation to determin k
* use the exponential relationship of half life, k=ln2/half life (t). often more accurate than drawing a tangent
how to determin the rate equation from rate concentration graphs
either from a rate conc graph, where order is determined by shape
0-horizontal line gradinet 0 so rate =k
1-stright line through the origin gradient = k
2-upward curve with increasing gradient
or
from clock reaction: rate is proportional to 1/t , with dta from a range of concentrations and there initital rate, plotting this data will produce the shapes listed above
how accurate is the clock reaction
accuracy is dependent on time as long as less than 15% of the reaction has taken place the aproximation is reasonably accurate
The rate determing step def
The rate determining step is the slowest step in the reaction mechanism of a
multi-step reaction.
predicting reation mechanisms
- the rate equation only includes reacting species involved in the rate determmining step
- the orders in the reaction rate must match the number of species in the rate determiing step
The mechanism proposed for the reaction above takes place in several steps. Suggest
two reasons why it is unlikely that this reaction could take place in one step.
stoichiometry in rate equation does not match stoichiometry in overall equation
✓
Collision unlikely with more than 2 ions/species/particles ✓
IGNORE more reactants in overall equation
If number of species is stated, ALLOW 3–5 only (rate equation contains 5 ions)
DO NOT ALLOW negative ions would repel (there is a mixture of positive and negative ions)
IGNORE more than two reactants collide as not related to rate equation
how do you use the arelius equation to calculate Ea and A(frequency factor, pre exponential factor)
Ea is calculated using the gradient (of the graph lnk against 1/T) where gradient =Ea/R
intercept = ln A
what are the units for Ea in the arelius equaltion
Jmol-1
