Chapter 18 Rates Etc Flashcards
What is rate of reaction, the formula and basic units
Rate of reaction shows how quickly a product is being made or how quickly a reactant is being used up
= change in concentration / time
Moldm-3s-1
What are orders of reaction basically
Orders of reaction show how the OVERALL rate of reaction is affected with changes of reactant concentrations
So if you double [A] and overall rate x2, then order of reaction RESPECT TO A is [A] ^1
What happens with 0 order
And why
If a reaction is 0 order with respect to a reactant, then the conc of reactant has NO EFFECT ON THE RATE OF REACTION
- anything to power 0 = 1
Rate is proportional to [A]^0
What happens with 1st order
Rate is proportional [A]^1
Means If conc doubles , rate inc by 2^1=2, so the same
What happens with second order
Rate is proportional to [A]^2
So if [A] x2, rate x2^2 =4, so it quadruples!
What is the RATE EQUATION THEN
What is K?
Rate = K [A]^m X [B]^n
K = RATE constant
What is overall order of equation?
Add all the powers together in the rate equation = overal order
Okay so how do you find the orders of reaction based on data they give you
WHAT KEY WORDING NEED TO Use
- isolate a constant reactant to see effect of changing reactant alone with conc
- say in/ comparing experiment 1 and 2 etc , for [y] and [z] being CONSTANT, the [X] doubles and the rate too, etc
- this means the order of reactions is 1 WITH RESPECT TO [X]
- keep doing that
WHAT KEY WORDING NEED TO Use when determining orders
- look at experiment x and y
- whilst conc one stays constant
- others DOUBLE TRIPLE
- so First second zero order WITH RESPECT TO etc
How to find units of rate constant?
Rearrange equation for k
Divide through l
Rate units remember = mol dm-3s-1
Why should we plot a conc time graph - what can we do
Conc time graph = can find gradients at points and plot a rate time graph = clear cut image of the order of the reaction with respect to that reactant
OR can find half life if it’s a FIRST order reactions and use this to find the k constant
Can basically tell what order of reaction it is
How to tell from coco time graph the order of reaction with respect to the reactsnt being explored
- if it is straight= 0 reaction, as the gradient is constant, and rate time graph is just the gradient
- if it is curved = 1 , gradient constantly decreasing
If it is even more steeply curved = 2 gradient changing by different amounts at exponential rate, think about it this is an x3 graph differentiate you get x2 that’s why on rate graph it’s curved too!
Assuming the reaction only has one reactant, why does a straight line in the concentration time graph gradient = k
Basically one reactant only, and straight line means constant gradient = rate, so so rate stays the same and the order is 0,
as rate = K [A]^0 now, it just = k, which si the rate which is just the gradient
The others are harder to find
Three different methods for continuous monitoring of concentrations to get data to plot conc time graph?
- monitoring gas collection (upturned beaker finding volume has in time)
- monitoring mass lass /time
- monitoring conc using colorimeter
How use corlimeter to find conc
1) identify what reactant is causing colour change
2) get standard solutions if know cones if this, use it agains the colrimeter with opposite wavelength and zero with distilled water, and find values for absorbance based off different concs
3) plot calibration curve of abrosbance/ conc
4) now do actual experiment and at set time intervals measure absorbances
5) now you can use calibration curve you plotted to find the concentration at different times,a no can plot a concentration graph
Now you can identify the order by looking or using half life or plotting a rate time graph !
What it’s t ^1/2
How can this be used , what is it known as if it is constant
The half life of a reaction is the time taken for the concentration to half and be used up
If this is known to be constant then it is undergoing EXPONENTIAL DECAY and can be used in formula to find k constant. If it is constant the the order is 1st order
What happens if the half life is constant (order ?)
This means the order with respect to reactant is 1ST
How can you find K CONSTANT using conc time graph if it’s first Oder
You know first order because it’s constant half life
Now you can just find rare at a point and re arrange for k, but INACCURATE
Instead use this formula —> k = ln2/ t^1/2
Wha this equation of k using constant half life?
K = lm2/t^1/2
What effect does TEMPERATURE have in the rate equation (what changes, constant order etc)
If temperature increases the RATE CONSTANT changes
However the ORDERS ALWAYS STAY THE SAME , only rate constant can change!
What shapes of lines are the orders on a RATE CONCENTRATION GRAPH
How is this formed
Rate concentration graph formed when using values of gradient for rate in concentration time graph AGAINST CONC AT THAT POINT
- if straight horizontal = 0 order
- if straight line like y= mx= 1st order
- if curved like y= x2 then 2nd order
How to find rate constants k using rate CONC graph And why
IMPORTANT
- 0 order = as rate = k [A]^0 rate = k so k just the value in y axis
- 1st order = as rate = k[A]^1, k = rate / [A] so K = VALUE OF GRADIENT
- 2nd order = as rate = K[A]^2, K = rate /[A]^2, so need to draw graph if RATE / CONC SQAURED, now it’s jut the GRADIENT OF THIS GRAPH
Sidenote but how can you find order and rate constant using log rate / log conc ?
Rate = k[A]^n
Log rate = nlog[A] +log k
Y = Mx +c
C = log k = rate constanr
M= n = order
What is an iodine clock used for
Iodine clocks are used to find the initial rate of reaction without drawing up graphs each time and using gradient tangent at t=0.
This allows you to easily compare effect of changing conc of one reactant and identifying effect on conc to determine ORDER AND THUS RATE CONSTANT
So what happens innthe iodine thisosulfate clock theory
In a mixture you have : KI (iodine ions ), H2so4 (H+ ions) H202, starch, thiolduftse ions with sodium and water
- first reaction is H202 + 2I - + 2H+ = I2 basically
- second is I2 and thiosulfate mixes to go back to 2I-
What’s happening is that the First reaction is SLOW but second is fast, thus any I2 made is instantly converted back to Ions before it can react with starch .
This continues to happen until all the THIOSULFATE IS USED UP, now no more can be converted and it becomes I2 WHICH REACTS WITH STARCH AND BECOMES blue black . At this point stop reaction time
What you then do is dilute the KI or whatever to do experiment at different concs , and get different times
The. You either actually find initial rates (through assumptions) by finding out amount of I2 produced per second by working back and seeing how much thiosfulate is used up in moles and divide thst by time and do for different concs, OR USE 1/T WHICH IS PROPORTIONAL TO RATE AND GIVES SAME IDEA
- then draw graph of 1/t as rate against conc, and based on the shape you can find ORDERS
HOwevee can only find K CONSTANT by doing the longer INITAL RATE METHOD!
Why use thiosukfate to delay?
I guess so that results can actually be measured or else they would happen to fast and also as a way to find out exact moles etc by working backwards
What are assumptions
IMPORTANT
That the rate you found is is basiacally the initial rate as you assuming only 15% of the reaction has happened, if more happens then you CANT ASSUME THE RATE IS BASICALLY CONSTANT FOR THIS TIME
What do you need to know actually for course for iodine clock
1/t draw graph find out orders
Longer version with initial rate using simple rate = conc / clock time to find orders and rate constant !
18.5
Recap = how does rate of reaction increase with temperature and why
What is the main factor
As a result what is increasing in the rate equation
Increases with increase of temperature
2) increase of temperature increases the proportion of particles with required Ea shown on Boltzmann which means more can react
- also increases frequent if collisions leading to more succesful collisions
- however main factor = ACTIVATION ENGERY IS MAINLY SURPASSED
3) the rate constant increases with temperature exponentially , orders never change !
Thus what is the exponential equation between rate constant and temperature then
K = A e ^-Ea/RT
A = pre exponential frequency factor (which barely contributes ) -Ea/RT = exponential factor showing activation energy plays such a big role and temperature
A is just how K would be say there was 0 activation energy needed
Okay what is Relationship between k constant and temp then
As temp increase, k increases exponentially , the Arrhenius exponential
Equation for Arrhenius exponential log form
What to find other variables b
Ln K = -Ea/R x 1/T + LN A
Graph of two variables lnK against 1/T
Y intercept = LnA
Gradient X R = Ea
Finally what to remmeber about are Haus equation like effect of k if I increase t or ea
Normally increasing ea will increase k, but because it’s -ea, it will decrease
Similar increase t should decrease as its 1/t , but now it’s - so it increases
18.4
Why do most reactions actually happen in MULTI STEP reaction pathway ?
- a reaction requires all molecules to collide, collide at correct orientation and with required Ea
- It is VERY UNLIKELY THAT REACTION WITH MORE THAN 2 MOLECULES can collide at the same time
- thus reactions overall take place in multi pathway reactions instead
What is the rate determining step then?
In a multi pathway reaction there are steps that have different speeds
The SLOWEST STEP IN THIS IS THE RATE DETERMINING STEP (kina like limiting factor, faster this the faster whole rate)
What are some rules when trying to make a reaction pathway for rate determining step etc
IMPORTANT
1) the reactants In the OVERAL RATE EQUATION are the only reactants to appear in the RATE DETERMINING SLOW STEP
2) the overall order of rate equation is = to the amount of molecules of reactants in rate determining steps
- so if 2 molecules of k and 1 of z , then it will be atleast k + k + z in the rate determining step
3) typically if made in step 1 then used in step 2,
4) INTERMEDIATES SHOULD CANCEL OUT ON BOTH SIDES such that what is left matvhes the OVERALL ORIGINAL EQUATION !
Again why does the rate determining step feature only the reactants in overall rate equation
As rate determining step controls the rate , only the substances in rate equation appear here= governs the rate!
This because substances that don’t appear in the rate equation have no effect on the rate of reaction at all, so they don’t determine the rate, thus they cant be in the rate determining step
Why can sometimes the order of something be effectively 0
If it is IN EXCESS/ or if it stays the same
Why are TWO reasons to why a reaction probably don’t happens in one step
Importsnt
Not likely reaction more than 2 things can happen at once
- IF THE STROICHE,TRY IN THE RWTE EQUATION FOESNT MATH. UP , it is unlikely that all the reactants react at once as a result!!
WHY CAN YOU NOT DETERMINE THE ORDER ?
Don’t know
- either too excess so varying conc is hard
- of reaction does not produce anything in the form that continuous monitoring is possible
