# Rates

• Created by: Hindleyc
• Created on: 26-11-18 19:34
What does the rate equation do?
links mathematically the rate of reaction to concentration of reactants
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what is r
symbol for rate- unit=mol/dm^3/s
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Square brackets?
Means concentration in unit mol/dm^3
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What is k
rate constant
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m and n?
reaction orders- usually 0,1,2
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equation
r=k[A]^m[B]^n
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0
reaction order 0 with respect to that reactant
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What does this mean?
As conc inc the rate remains the same- change in conc has no effect on the rate
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1
first order- the rate is directly proportional to the concentration eg x2 rate x2 conc
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2
second order- the rate is proportional to the square of the concentration eg if we double conc the rate inc by 2^2=4
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How is total order for a reaction worked out?
adding all the individual orders together (m+n)
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What does each species in a rxn have?
Its own order
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What does the order of reaction with respect to a particular reactant tell us?
How changing the concentration of that reactant changes the rate of reaction
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What does the units of k depend on and how must it be worked out?
overall order of reaction. from the rate equation
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What is the value of k independent from
concentration and time- it is constant at a fixed temperature
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what does the value of k refer to? what does it increase with?
A specific temperature and it increases if we increase temperature
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why?
more molecules have energy greater than or equal to activation energy therefore more frequent and successful collisions
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k=
rate/conc [ ]
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Whats a continuous rate method?
when we follow 1 experiment over time recording the change In concentrations
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rate of reaction
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When is reaction fastest?
at start where the gradient is steepest
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When does the rate drop?
As reactants start to get sued up and their concentration drops
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What will eventually happen to graph?
Become horizontal and the gradient become 0 which represents the reaction having stopped
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When can you measure change in volume of a gas
if there is a change in no. moles of gas in a reaction
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What is a common way of following this?
gas syringe
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When does it work well? problems?
when measuring continuous rate-but typical gas syringe only measures 100ml of gas so you don't want a reaction to produce more than this volume
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What needs to calculated carefully?
quantities of reactants
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What is the initial rate?
rate at start of reaction where it is fastest
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how can it be calculated ?
gradient of a continuous monitoring conc vs times graph at time=0
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Why is initial rates preferred
we know the conc at start of the reaction
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What happens in a reaction where there are several reactants and the conc of one reactants is kept in a large excess?
then that reactant will appear not to affect rate and will be pseudo - 0 order
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why
its conc stays virtually and doesn't affect rate
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The higher the conc/ temp/ SA the faster the...
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if the Mg is in excess of the acid, final volume of gas produced will be proportional to
the amount of moles of acid
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What will different volumes of the same initial concentration have the same of?
initial rate (if other conditions are the same but will end at diff amounts)
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What do you need calc/compare why
initial moles of reactants to distinguish b/w diff finishing volumes eg amount of product proportional to moles of reactant
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how can initial rate be measured?
calculated from taking the gradient of a continuous monitoring conc v time graph at time = zero
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How else can be calc using clock reactions
time taken to reach fixed conc is measured - often 2 successive reactions and end point achieved when 1 limited reactant runs out resulting in a sudden colour change
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How can you use this to determine order of reactant with respect to that reactant
by repeating experiments several times varying conc of a reactant keeping other constant conc
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How can you work out orders from experimental initial rate data?
do series of experiments where initial conc of reactant changed one at a time and measure initial rate at each time
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How do you work out rate order graphically
When conc of one reagent is changed and reaction rate measured
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rate=
k[y]^n
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comparing logged to y=mx+c what is m, y intercept
order n, log k
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What will have biggest percentage errors?
experiment with high conc and quick time
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what do you do to calc orders when 2 reactants are changed simultaneously
effect of both individual changes on conc are multiplied together to give overall change on rate
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Rate=
change in conc/time
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Calculating k?
Using initial rates data put values of any experiment into rate eqn rearrange to give k
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What is k the same for?
all experiments done at same temp
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as temp inc
k rate constant inc
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half life?
time for conc of reactant to half - 1st order= constant half life
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What can initial rate of reaction be represented as?
1/t
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For 0 order reactions what does the rate stay? why
constant as reactant used up meaning conc of reactant has no effect on rate
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so rate =
k
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what does this mean gradient is
rate constant
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Arrhenius eqn
k=Ae^-Ea/RT
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Ea/RT show>
proportion of molecules able to react
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A, R , Ea?
constant, gas constant , activation energy
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lnk=
constant - Ea/RT
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what is k proportional to?
ROR- ln k replaced by ln rate
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how to plot graph of ln rate
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overall eqn
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which step controls overall ROR
slowest step
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what is this known as
rate determining step
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what is the number of moles of each substance in slowest step the same as?
order for each substance
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what does rate eqn give us
info on rate determining step
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what does order of reagent tell us
how many molecules in RDS
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When is it multistep?
Any of reactants in balanced eqn not in rate (any 0 order). more than 2 reactant molecules
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steps
1. slow step using rate, 2. anything not in balanced step, 3. any reactants not used
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any step after rate determining step..
won't affect rate
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What does s concentration v time show?
Change in concentration of a reaction as the reaction progresses
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How is a conc v time graph orientated
Conc on y axis and time on x axis
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What can be drawn on a conc v time graph?
Tangent to calculate rate
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What else can be used
Half life to confirm whether the reaction is first order or not- constant ?
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What does a rate v concentration graph show
Change in rate when concentration of a particular reactant is changed
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How is it orientated
Rate on y axis and conc on x axis
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What can shape be used for
To determine the order or reaction
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## Other cards in this set

### Card 2

what is r

#### Back

symbol for rate- unit=mol/dm^3/s

Square brackets?

What is k

m and n?