# Rates

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- 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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What does the gradient represent?

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...

rate- steeper the gradient

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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

against 1/t so Ea=gradient

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All steps added together

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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## Other cards in this set

### Card 2

#### Front

what is r

#### Back

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

### Card 3

#### Front

Square brackets?

#### Back

### Card 4

#### Front

What is k

#### Back

### Card 5

#### Front

m and n?

#### Back

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