Chapter 18 Rate Orders Flashcards
What is the equation for rate?
(Quantity reacted or produced) / time
Change in Concentration / Change in time
What do square brackets mean?
Concentration of
What is the relationship between rate and concentration?
Rate is proportional to concentration to the power of the order
What is order of a reactant?
The measure of impact of changing concentration on rate
What does zero, first, and second order mean in terms of concentration and rate?
Zero- changing concentration has no effect upon rate
First- the change in concentration is equally reflected in the rate
Second- the change in concentration squared is reflected in the rate
What is the general form for the rate equation?
rate= k[m]^a [n]^b
How do you find the overall order a reaction?
Add the powers of the reactants in the rate equation
How do you find the units of k?
Substitute into the rearranged equation for k, and cancel
How can rate orders be established? Where do catalysts fit in?
Only experimentally, not through the stoichiometric equation
Catalysts can appear in the rate equation
From experimental data, how can you determine the order of a reactant?
Pick two rows where only one concentration changes, and see how the change changes rate, determine order
ENSURE ONLY ONE CHANGES, OR USE PREVIOUS INFO
What are 3 methods for continuous monitoring?
With a gas- Measure the volume of gas collected or mass loss as gas escapes in a certain time
With acid/base/redox- Take samples at regular intervals and calculate conc.
With a colour change- colorimetry, take samples regularly and compare to calibration curve
What is the procedure for colourimetry? [6]
Prepare standard solutions of the coloured reactant. (This can be done by mixing 1 molar solution with water. e.g 0.8 sol + 0.2 water, 0.6 sol + 0.4 water. Conc= original conc x vol coloured / vol total )
Choose a colour complimentary to the coloured substance, opposite on the colour wheel, for the filter
Zero the colorimeter with water
Measure the absorption readings for the different concentration of the coloured substance and plot a calibration curve. Increased conc increased absorption
Carry out the reaction and take samples at regular intervals. Measure the absorption of these samples in the colorimeter.
Convert these readings to concentrations using the calibration curve and plot a concentration time graph. Which can be used to find order.
What is the shape for a zero order concentration-time graph? How can k be calculated?
Constant gradient downwards
Gradient= k
What is the shape for a first order concentration-time graph? How can k be calculated?
Reciprocal graph- half lives constant, calculate at least 3 half lives to verify
k= ln2/half life
Or draw a tangent at a point, calculate the gradient for rate, rearrange the rate equation and use the concentrations to calculate k
What is the different between first and second order concentration-time graphs?
Second order steeper at first and tails of more slowly
Half lives not constant for second order
What is half life and how can it be calculated?
The amount of time taken for the concentration of a reactant to half
Look the time elapsed between the start concentration and half of this concentration
Repeat three times to verify half life
What type of mathematical graph is first order concentration-time graphs?
Exponential decay
What is the shape for a zero order rate-concentration graph? How can k be calculated?
Horizontal line, does not go through the origin
Y intercept= k
What is the shape for a first order rate-concentration graph? How can k be calculated?
y=mx + c format straight line, through the origin upwards
Gradient= k
What is the shape for a second order rate-concentration graph? How can k be calculated?
Quadratic curve through the origin
Plot a graph of rate against concentration squared
k= gradient of this second graph
Or, use values from the graph to find k from rearranging the rate equation, assuming orders of others are known
Rate against Conc
Rate on y
Conc on X
What are the equations used in iodine clock reactions?
S2O82- + 2I- –> 2 SO4 2- +I2
I2 + 2 S2O3 –> 2I- + S4O6 2-
Describe how the reactions in the iodine clock practical lead to initial rate determined?
Iodide ions are colourless. Peroxydisulphate ions reacts with these ions to form iodine, orange brown, and 2 sulphate ions
The iodine produced is immediately used up by the small amount of Thiosulphate ions to produce Iodide ions which are colourless and tetrathionate ions.
When the thiosulphate ions run out, iodine is no longer used up, giving a sharp end point
Rate= 1/t
What is the procedure for initial rates reaction with iodine clocks?
Add 5cm3 iodide ions, 1cm3 of thiosulphate ions, and 1cm3 of starch to a conical flask. In a separate flask, add 5 cm3 of peroxydisulphate
Add the peroxydisulphate to the conical flask and immediately start timing
Stop the timer when the blue/black colour appears.
Repeat the experiment but use 4cm3 of peroxydisulphate and 1cm3 of water. Then 3 and 2. And so on, varying the concentrations of the ions.
Repeat the experiments with 5cm3 of Peroxydisulphate but varying amounts of Iodide ions, working with varying amounts of water as with the peroxydisulphate.
Work out the concentrations of the peroxydisulphate and Iodide by Bottle conc x vol of iodide / total vol of reactants
Calculate rate by 1/t
Plot a graph of rate against concentration for each of the reactants
How accurate are clock reactions and how can you make them more accurate?
If the initial rate is measured within 15% of the reaction progress, it is accurate as the change in concentration does not have a significant impact upon rate so similar to initial.
Use less thiosulphate ions or what is used to delay the colour change
What is a reaction mechanism?
The series of steps which make up an overall equation
Why do reactions proceed in multiple steps?
It is highly unlikely that multiple particles which have energy greater than the activation energy collide all at once with the correct orientation
What is the rate determining step?
The slowest step in a reaction mechanism
How can the rate equation help predict a mechanism?
Only the species in the rate equation appear in the rate determining step
The order of the species match the number of the species present in the rate determining step
What do successful mechanism predictions require?
The series of steps add and eliminate to produce the stoichiometric equation
The species in their molar quantities as in the rate equation appear in the rate determining step
Many mechanisms are possible if these steps are followed
Often, the first step will produce an unstable intermediate
Think about common products e.g water, and what products need to be formed at some point from the equation
How does temperature affect rate constants? In most reactions- specifics linking to k
As temperature increases, the proportion of particles energy greater than the activation energy increases, and the speed and so collision frequency increases (less important)
Usually, a 10 degrees increase doubles the rate, doubles k
What is the pre-exponential factor?
A
Takes into account frequency of collisions with the correct orientation
What is the exponential factor?
e^’Ea/RT (T in kelvin and Ea Joules)
Takes into account the proportion of molecules with energy greater than Ea
How can the arrhenius equation be plotted with reduction to linear form?
Plot a graph of lnk against 1/T
Gradient= Ea/RT
Y intercept= ln A (don’t use)
Tips for plotting the linear form of arrehnius?
Scales can include standard form or x10^-3 units, use them
Units and Titles for Axis, include with calculations
Don’t start from 0, but can’t use y intercept
How can you calculate A with an arrhenius graph?
Y intercept in, e to the power of it
But if graph not from 0, substitute in the gradient and rearrange for ln A or A
How can you calculate A with an arrhenius graph?
Y intercept in, e to the power of it
But if graph not from 0, substitute in the gradient and rearrange for ln A or A
CHECK UNITS IN TABLES, CAN INCLUDE X 10^-3 E.G
CHECK UNITS IN TABLES, CAN INCLUDE X 10^-3 E.G
What does the original arrhenius graph look like?
Exponential
What should the reduction arrhenius graph look like?
Negative gradient usually below the x axis, on either side of the y axis
Why should other reactant not being tested be kept well in excess?
To keep its concentration effectively constant, 0 order
Even when used up in the reactions, it so in abundance still that it wouldn’t affect the rate, only the species being tested
Why would absorbance decrease with the progress of a reaction?
The coloured reactant is being used up, meaning the colour will decrease so absorbance decreases
If given data, how can you convert from absorbance to concentration?
Initial absorbance, time 0 is the original concentration of the coloured compound
Initial conc= ( vol in reaction mix / vol of all reactions in mix) x bottle conc
From their, divide and multiple to various absorbances
e.g if at t=0, absorbance=80, want 20, divide by 4
How do you know a mechanism is consistent with the overall reaction?
The mechanism steps add and cancel to give the stoichiometric equation
The species in the rate determining step match the species in the rate equation, in molar quantities relating to the order
What are two pieces of evidence for reactions occurring in multiple steps?
The rate equation is not consistent with the stoichiometric equation
There is a very small chance that multiple particles will collide at once with the correct orientation and all having energy greater than the activation energy
What condition must be kept constant when comparing rates with varying concentrations?
Temperature
Why are you able to see how one reactants concentration alters the concentration of the products if more than one reactant is being used?
Reactant concentrations will decrease over the progress of the reaction as they are used up
This decreases the rate of the reaction
But, if the concentration of the other reactant is very much in excess, the decrease of the other reactant is virtually meaningless, as it is still so much higher than what it is needed
Other effectively constant throughout the experiment
Do first order conc time graphs start at 0?
Yes
Even though close to negative recipricol
When can the rate equation not mirror the RDS?
Intermediates are not part of the rate equation
If some of the reactants of the rate equation react to produce an intermediate, this intermediate can react with the other species in the rate equation
This final step of the intermediate + missing species from the rate equation can be considered the RDS
What are the units for activation energy in the arrhenius equation?
Joules
Important when calculating A
What is the relationship between time and the concentration of a reactant that is second order?
1/T directly proportional to [A]^2
If using continuous monitoring, what must be done to the samples before carrying out a titration for conc?
Stop the reaction
By cooling it or adding a substance which reacts with one of the reactants
To prevent the reaction still continuing and concentrations continuing to change
How do you determine the units of K from a graph?
If it is Zero order, compare back to rate equation
If purely at that one graph, then you need to rearrange that specific rate equation for K
How do you determine the units of K from a first order graph using half lives?
Normally it is s-1
As the graphs when these will be given will be continuous monitoring with 1 reactant
If this reactant is zero order, needs to Match K rate equation, same with 2nd
If multiple reactants are involved, this K value will not actually be right, and instead is a K value for when the other reactants are in such large excess they are Zero order
Using concentration is the only way you can calculate K here then and using rate
Why is the reaction between peroxydisulphate and Iodide slow? How does Fe2+ catalyse it?
Reaction between two negative ions which repel each other
So a very high activation energy
Fe2+ takes the electron from the iodide, and then gives it to the peroxydisulphate, as it has an electrode potential in between the two, and variable oxidation states
