Chapter 18 Flashcards
Reaction rates
Changes in the quantities of reactants or products over time
How can rate of reaction be calculated (simple)
Rate = (quantity reacted or produced)/time
BUT FOR CONSISTENCY
Rate = change in concentration/change in time
If time is measured in seconds, what are the units of rate?
mol dm^-3 s^-1
What is the shorthand way of writing the concentration of A?
[A] ; units of concentration mol dm^-3
What does changing the concentration do?
Changes the rate of a reaction ; rate of a reaction is proportional to the concentration of a particular reactant raised to a power
rate (is directly proportional to) [A]^n
In the proportional rate equation, what does the power symbolise?
It is the order of the reaction for that reactant
In a reaction, different reactants can…
Have different orders and each may affect the rate in different ways ; common orders are zero order, first order and second order
What is zero order?
When the concentration of a reaction has no effect on the rate, the reaction is zero order with respect to the reactant
Rate (is directly proportional to) [A]^0 (which is equal to 1)
Concentration does not influence the rate
First order?
This is when the rate depends on the reactant’s concentration raised to a power of 1
If the concentration of A is doubled, so does the reaction rate (directly proportional to each other)
Second order?
Rate depends on the reactant’s concentration raised to the power of two
Rate is proportional to [A]^2
If the concentration is doubled then the reaction rate increases by a factor of 2^2 = 4
Same thing when tripled then it will increase by a factor of 3^2 = 9
Rate equation
Gives the mathematical relationship between concentrations of the reactants and reaction rate Rate = k[A]^n[B]^m where k = rate constant n= order of reaction with respect to A m = order of reaction with respect to B k = proportionality constant
Overall order of reaction
Overall effect of the concentrations of all reactants on the rate of reaction
Overall order = sum of orders with respect to each reactant (m + n)
Zero order reactants?
They are usually omitted from the rate equation
Units of rate constant
Rearrange the equation to make k the subject and cancel out common units
How are orders of reactions determined?
Must be determined experimentally by monitoring how a physical quantity changes over time ; they cannot be found directly from the chemical equation
It is important that the rate is always measured after the same time ; ideally as close to the start of the experiment as possible - initial rate (instantaneous rate at t=0)
How to investigate the rate equation?
Compare concentrations of reactants ; if they double and the rate also doubles then first order etc
Then sun in the other values (concentrations and initial rate) to find k
How are concentration time graphs plotted?
From continuous measurements taken during the course of a reaction ; continuous monitoring
Continuous monitoring of reactions that produce a gas?
Monitoring by gas collection
Monitoring by mass loss
Not all reactions produce gases?
Another useful property is therefore colour change ; monitored using a colorimeter or estimated by the eye
How does a colorimeter work?
Wavelength of light passing through a coloured solution is controlled using a filter ; amount of light absorbed by a solution is measured
How is filter of colour chosen?
COMPLEMENTARY COLOUR TO THE COLOUR BEING ABSORBED IN THE REACTION
How would you carry out a colorimeter practical?
Get standard solutions of iodine and measure absorbency readings of these standard solutions - now you have w calibration curve plotted
When measuring, you must first select the complementary filter and zero the colorimeter with water (standardise)
Now actually carry out the reaction and take absorbency readings of the reacting mixture at regular intervals
Use calibration curve to measure concentration of iodine at each of these intervals
Plot a second graph of concentration against time
Gradient of concentration time graph
= rate of reaction
How can the order with respect to a reactant be deduced?
From the shape of a concentration-time graph for zero and first order reactions
When can order with respect to a reactant be obtained?
Only if all other reactant concentrations remain effectively constant
Zero order
A zero order reaction produces a straight line with a negative gradient ; reaction rate does not change at all during the course of the reaction and the value of the gradient is equal to the rate constant k (straight line makes a zero order relationship easy to identify)
First order concentration-time graph
Produces a downward curve with a decreasing gradient over time ; as the gradient decreases with time, the reaction gradually slows down
The time for the concentration of the reactant to halve IS CONSTANT ; this time is called the half-life and the rate constant of a first order reaction can be determined using its value
Second order concentration time graph
Not required but also a downward curve ; steeper at the start but tailing off more slowly
Half life
Time taken for the concentration of a reactant to decrease to half its original value
t (subscript - at the bottom) 1/2
First order reactions half-life?
Exponential decay with the concentration halving every half life ; a first order relationship can be confirmed from a concentration-time graph by measuring successive half-lives ; if they are the same then the reaction is first order
What is [A] (subscript - at the bottom) 0
Initial concentration of A at t=0
What does a tangent on concentration time graph give?
The rate of reaction at a particular concentration
How to determine rate constant from a concentration time graph for a first order reaction?
1) Calculate rate constant from the rate ; a tangent to the curve is drawn at a particular concentration - gradient is calculated to give the rate of reaction. Rearranged the rate equation and now sub the value of the rate (gradient) and concentration at point calculated to get k (remember units)
2) For a first order reaction ; make use of the exponential decay
k = ln2/half life
MUCH MORE ACCURATE THAN DRAWING TANGENT TOO
Rate concentration graphs?
Can be plotted from measurements of rate of reaction at different concentrations ; they offer a route into the direct link between rate and concentration in the rate equation
Zero order rate-concentration graph?
A horizontal straight-line with zero gradient
rate = k[A]^0
Thus the intercept on the y-axis gives the rate constant k
Reaction rate does not change with increasing concentration
First order reaction?
A first order reactant produces a straight line graph through the origin
Rate = k[A]^1 so rate =k[A]
Rate is directly proportional to concentration for a first order relationship
Rate constant = gradient of straight line of this graph
Second order
Second order reactant produces an upward curve with increasing gradient
rate = k[A]^2
BY PLOTTING A SECOND GRAOH OF RATE AGAINST CONCENTRATION SQUARED, THE RESULT IS A STRAIGHT LINE THROUGH THE ORIGIN AND THUS THE GRADIENT OF THIS GRAPH = k
Initial rate
Instantaneous rate at the start of a reaction when t=0 ; can be found by measuring the gradient of a tangent drawn at t=0 ON A CONCENTRATION-TIME GRAPH
Clock reaction?
More convenient way of obtaining the initial rate of a reaction by taking a single measurement ; time from the start of an experiment is measured for a visual change to be observed (often a colour or precipitate)
What are we assuming with clock reactions?
That there is no significant change in rate during this time ; average rate of reaction over this time will be the same as the initial rate
Initial rate = 1/t
Clock reaction is repeated several times with different concentrations and values of 1/t are calculated for each experimental run
Iodine clock?
A type of clock reaction relies on the formation of iodine ; as aqueous iodine is coloured orange brown, the time from the start of the reaction and the appearance of the iodine colour can be measured - starch is added to form a complex with iodine which is an intense dark blue-black colour
Iodine clock - how does it work?
Separate experiments are carried out using different concentrations of one of the reactants and all other concentrations are kept constant
COLOIR CHANGE IS DELAYED BY INCLUDING A SMALL AMOUNT OF SODIUM THIOSULFATE WHICH REMOVES IODINE AS IT IS FORMED (IODINE IN EXCESS and as soon as thiosulfate ions used up, blue black colour appears)
Solution is colourless to start and the time t is measured for the blue black colour of the starch-iodine to appear
Initial rate is proportional to 1/t
GRAPH of 1/t (rate) against concentration is then plotted and the shape is matched
More experiments are carried out in which the concentration of one of the other reactants is changed - from rate concentration graphs, order with respect to each reactant can be calculated
How accurate are clock reactions?
You are measuring the average rate during the first part of the reaction - can assume that the average rate of reaction = initial rate
Shorter the period of time over which average is calculated, the less the rate changes over that time period
CAN BE EXEMPLIFIED THROUGH DELAYS IN TIME ON CONCENTRATION-TIME GRAPHS ; RATE FLUCTUATES MORE FROM THE INITIAL RATE THE LONGER TIME IS
Initial rate measure during a clock reaction?
Is an approximation but is still reasonably accurate provided that less than 15% of the reaction has taken place ; can see that the longer time is, more than 50% of the reactant has been used up soooo not goos
Stoichometry
Given by balancing numbers ; relative amounts of the species in the reaction - overall chemical equation compares the reactants and products
When do multi step reactions come into play?
When there are more than 2 mol on the reaction side (more than 2 species), it is very unlikely for molecules to collide with the specific orientation and correct activation energy for a reaction to take places
So more likely to take place in a series of steps ; this is the reaction mechanism
Slowest step in a reaction mechanism?
Rate determining step
How to predict reaction mechanisms?
Rate equation only includes reacting species involved in the rate-determining step
The orders in the rate equation match the number of species involved in the rate-determining step
Rate determining step provides important evidence in supporting or rejecting a proposed reaction mechanism
What is an intermediate?
The species that is formed in the first step and used up in the second (fast) step - not in overall equation
Example of a reaction that can b investigated experimentally?
Hydrolysis of haloalkanes by hot aqueous alkali
(CH3)3CBr -> (CH3)3C+ + Br- (rate determining as only (CH3)3CBr involved in rate equation)
(CH3)3C+ + OH- -> (CH3)3COH
As temperature increases?
The rate increases and the value of the rate constant k will also increase ; for many reactions, each 10 degrees rise in temperature doubles the rate constant and doubles the rate of the reaction
Typical variation of rate constant k (y axis) and T
Pg 289 ; exponential increase
What two factors contribute to increased rate and rate constant?
1) Increasing the temperature shifts the Boltzmann distribution to the right, increasing the proportion of particles that exceed Ea
2) As temperature increases, particles move faster and collide more frequently
Increased frequency of collisions is comparatively small compared with the increase in the proportion of molecules that exceed Ea from the shift in the Boltzmann distribution
How can rate constants be determined?
Experimentally at different temperatures
Arrhenius equation
k (rate constant) = A e^-Ea/RT
where A = pre-exponential factor (frequency factor)
R = gas constant of 8.314 J/mol/K
T = temperature in KELVIN
everything except A is the exponential factor (linked to activation energy and temperature)
Exponential factor?
Represents the proportion of molecules that exceed Ea and that have sufficient t energy for a reaction to take place ; pre-exponential term A takes into account the frequency of collisions + correct orientation
Does A fluctuate?
Increases slightly with temperature as frequency of collisions increases but is constant over a small temperature range - frequency factor essentially gives the rate if there were no activation energy
How to convert Arrhenius equation to y=mx+c?
ln k = -Ea/R (1/T) + lnA Where y = lnk M = -Ea/R x = 1/T (in kelvin) y-intercept = lnA
SEARCH UP/LEARN HOW TO EXTRAPOLATE
We are looking age when THE LINE CROSSES THE Y-AXIS when x=0
