rate of reaction
DESCRIPTION
Chapter 1. Rate of Reaction. Reaction Rates. What makes “superglue” bond instantly while Elmer’s glue does not? What factors determine how quickly food spoils? Why do “glow sticks” last longer when stored in the freezer? How do catalytic converters remove various pollutants from car exhaust?. - PowerPoint PPT PresentationTRANSCRIPT
RATE OF REACTIONChapter 1
Reaction Rates
– What makes “superglue” bond instantly while Elmer’s glue does not?
– What factors determine how quickly food spoils?– Why do “glow sticks” last longer when stored in
the freezer?– How do catalytic converters remove various
pollutants from car exhaust?
What are “kinetics”?
• The study of the speed or rate at which chemical reactions occur
Ch 1.1 A2
Wha
t ar
e ch
emic
al k
ineti
cs a
nd w
hat
affec
ts th
em?
Besides information about the speed at which reactions occur, kinetics also sheds light on the reaction mechanism (exactly how the reaction occurs).
What are “kinetics”?Ch 1.1 A2
Wha
t ar
e ch
emic
al k
ineti
cs a
nd w
hat
affec
ts th
em?
How fast a chemical reaction goes is affected by many factors. List four of these.
TemperaturePressureConcentrationParticle sizeCatalyst
Discuss in your groups how each factor affects the reaction rate
Chemical reaction ratesCh 1.1 A2
How
do
fact
ors a
ffect
reac
tion
rate
?Temperature:Food spoils more quickly at room temperature than in a refrigerator
bacteria grow faster at RT than at lower temperatures
simulation
At higher temperatures, reactant molecules have more kinetic energy, move faster, and collide more often and with greater energy.
Factors affecting reaction ratesCh 1.1 A2
How
do
fact
ors a
ffect
reac
tion
rate
s?Concentration:
As the concentration of reactants increases, so does the likelihood that reactant molecules will collide, the rate of reaction generally increase.
There are fewer red particles inc the same volume so there is less chance of a collision
There are more red particles in the same volume so there is more chance of a collision so the reaction goes faster
Factors affecting reaction rates
Catalysta substance that increases the rate of a reaction
without being consumed in the reactionEnzymes
– biological catalysts– proteins that increase the rate of biochemical
reactions
Ch 1.1 A2
How
do
fact
ors a
ffect
reac
tion
rate
s?
Surface areaas surface area increases the rate of reaction generally increases
Factors affecting reaction ratesCh 1.1 A2
How
do
fact
ors a
ffect
reac
tion
rate
s?Surface area
As surface area increases the rate of reaction generally increases
The particles on the surface can react
When cut into smaller pieces the particles on the inside can react
Chemical reaction ratesCh 1.1 A2
Wha
t do
es “
rate
of
reac
tion
mea
n?
A → B
Concentration of [A] decreases with time as the concentration of [B] increases.
Ways to measure the rate of reaction
Ch 1.1 A2
Wha
t do
reac
tion
rate
gra
phs l
ook
like?
Gas Volume – you can measure the rate of reaction by measuring how much gas is released
Colour change – use a colorimeter to measure the change in a colour change reaction.
Electrical conductivity – as the number of ions change during the reaction, the conductivity changes.
Practical
Design a practical to show that concentration of HCl affects the rate of reaction with calcium carbonate.
HCl (aq) + CaCO3(s) CaCl2(aq) + H20(l) + CO2(g)
Chemical reaction ratesCh 1.1 A2
Wha
t do
es “
rate
of
reac
tion
mea
n?
A → B
Concentration of [A] decreases with time as the concentration of [B] increases.
Chemical reaction ratesCh 1.1 A2
Wha
t do
reac
tion
rate
gra
phs l
ook
like?
Predict the general shape of the graph you would expect for A → Product
Chemical reaction ratesCh 1.1 A2
Wha
t do
reac
tion
rate
gra
phs l
ook
like?
Predict the shape of the graph for the reaction
A + 2B →C
Describe what is happening in this graph in terms of reaction rate and concentrations
Chemical reaction ratesCh 1.1 A2
Wha
t do
es “
rate
of
reac
tion
mea
n? The steeper the curve the faster the rate of reaction
Why is the initial slope of the graph the steepest?
The rate of reaction is highest here because there is more likelihood of collision
Chemical reaction ratesCh 1.1 A2
Wha
t do
es “
rate
of
reac
tion
mea
n?Why does the slope of the reaction decrease with time?
The rate of reaction slows down over time because there are fewer reactant particles left.
Chemical reaction ratesCh 1.1 A2
Wha
t do
reac
tion
rate
gra
phs l
ook
like?
Why does [B] fall faster than [A]?
For each molecule or atom of A, two B particles of B are used up.
Chemical reaction ratesCh 1.1 A2
Wha
t do
es “
rate
of
reac
tion
mea
n?SummaryThe reaction rate for a chemical reaction can be expressed as either:
the increase in concentration (or number of moles) of a product as a function of time.
the decrease in concentration (or number of moles) of a reactant as a function of time
Finding the rateCh 1.1 A2
How
do
you
find
reac
tion
rate
s?
The speed of an object or event is the change that occurs in a given time interval.
Speed of a car = change in distancetime interval
= DdDt
Remember, the term change always refers to final value minus initial value.
Finding the rateCh 1.1 A2
How
do
you
find
reac
tion
rate
s?
Similarly, the rate (or speed) of a reaction can be determined:
Rate = change in concentration (or moles) of product time interval
Rate = D (conc. or moles)Dt
Finding the rateCh 1.1 A2
How
do
you
find
reac
tion
rate
s?
Reaction RatesConsider the chemical reaction:
A B
Time = 0.10. mol A
t = 20. min5.0 mol A5.0 mol B
t = 40. min2.0 mol A8.0 mol B
Finding the rateCh 1.1 A2
How
do
you
find
reac
tion
rate
s?
Finding the rateCh 1.1 A2
How
do
you
find
reac
tion
rate
s?
02468
1012
0 20 40 60 80
time (min)
mol
es A
or B
moles A
moles B
If the number of moles of A and B are measured and plotted, a graph such as this one can be obtained
This data can be used to find the reaction rate.
Finding the rateCh 1.1 A2
How
do
you
find
reac
tion
rate
s?
In this reaction:
Average rate of appearance of B = change in # of moles of B
change in time
= D (mol B)D t
We can calculate the average rate for any time interval involved in the reaction.
Finding the rateCh 1.1 A2
How
do
you
find
reac
tion
rate
s?
the rate of appearance of B over the first 20 minutes of reaction:
Average rate of appearance of B = D (mol B)
D t
= 5.0 mol B – 0.0 mol B20. min – 0. min
= 0.25 mol/min
Time = 010 mol A
t = 20 min5.0 mol A5.0 mol B
Finding the rateCh 1.1 A2
How
do
you
find
reac
tion
rate
s?
In this reaction, the concentration of butyl chloride, C4H9Cl, was measured at various times, t.
C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq)
Finding the rateCh 1.1 A2
How
do
you
find
reac
tion
rate
s?
The average rate of the reaction over each interval is the change in concentration divided by the change in time:
C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq)
Finding the rateCh 1.1 A2
How
do
you
find
reac
tion
rate
s?
Finding the rateCh 1.1 A2
How
do
you
find
reac
tion
rate
s?
The average rate decreases as the reaction proceeds.
What do you notice about the average rate?
Why? As the reaction goes forward, there are fewer collisions between reactant molecules.
ExampleCh 1.1 A2
How
do
you
find
reac
tion
rate
s?
:
Given the following data, what is the average rate of the following reaction over the time interval from 54.0 min to 215.0 min?
CH3OH (aq) + HCl (aq) → CH3Cl (aq) + H2O (l)
Time (min) [HCl] (M)0.0 1.85
54.0 1.58107.0 1.36215.0 1.02
Finding the rateCh 1.1 A2
How
do
you
find
reac
tion
rate
s?
Given: [HCl]54 min = 1.58 M [HCl]215 min = 1.02 M
Find: avg. rate of disappearance of HCl
Avg. rate = - D [HCl]D t
= - (1.02 M - 1.58 M) 215 min - 54 min
= 0.0035M / min
Finding the rateCh 1.1 A2
How
do
you
find
reac
tion
rate
s?• A plot of concentration vs. time for this reaction yields a curve like this.
• The slope of a line tangent to the curve at any point is the instantaneous rate at that time.
C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq)
Finding the rateCh 1.1 A2
How
do
you
find
reac
tion
rate
s?
The rate of reaction (and reaction order) can only be worked out experimentally.
You can calculate the rate (gradient) at different points, and produce a rate-concentration graph.
Finding the rateCh 1.1 A2
How
do
you
find
reac
tion
rate
s?
Rate OrderThe order of a reaction will tell you how the change in concentration affects the rate of reaction.
If you double the concentration of a reactant, and the rate stays the same, the reaction is zero order with respect to the reactant
If you double the concentration of a reactant, and the rate doubles, the reaction is first order with respect to the reactant
If you double the concentration of a reactant, and the rate quadruples, the reaction is second order with respect to the reactant
What this tells us…The order of reaction for a reactant tells us how it will affect the rate of reaction.
A + B C
By completing rate experiments, you may find that doubling the concentration of A, the rate remains constant…BUTdoubling the concentration of B, the rate also doubles.
We can say it is zero order with respect to [A]and first order with respect to [B].
Finding the rateCh 1.1 A2
How
do
you
find
reac
tion
rate
s?
For a generalized chemical reaction:
w A + x B y C + z D
the general form of the rate law is:
Rate = k[A]m [B]n
where k = rate constantm, n = reaction order
First Order ReactionsCh 1.1 A2
How
do
you
find
reac
tion
rate
s?
Expt [A] (M) Rate (M/s)
1 0.50 1.00
2 1.00 2.00
3 2.00 4.00
x2 x2
x2 x2
As [A] doubles, the rate doubles
[A] rate
• First Order Reaction– Overall reaction order = 1– Rate = k[A]
Second Order ReactionsCh 1.1 A2
How
do
you
find
reac
tion
rate
s?
Expt Initial [A] (M) Initial [B] (M ) Rate (mol dm-3 s-1)
1 0.1 0.2 1.6 x 10-2
2 0.1 0.4 3.2 x 10-2
3 0.2 0.2 6.4 x 10-2
x1 x2 x2
[A] stays the same [B] doubles
x2 x1 x4
the rate doubles [B] rate
[A] doubles [B] stays the same
the rate is x4 [A]2 rate
Second Order ReactionsCh 1.1 A2
How
do
you
find
reac
tion
rate
s?
[A] doubles [B] stays the same
[A] stays the same [B] doubles
the rate doubles [B] rate
the rate is x4 [A]2 rate
What is the rate equation for this reaction?
Rate = k[A]2 [B]
The reaction is second order in respect of A and first order in respect of B. The overall reaction order is 3.
Initial [X]/M Initial [Y]/M Initial [Z] / M Initial rate/ mol dm-3 s-1
0.10 0.10 0.10 2.40 x 10-3
0.10 0.10 0.30 7.20 x 10-3
0.05 0.10 0.10 2.40 x 10-3
0.10 0.40 0.10 3.84 x 10-2
Second Order ReactionsCh 1.1 A2
How
do
you
find
reac
tion
rate
s?x1 x3 x3
[Z] triples [X] &[Y] stay the same
X0.5 x1 x1
the rate trebles [Z] rate
[X] halves [Y] & [Z] stay the same
the rate is the same
[X]0 rate
NEx1
[Y] quadruples [X] & [Z] stay the same
the rate goes up by 16 (ie 42 ) [Y]2 rate
x1 x4 x1 x1
Second Order ReactionsCh 1.1 A2
How
do
you
find
reac
tion
rate
s?
What is the rate equation for this reaction?
Rate = k[Y]2 [Z]
The reaction is second order in respect of Y and first order in respect of Z. The overall reaction order is 3.
[X] halves [Y] & [Z] stay the same
[Z] triples [X] &[Y] stay the same
the rate trebles [Z] rate
the rate is the same
[X]0 rate
[Y] quadruples [X] & [Z] stay the same
the rate goes up by 16 (ie 42 ) [Y]2 rate
SummaryCh 1.1 A2
How
do
you
find
reac
tion
rate
s?
Rate Constant (k)a proportionality constant that relates the concentration of reactants to the reaction rate
Reaction Orderthe power to which the concentration of a reactant is raised in a rate law
Overall reaction orderThe sum of all individual reaction orders