Chapter: 6 Kinetics Title: Lesson 1 Rates of Reaction Learning Objectives: – Understand the term...
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Chapter: 6 Kinetics Title : Lesson 1 Rates of Reaction Learning Objectives: – Understand the term ‘rate of reaction’ – Meet three possible approaches for measuring rates of reaction
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Transcript of Chapter: 6 Kinetics Title: Lesson 1 Rates of Reaction Learning Objectives: – Understand the term...
Chapter: 6 KineticsTitle: Lesson 1 Rates of Reaction
Learning Objectives:– Understand the term ‘rate of reaction’
– Meet three possible approaches for measuring rates of reaction
Main Menu
The Rate of Reactions In simple terms, the rate of a reaction can be
thought of as its speed
Some reactions are very fast
Some reactions are very slow
Discuss: Think of examples of ‘fast’ and ‘slow’
reactions Why is ‘speed’ not a good word to use in this
context?
Chemical reaction ratesCh 1.1 A2
Wha
t do
es “
rate
of
reac
tion
mea
n?
A → [B]Product
[A] decrease with time as [B (product)] increases.
Chemical reaction ratesCh 1.1 A2
Wha
t do
reac
tion
rate
gra
phs
look
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
look
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
look
like
?
Why does [B] fall faster than [A]?
For each molecule or atom of A, two B particles of B are used up.
Main Menu
Defining the Rate of Reaction The rate of reaction is:
Where: ∆[R] is change in concentration of
reactants ∆[P] is change in concentration of products ∆t is change in time
Definition – Rate of reaction is the change in concentration of reactants or products per unit time.
Units: mol dm-3 s-1 (moles per decimetre cubed per second)
The negative sign in the reactant expression indicates the reactant concentration is decreasing, but by convention, rate is expressed as a positive value.
t
P
t
RRate
][][
Main Menu
Using a graph to find rate of reaction
Steepness/gradient is the measure of the change in the concentration per unit time , or the rate of reaction.
The gradient for the curve is not constant and so can only be given for a particular value of time.
Note: Even though the graph gradient is negative, the rate is expressed as positive.
Gradient = Change in y Change in x
Rate is greatest at the start when the reactant concentration is greatest.
As rate varies from this point onwards, it is common to compare initial rates of reactions by taking the tangent at t = 0.
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)
mo
les
A o
r 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?
The average rate of appearance of B during the second 20 minutes of the reaction:
Avg. rate = 8.0 mol B – 5.0 mol B40. min – 20. min
= 0.15 mol/min
t = 20 min5.0 mol A5.0 mol B
t = 40. min2.0 mol A8.0 mol B
The rate of a reaction can also be expressed as the disappearance of A as a function of time.
For this particular reaction, when 1 mole of B is formed, 1 mole of A must disappear.
A → B
HenceDB/Dt = - DA/Dt
Finding the rateCh 1.1 A2
How
do
you
find
reac
tion
rate
s?
For reactions with 1:1 stoichiometry:
• Avg. rate = D (moles product)D t
= - D (moles reactant)D t
Finding the rateCh 1.1 A2
How
do
you
find
reac
tion
rate
s?
• For most reactions, the reaction rate is expressed as a change in concentration of a particular reactant or product
Average Rate = D [Product] = - D [Reactant]D t D t
With concentration in mol dm-3
Finding the rateCh 1.1 A2
How
do
you
find
reac
tion
rate
s?
Main Menu
Measuring Reaction RatesTechnique Apparatus/Notes
Collecting gas Using a gas syringe or inverted measuring cylinder filled with water
Collecting gas (low solubility in water)
Using displacement of water from an inverted burette
Mass loss Reaction conducted on a balance…if it produces a gas the mass will decrease
Colour change Measured using a colorimeter
Conductivity Conductivity will change depending on the concentration of the ions and charges in the reactants/products.
pH Monitored using a pH probe
Titration* Titration of reactants/products against a known standard. Cannot be done continuously, only at set time intervals.
Obscured cross* Useful if reaction produces a precipitate (for example sodium thiosulphate and acid)
Iodine clock* Produces a sudden colour change from colourless to black…. Monitors any reaction producing iodine
* These reactions are not continuous or have an ‘end point’...
Main Menu
Main Menu
Solutions
Main Menu
Main Menu
Review
Rates determined by monitoring a change
t
P
t
RRate
][][
