M5 Rate of Rxn III Speed of Reaction 2.pptx

© 2004-09 Dorje Gurung
M5 Rate of Rxn III : Speed
of Reaction (Part II)

M5 Rate of Rxn: Speed of Rxn II
Slide 2 of 23
Learning Objectives
Concepts:
– speed (rate) of reaction, concentration, catalyst, surface area,
temperature, activation energy
Skills:
– Describe a practical method for investigating the speed of a reaction
involving gas evolution by collecting mass data over time and graphing
the result.
– Analyze and interpret data obtained from experiments on speed of
reaction
– Devise experiments to test the effect of any four of the factors on speed
of reaction
– Be able to determine rate of reaction from a mass-time graph

Slide 3 of 23
Preamble
You have learned that concentration of a reactant particles, temperature,
surface area of the reactants, and catalyst affect the speed of
reaction.
But how can we actually follow the progress of a chemical reaction to
confirm this?
Whereas speed of a vehicle is change of distance per unit time, speed of
a chemical reaction is a measure of change of a measurable quantity
per unit time.
Speed of reaction is a measure of how fast a reactant is being used up or
how fast a product is being produced.
We must therefore be able to follow the progress of a reaction by looking
at a quantity that changes over time.

Slide 4 of 23
Following the Progress of a Chemical
Reaction
As you know, we have two choices:
1. Record the change in quantity of a reactant as the reaction progresses.
– Again, the quantity measured could be mass or volume or concentration of the
reactant.
– (We will not be looking at any example of this kind because you are not likely to
encounter it this in your MYP chemistry course.)
1.Record the change in quantity of a product as the reaction progresses.
– The quantity measured could be volume or mass or concentration of the product
– We will be looking at methods involving collection and recording of volume of gas
produced first and then mass of reaction vessel.)
The method chosen depends on the chemical reaction being studied.
– Reactions that produce a gas for example can be followed by measuring the
change in volume of gas evolved or mass of the reaction vessel.
– To be sure, there are other ways of following the progress of a reaction.

Slide 5 of 23
2. Progress of a Chemical Reaction: Mass
Change of Reaction mixture
The reaction between marble chips and hydrochloric acid once again:
CaCO3 (s) + 2HCl (aq)  CaCl2 (aq) + H2O (l) + CO2 (g)
Since one of the products is a gas (CO2) if allowed, it will escape from a
reaction container causing the mass to decreases over time.

Slide 6 of 23
2. Progress of a Chemical Reaction: Mass Change
of Reaction mixture
A setup shown below can be used for this.
Here too, if the speed of
the reaction is to be
determined, then mass
would be recorded at a
regular interval and the
data collected could be
analyzed in one of two
ways.

Slide 7 of 23
Analysis: Mass Change of Reaction mixture
You can either plot a graph of mass of reaction vessel against time as
shown below
or…
mass
of
reaction
vessel
/cm
3
Time/sec.

Slide 8 of 23
Analysis: Mass Change of Product
…plot a graph of mass of CO2 evolved against time.
mass
of
carbon
dioxide
/cm
3
Time /sec.
Either way, the slope of the
curve shows the speed of
the reaction.

Slide 9 of 23
Analysis: Effect of Surface Area
Comparison of speed of reaction between two different surface areas of marble
chips could be carried out by graphing the results of the two experiments on
the same graph.
Time /sec.
two 3g chips
four 3g chips
mass
of
reaction
vessel
/g
Speed higher when 4
chips used
because:
• Curve is steeper,
and
• Curve levels off
earlier—reaction
ends earlier
(Here however, the
limiting reagent
would be the
acid.)

Slide 10 of 23
Other ways of Following progress of a
reaction
You can count the number of bubbles evolved at fixed interval of time
and then plot that.
Or you can count the number of bubbles evolved in a fixed time if speed
of reaction between two or more experiments are being compared.
Concentration of one of the reactants can be determined at regular
intervals and analyzed to obtained information about speed of the
reaction, but that requires a lot more sophisticated methods and
instruments.
The only thing to remember is that regardless of the method used, if a
factor is being studied, for example concentration, that is the only
variable that should be changed from one experiment to another
otherwise the investigation will not be a fair test.

Slide 11 of 23
Determining Speed from Mass-time Graph
Time /sec.
mass
of
reaction
vessel
/g
Rise
Run
(x2, y2)
(x1, y1)
2 1
2 1
Rise
Slope = =
Run


y y
x x
How do you keep the coordinates
straight?
The coordinates for where the
tangent touches the curve is
(x1, y1) and the other one (x2,
y2).
Speed of reaction =  Slope at t=0
The negative sign is required
because rate has to be a
positive value.

Slide 12 of 23
Determining Speed from Mass-time Graph
2 1
2 1
Speed =



y y
x x
2 1
180.20 180.85
Speed =
60 0
= 1.08 10 



 gs
0
Time/sec.
60 120 180 140
mass
of
container
/g
180.6
180.4
180.8
180.2
(60, 180.20)
(0, 180.85)
NB: Students often make the
mistake of firstly drawing a
very small tangent, and
secondly, equating y2 to “0”
when it starts above “0” as is
the case on the left.

Slide 13 of 23
Summary
Progress of a reaction can be followed by recording mass or volume or
concentration change of a reactant or product over time
Plots of the quantity measured over time furnishes information about the
speed of the reaction.
If the effect of a factor on speed is being studied, then both the relative
slope of the plots and the time taken for the reaction to go to
completion indicate the relative speeds of the reaction.
Study of the effect of a factor on the rate of a reaction can also be
carried out by determining the time taken for a fixed change in the
quantity being measured, such as mass or volume.
– The inverse (reciprocal) of the time taken in this case gives the relative
speeds.

Slide 14 of 23
Practice Questions
1. N02/6/3. Dilute nitric acid
was added to a large
amount of magnesium
carbonate in a conical
flask as shown.
The flask was placed on a
balance and the mass of
the flask and contents
recorded every minute.
Write a balanced equation
for the reaction.

Slide 15 of 23
Practice Questions
The results are shown in the table.
(a) Plot the results on the grid and draw a smooth line graph. [3]

Slide 16 of 23
Practice
Questions

Slide 17 of 23
Practice Questions
(b) (i) Which result appears to be inaccurate? Why have you selected this
result? [2]
(ii) Suggest a source of experimental error that may have caused
that result to be inaccurate. [1]
(c) Why does the mass of the flask and contents decrease? [1]

Slide 18 of 23
Practice Questions
(d) Suggest the purpose of the cotton wool. [1]
(e) At what time did the reaction finish? [1]
(f) Determine the rate of the reaction by determining the slope of the
tangent at time=0.
(g) On the grid, sketch the graph you would expect if the experiment
were repeated using nitric acid at a higher temperature. [2]

Slide 19 of 23
Practice Questions
2. J01/2/6g. A student investigated the reaction between calcium
carbonate and hydrochloric acid. Balance the quation.
CaCO3(s) + …HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g)
(ii) The student
measured the rate
of this reaction,
using the apparatus
shown to the right.
Describe how the
student could use
this apparatus to
follow the rate of
reaction. [2]

Slide 20 of 23
Practice Questions
(iii) The student wanted to find out if the concentration of hydrochloric acid
affected the rate of reaction. The student repeated the experiment using
hydrochloric acid of different concentrations.
State two things the student should keep the same in each experiment.
[2]

Slide 21 of 23
Practice Questions
(iv) Draw representative plots on the same graph for results on study of
effect of concentration using 1M and 2M hydrochloric acid with everything
else kept constant. [3]

Slide 22 of 23
Practice Questions
3. N00/2/5c. (a) The apparatus below was used to measure the volume
of gas produced when zinc reacted with hydrochloric acid.
A student measured
the volume of gas
produced during the
first minute of the
reaction.
The student
repeated the
experiment altering
one condition at a
time.

Slide 23 of 23
Practice Questions
What effect would each of the following have on the volume of gas
produced during the first minute of the reaction.
(i) decreasing the concentration of acid
(ii) increasing the temperature
(iii) using finely divided zinc instead of lumps of zinc [3]
(b) Using the collision theory explain each effect. [3]

M5 Rate of Rxn III Speed of Reaction 2.pptx

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