G11 Elite Science Lab Assessment
Kinetics Lab
Purpose: To determine the rate law and order of a reaction.
Background:
The factors that affect the rate of a chemical reaction are important to understand due to
the importance of many such reactions to our health, well-being, and comfort. The rate of
a reaction is governed by the collision theory. For a reaction to occur, there must be
collisions between reactant molecules. This collision must have enough energy to break
and form the appropriate bonds as well as have the correct orientation when colliding.
When all these things happen, a chemical reaction occurs. If we can increase the amount
of these favorable collisions, then we increase the rate of the reaction. However, the rate
of nearly every reaction changes over time as reactants are depleted, making effective
collisions less likely to occur.
Chemists often write chemical equations for reactions as a single step that shows only the
net result of a reaction. However, most chemical reactions occur over a series of
elementary reactions. The complete sequence of these elementary steps is called a
reaction mechanism. The reaction mechanism is the step-by-step process by which
reactants actually become products. It is the “how” of the reaction, whereas the overall
balanced equation shows only the “what” of the reaction. In kinetics, the rate of a
reaction with several steps is determined by the slowest step, which is known as the ratedetermining, or rate-limiting, step. The overall reaction rate depends almost entirely on
the rate of the slowest step. If the first step is the slowest, and the entire reaction must
wait for it, then it is the rate-determining step.
All chemical reactions have a specific rate defining the progress of reactants going to
products. This rate can be influenced by several factors and the physical properties of the
reactants. The rate law defines the role of each reactant in a reaction and can be used to
mathematically model the time required for a reaction to proceed. The general form of a
rate equation is shown below:
where [A] and [B] are concentrations of different molecular species, m and n are reaction
orders, and k is the rate constant. The rate constant, however, is fixed for any single
reaction at a given temperature. The reaction order illustrates the number of molecular
species involved in a reaction. It is very important to know the rate law, including rate
constant and reaction order, which can only be determined experimentally. In this
experiment, we will explore one method for determining the rate law and use it to
understand the progress of a chemical reaction.
Pre-Lab: Answer the following questions to prepare for your experiment. (4 points)
1.
Apart from catalyst, list three other factors that may affect the speed of chemical
reactions.The factors affecting the speed of the chemical reaction are the nature
of the reactants, the reactants concentration, and the reaction temperature
1.
Explain why the rate of disappearance of NO and the rate of formation of N2 are not
the same in the reaction below,
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2NO(g) + 2H2(g) → N2(g) + 2H2O(g). [2]
because the rate of disappearance means the concentration is going down with
negative sign and ratio between NO and N 2 , the NO must use 2 moles for each
mole of N 2 produced.
1.
How does an increased surface area influence the rate of a chemical reaction?
[1]
. The rate of a chemical reaction can be raised by increasing the surface area of
a solid reactant so more particles are exposed to the other reactant and it will
immediatly increase
Part 1: (3 points)
The decomposition of hydrogen peroxide H2O2, in aqueous solution proceeds very
slowly. The decomposition takes place according to the reaction below.
2H2O2 → 2H2O + O2
To determine the best catalyst for this decomposition, Zainab used three catalysts:
potassium iodide KI, potassium chloride KCl and iron(III) chloride FeCl3. Answer the
following questions using the graphs of generated oxygen below:
O2 graph using KI
O2 graph using KCl
O2 graph using FeCl3
•
Which of the three catalysts will you recommend for the reaction? Explain your
answer in terms of O2 graphs.
[2]
O2 Graph Using KI cuz it has increased more than K CI and FeCl3
• A newly discovered catalyst X was found to be more efficient than KI, describe
and explain the nature of the graph for catalyst X compared to that obtained for
catalyst KI. [1]
The graph for Catalyst X will be diffrent compared to O2 KI
and this means wether it increased or decreased than KI
Part 2: (7 points)
Procedure
Click the link below or copy it into your browser (it may take some moments depending
on your network) .
https://www.youtube.com/watch?v=Gl6LVl7oAlU
•
•
Watch the YouTube video closely.
Answer the following questions after watching the video:
• Assuming you are the teacher, and you provided your students with 0.25 M
solution of sodium thiosulfate Na2S2O3, write the procedures/steps needed by
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the students to run the experiment [4]
The students have to put 0.25 M of solution in beaker 1 and the volume
of Na2S2O3 going to be 50 and H2O is 0 and after they will run the
experiment the reaction time will going to be 27.44s
•
Calculate and fill the column for the concentration of sodium thiosulfate
Na2S2O3 in the table below [2]
Beaker
Volume of
Na2S2O3
(mL)
Volume of
H2O (mL)
[Na2S2O3],
M
Reaction
time (sec)
1
2
3
4
5
50
40
30
20
10
0
10
20
30
40
0.25
0.32
0.41
0.50
0.80
27.44s
32s
35.1s
60s
159.1s
•
Reaction
rate
(1/time,
sec–1)
0.04
0.031
0.0285
0.0167
0.00624
In terms of rate of reaction, explain the inverse relationship the exist between
the concentration and time for the reaction [1]
as we increase the concentration of Na2S2O3 Sodium thiosulfate the
reaction time decrease
Part 3: (8 points)
Procedure
Click the link below or copy it into your browser (it may take some moments depending
on your network) . https://www.youtube.com/watch?v=xydwgATy8lo
•
•
Watch the YouTube video closely and take notes of how the questions were solved.
Answer the following questions after watching the video:
• What happens if you alter the conditions for more than a reactant at a time?
[2]
we wont get the real answers because we have to alter the conditions 1
time for each reactant at the same time and to determine the rate law
using initial rates we should run the reaction for a short amout of time
and alter the concentration of one reactant at time
•
Why do you need to determine the order of each reactant? [1]
It determines how the amount of a compoundspeeds up or retards a
reaction and to know the functional relationship between concentration
and the rate law
3) The following data were collected for the reaction of A + B → C
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Run
1
2
3
•
Initial [A]
0.100
0.200
0.100
Initial [B]
0.050
0.050
0.100
Initial Rate M/s
0.0400
0.0800
0.160
Write the general rate equation for the reaction. [1]
the rate law of expression is Rate= k[A]^1 [B]2
•
What is the order for each reactant in the reaction? [1]
Initial [A] is first order 2=2
Initial [B] is second order 2=4
•
First order 1
Second order2
What is the overall order reaction order? [1]
1+2 = 3 the reaction is 3rd order overall
•
Now that you have determine the reaction order for each reactant, write the
rate law for the reaction.[1]
Run1= 0.04M/s = k[0.1] X [0.050]^2 =
Run1= 0.04M/s = k 2.5 x10^-4
Run1= 0.04 divide 2.5 x10^-4
k= 160
Run2= 0.08M/s = k[0.2] X [0.050]^2 =
Run2= 0.08M/s = k 5 x10^-4
Run2= 0.08 divide 5 x10^-4
k= 160
Run3= 0.160M/s = k[0.1] X [0.1]^2 =
Run3= 0.160M/s = k 1x10^-3
Run3= 0.160 divide 1 x10^-3
k= 160
• Calculate the value of k with correct unit. [1]
k=rate divide [A]1 X [B]2
k = 160
the 3 trials are the same 160
Total Score
22 points
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