Intro to Kinetics
Introduction
Chemical kinetics is the study of the speed of a reaction
and its mechanism. Chemical reactions take place at various
speeds. Think of a firecracker going off and iron rusting.
Each of these is a chemical reaction (or a series of chemical
reactions) and the speed of the one reaction is many orders
of magnitude greater than the speed of the other reaction.
For chemical reactions, the speeds are measured in units of
moles of reactant consumed per unit of time , and the term
rate is used, rather than speed. When reactions take place in
solutions, or for gases, then rate is usually given in units of
moles per liter of reactant consumed per unit of time. In
Chem 101B, we will use this latter definition exclusively. To
say it in another way, the rate of a reaction is the change
in concentration of the reactants per unit of time.
the air, it ignites when brought to a flame. A gas supports
combustion if a glowing, wooden splint continues to
glow, or even bursts into flame when placed in the gas. If
a burning wooden splint goes out when placed in a gas, it
does not support combustion. Common combustible gases
are hydrogen, H2, and methane, CH4. Common gases that
support combustion are oxygen, O2, and dinitrogen oxide,
N2O. Common gases that do not support combustion are
carbon dioxide, CO2, and nitrogen, N2.
Various Factors Affecting Rates
Part 1: Mixing
Half fill two small test tubes with deionized water. Drop
one small crystal of potassium permanganate (KMnO4)
into each test tube. Set one tube aside in a beaker for the
remainder of the experiment. Swirl the other tube for a
minute. Put it next to the first tube. As you do the rest of
the experiment, occasionally look at the unswirled tube and
note how the color is developing compared to the swirled
tube. Make brief notes in the observation table. After the
entire experiment is finished, dump liquids into the waste
container provided.
In today's experiment, you will investigate some of the
factors that influence rates of reactions for a variety of substances. In following experiments, you will quantitatively
measure the effects of concentration and temperature on a
specific reaction. Reaction mechanism, the series of steps
involved as reactants change to products, will be discussed
in lecture.
Experiment
Work in pairs
Supplies:
• A kit from the storeroom containing:
1-hole rubber stopper
1 bent glass tube (gas delivery tube)
1 straight glass tube
1 solid glass rod
1 splint
1 cotton square
• From the cart
mortar and pestle
• From your drawer
forceps
1-125 ml flask
1 watch glass
1 medium rubber hose
small test tubes
beakers to hold small test tubes
Part 2: The Nature of the Reactants
Magnesium and calcium are both in column 2 of the periodic
table. Take one piece of calcium and three or four pieces of
magnesium turnings to your work area. Add deionized water
to two of the small test tubes to fill them one-third full.
This paragraph gives instructions so you will be ready to do
what is described in the following paragraph. Have a third
test tube ready to collect any gases evolved. You will do this
by inverting the third test tube and holding it a few millimeters above the test tube where gas evolution is vigorously
taking place, and then, after about 15 seconds of collecting,
moving the still-inverted test tube so that its mouth comes
in contact with the flame on a Bunsen burner.
Now add the magnesium to one test tube, and the calcium
to the other. Set the test tubes in a 50 ml beaker. Watch each
tube for evidence of reaction. If you see evolution of gas,
wait until it becomes rapid evolution (might take a while),
then collect and test as noted. Is there any evidence of gas
evolution in the other test tube? Warm the tube gently and
see if there is any evolution of gas at a higher temperature
(note that boiling water gives off water vapor. If gas is
evolved, it will continue to form on the metal even after
You will be asked to identify various gases during the
experiment. You will see whether or not the gas supports
combustion, and whether or not the gas is combustible.
A gas is combustible if, in the presence of the oxygen in
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heating is stopped). Do the activities of these metals correspond with their position on the periodic table? Briefly
explain. Dispose of the liquids in the sink and any solids
in the garbage.
Part 3: Surface Area
This reaction is called a heterogeneous reaction. Solid, liquid,
and gas are the three phases of matter. When a substance
in one phase reacts with a substance in another phase, it is
labeled a heterogeneous reaction. Here it will be a solid
reacting with ions in solution. Set up an apparatus as shown
below. Fill the test tube brim full with water, then invert it
while holding your finger over the opening and place it in
the water in the beaker.
small
test tube
250 ml
200 ml
mark
125 ml
Do not place the inverted and water filled test tube onto
the tube leading from the flask until you have loaded the
flask with the contents as directed and secured the stopper
in the flask. Make sure the stopper is firmly in the flask.
Give it a good twist.
a) Put 3 grams of marble chips (CaCO3) in the flask. Add
25 ml of 1M HCl to the flask. Insert the stopper, and then
set the test tube on the gas delivery tube leading from the
reaction flask. Record how many seconds it takes to fill the
test tube.
b) Rinse the flask and marble chips with water, and transfer
the chips to the waste container in the fume hood. Obtain
three more grams of marble chips and pulverize them in a
mortar with a pestle. Your hand should cover the mortar, and
the pestle should slide between your thumb and first finger
as you pound the chips to a smaller size. Break them into
the size of grains of sugar. Pour the crushed chips from the
mortar onto a piece of paper. Proceed as in Part 3 a), again
recording the time. Before you take the flask and stopper
apart, test the gas in the flask to see if it supports combustion. To do this, light a wooden splint. As soon as your
partner takes the stopper out of the flask, insert the burning
end of the splint into the flask (but not into the liquid at
the bottom of the flask). What gas was in the flask? Dump
the flask contents into the sink. You will use this apparatus
again in Part 6.
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Part 4: Temperature
You will test ignition temperatures for methyl alcohol and
for cotton.
a) Place 20 drops of methyl alcohol into a watch glass. Set
the Bunsen burner so that it is burning with a hot flame (air
inlet open full, with a well-defined inner cone about 2–3 cm
in height). Use the glass rod in the kit from the store room.
Place the tip of the rod just above the tip of the inner blue
flame for 10 seconds, then quickly touch the hot tip of the
rod to the alcohol. If the alcohol did not ignite, place the
rod back in the flame, and heat it until the flame is glowing
yellow and the tip almost looks as if it might melt. Again
touch the tip of the rod to the alcohol. Allow the alcohol to
burn away. Record what you observe.
b) Obtain 10 ml of a 50:50 v:v isopropyl alcohol/water mix
from the reagent bottle so labeled (this means half and half
by volume). Use a 50 ml beaker to take the mix to your work
area. Obtain a small square of cotton, and using a forceps,
immerse the cotton into the liquid. Take the cotton out of the
liquid with the forceps and allow excess liquid to drain off
back into the beaker. Ignite the liquid by quickly putting the
cotton into a flame and then immediately removing it from
the flame so that it will burn on its own. Record what you
see. As soon as the flame dies, feel the cotton. Is it warm?
Is it moist?
Part 5: Concentration
Take 4 magnesium turnings to your work area. Place about
1 ml of 1M HCl into one small test tube, and about 1 ml of
0.01M HCl into another small test tube. Put 2 turnings of
magnesium into each test tube and observe the rate of the
reaction. Record your observations. Dispose of the liquid
in the sink and the solid in the garbage.
Part 6: Catalysts
Use the apparatus from Part 3
a) Add 10 ml of 3% H2O2 to the flask. Insert the stopper
and then place the inverted and water filled small test tube
onto the delivery tube. Swirl the flask for 30 seconds. Is
gas being delivered to the test tube?
b) Take the test tube off of the delivery tube. Refill it with
water if necessary. Take the stopper out of the flask and add
5 drops of 3M CuCl2 solution. Quickly place the stopper
back on the flask, put the test tube on the delivery tube, and
swirl the flask. Record the time it takes to fill the test tube.
Dump the liquid in the flask into the sink. Rinse the flask.
c) Add another 10 ml of 3% H2O2 to the flask. Have the
test tube ready as before. Add 5 drops of 3M FeCl3 solution to the flask and proceed to time the filling of the test
tube. Before you take the flask and stopper apart, test the
gas in the flask to see if it supports combustion. To do this,
light a wooden splint, and after it is securely lit, blow out
the flame. The charred end of the splint should be glowing
but not burning. As soon as your partner takes the stopper
out of the flask, insert the glowing end of the splint into
the flask (but not into the liquid at the bottom of the flask).
Pull the splint out, blow it out, and reinsert it. Record your
observations. Dump the liquid into the sink. Rinse the flask
and pull the pieces from the kit apart.
Observation T able
1: Mixing
describe appearance
of each solution
2: Nature of Reactants
Behavior of metals
Activities and periodic table
Behavior of gas collected
gas is ______
3: Surface Area
a) Time________sec
b) Time________sec
b) is _______ times as fast as a)
Behavior of splint?
gas is ______
4: Temperature
a)
b)
Why didn’t the cotton ignite?
5: Concentration
6: Catalysts
give times and
observations
a)
b)
c)
Behavior of splint?
gas is ______
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Name_________________________________________ Grade___________ Date ___________
Questions
1. If a cube of CaCO3 has an edge length of 1 cm, what is its volume? What is its surface area?
2. If the cube in question 1 is pulverized into cubes each with an edge length of 0.05 cm, what is the volume of one
of those smaller cubes? What is the small cube's surface area? How many small cubes could be obtained from the
larger cube (assume that the volume of marble remains unchanged)? What is the total surface area of the smaller
cubes? How many times larger is this than the surface area of the large cube?
3. In Part 3 of the experiment, assuming that the reaction takes place on the surface of the CaCO3, how many times
larger must the surface area have been in Part b) compared to Part a)? How does this compare to the answers you got
in question 2?
4. Activation Energy is the minimum amount of energy necessary for molecules to collide and react. If they collide
with less than the activation energy, they simply bounce off one another unchanged. In Part 4, you ignited methyl
alcohol. This is a combustion reaction where the alcohol, CH3OH, reacts with O2 to produce CO2 and H2O. Show
the balanced equation:
Alcohol and oxygen react together in an exothermic reaction, and yet, the alcohol can sit in the air surrounded with
oxygen and not ignite at room temperature. Explain this phenomenon using activation energy in your answer.
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