Intro to Kinetics - Ohlone College

Intro to Kinetics

Introduction the air, it ignites when brought to a flame. A gas supports
Chemical kinetics is the study of the speed of a reaction combustion if a glowing, wooden splint continues to
and its mechanism. Chemical reactions take place at various glow, or even bursts into flame when placed in the gas. If
speeds. Think of a firecracker going off and iron rusting. a burning wooden splint goes out when placed in a gas, it
Each of these is a chemical reaction (or a series of chemical does not support combustion. Common combustible gases
reactions) and the speed of the one reaction is many orders are hydrogen, H2, and methane, CH4. Common gases that
of magnitude greater than the speed of the other reaction. support combustion are oxygen, O2, and dinitrogen oxide,
For chemical reactions, the speeds are measured in units of N2O. Common gases that do not support combustion are
moles of reactant consumed per unit of time , and the term carbon dioxide, CO2, and nitrogen, N2.
rate is used, rather than speed. When reactions take place in
solutions, or for gases, then rate is usually given in units of Various Factors Affecting Rates
moles per liter of reactant consumed per unit of time. In
Chem 101B, we will use this latter definition exclusively. To Part 1: Mixing
say it in another way, the rate of a reaction is the change Half fill two small test tubes with deionized water. Drop
in concentration of the reactants per unit of time. one small crystal of potassium permanganate (KMnO4)
into each test tube. Set one tube aside in a beaker for the
In today's experiment, you will investigate some of the remainder of the experiment. Swirl the other tube for a
factors that influence rates of reactions for a variety of sub- minute. Put it next to the first tube. As you do the rest of
stances. In following experiments, you will quantitatively the experiment, occasionally look at the unswirled tube and
measure the effects of concentration and temperature on a note how the color is developing compared to the swirled
specific reaction. Reaction mechanism, the series of steps tube. Make brief notes in the observation table. After the
involved as reactants change to products, will be discussed entire experiment is finished, dump liquids into the waste
in lecture. container provided.

Experiment Part 2: The Nature of the Reactants
Work in pairs Magnesium and calcium are both in column 2 of the periodic
Supplies: table. Take one piece of calcium and three or four pieces of
• A kit from the storeroom containing: magnesium turnings to your work area.Add deionized water
1-hole rubber stopper to two of the small test tubes to fill them one-third full.
1 bent glass tube (gas delivery tube)
1 straight glass tube This paragraph gives instructions so you will be ready to do
1 solid glass rod what is described in the following paragraph. Have a third
1 splint test tube ready to collect any gases evolved. You will do this
1 cotton square by inverting the third test tube and holding it a few milli-
• From the cart meters above the test tube where gas evolution is vigorously
mortar and pestle taking place, and then, after about 15 seconds of collecting,
• From your drawer moving the still-inverted test tube so that its mouth comes
forceps in contact with the flame on a Bunsen burner.
1-125 ml flask
1 watch glass Now add the magnesium to one test tube, and the calcium
1 medium rubber hose to the other. Set the test tubes in a 50 ml beaker. Watch each
small test tubes tube for evidence of reaction. If you see evolution of gas,
beakers to hold small test tubes wait until it becomes rapid evolution (might take a while),
then collect and test as noted. Is there any evidence of gas
You will be asked to identify various gases during the evolution in the other test tube? Warm the tube gently and
experiment. You will see whether or not the gas supports see if there is any evolution of gas at a higher temperature
combustion, and whether or not the gas is combustible. (note that boiling water gives off water vapor. If gas is
A gas is combustible if, in the presence of the oxygen in evolved, it will continue to form on the metal even after

1

heating is stopped). Do the activities of these metals cor- Part 4: Temperature

respond with their position on the periodic table? Briefly You will test ignition temperatures for methyl alcohol and

explain. Dispose of the liquids in the sink and any solids for cotton.

in the garbage. a) Place 20 drops of methyl alcohol into a watch glass. Set

Part 3: Surface Area the Bunsen burner so that it is burning with a hot flame (air

This reaction is called a heterogeneous reaction. Solid, liquid, inlet open full, with a well-defined inner cone about 2–3 cm

and gas are the three phases of matter. When a substance in height). Use the glass rod in the kit from the store room.

in one phase reacts with a substance in another phase, it is Place the tip of the rod just above the tip of the inner blue

labeled a heterogeneous reaction. Here it will be a solid flame for 10 seconds, then quickly touch the hot tip of the

reacting with ions in solution. Set up an apparatus as shown rod to the alcohol. If the alcohol did not ignite, place the

below. Fill the test tube brim full with water, then invert it rod back in the flame, and heat it until the flame is glowing

while holding your finger over the opening and place it in yellow and the tip almost looks as if it might melt. Again

the water in the beaker. 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

small area. Obtain a small square of cotton, and using a forceps,
test tube immerse the cotton into the liquid. Take the cotton out of the
liquid with the forceps and allow excess liquid to drain off
250 ml back into the beaker. Ignite the liquid by quickly putting the

200 ml 125 ml cotton into a flame and then immediately removing it from
mark 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

Do not place the inverted and water filled test tube onto magnesium into each test tube and observe the rate of the
the tube leading from the flask until you have loaded the reaction. Record your observations. Dispose of the liquid
flask with the contents as directed and secured the stopper in the sink and the solid in the garbage.
in the flask. Make sure the stopper is firmly in the flask. Part 6: Catalysts
Use the apparatus from Part 3
Give it a good twist.
a) Add 10 ml of 3% H2O2 to the flask. Insert the stopper
a) Put 3 grams of marble chips (CaCO3) in the flask. Add and then place the inverted and water filled small test tube
25 ml of 1M HCl to the flask. Insert the stopper, and then onto the delivery tube. Swirl the flask for 30 seconds. Is

set the test tube on the gas delivery tube leading from the gas being delivered to the test tube?
reaction flask. Record how many seconds it takes to fill the 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
test tube.
5 drops of 3M CuCl2 solution. Quickly place the stopper
b) Rinse the flask and marble chips with water, and transfer back on the flask, put the test tube on the delivery tube, and

the chips to the waste container in the fume hood. Obtain swirl the flask. Record the time it takes to fill the test tube.
three more grams of marble chips and pulverize them in a 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
mortar with a pestle. Your hand should cover the mortar, and test tube ready as before. Add 5 drops of 3M FeCl3 solu-
the pestle should slide between your thumb and first finger tion to the flask and proceed to time the filling of the test
as you pound the chips to a smaller size. Break them into
the size of grains of sugar. Pour the crushed chips from the tube. Before you take the flask and stopper apart, test the
mortar onto a piece of paper. Proceed as in Part 3 a), again gas in the flask to see if it supports combustion. To do this,
recording the time. Before you take the flask and stopper light a wooden splint, and after it is securely lit, blow out
apart, test the gas in the flask to see if it supports combus- the flame. The charred end of the splint should be glowing
tion. To do this, light a wooden splint. As soon as your but not burning. As soon as your partner takes the stopper
partner takes the stopper out of the flask, insert the burning out of the flask, insert the glowing end of the splint into
end of the splint into the flask (but not into the liquid at the flask (but not into the liquid at the bottom of the flask).
the bottom of the flask). What gas was in the flask? Dump Pull the splint out, blow it out, and reinsert it. Record your
the flask contents into the sink. You will use this apparatus observations. Dump the liquid into the sink. Rinse the flask
and pull the pieces from the kit apart.
again in Part 6.

2

1: Mixing Observation T able
describe appearance
of each solution Behavior of metals

2: Nature of Reactants

Activities and periodic table

Behavior of gas collected

3: Surface Area a) Time________sec b) is _______ times as fast as a) gas is ______
4: Temperature b) Time________sec gas is ______
Behavior of splint?

a)

b)

Why didn’t the cotton ignite?

5: Concentration

6: Catalysts a)
give times and b)
observations c)
Behavior of splint?

gas is ______

3

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.

4