4 observation sheets
1 textbook for crushing chalk (students provide)
pipettes
SET-UP
Instruct the students to do the following:
1. Label the test tubes “A” through “I”.
2. Measure 10 mL of regular strength vinegar and pour into test tubes “A” through “E”.
3. Measure 5 mL of regular strength vinegar and pour into test tube “F”. Measure 5 mL of
water and pour into test tube “F”. This test tube now contains a 50% vinegar solution.
4. All “D” test tubes should be placed in the container of hot tap water. The GTF or teacher will give instructions on how to do this. The vinegar in the test tube will take about 5 minutes to warm up.
5. Measure 10 mL of hydrogen peroxide and pour into test tube “G”. Repeat for test tubes “H and I”.
II. DEMONSTRATION OF THE EFFECT OF SURFACE AREA
Ask students, What is surface area?
Accept logical responses.
(Surface area is the exposed surface of an object. Some students do not understand the
concept of surface area. You might tell them that the desks in the room are spread out
over a lot of the floor’s surface area. If the desks were pushed together in the center of the room, they would be concentrated on a smaller amount of the floor’s surface area.)
Tell students that the next two demonstrations will illustrate the effect of surface area or
particle size on the rate of a reaction.
Materials
1 coffee can with lid (lined with aluminum pie plate) containing:
1 box of matches
1 vial of lycopodium "dust” powder
1 pipette (jumbo size)
1 tea light candle
1 aluminum pan
A. DUST IN A FLAME
• Show the students the lycopodium "dust” powder.
• Place a small pile of powder on the aluminum pan and attempt to light it with a match.
(Depending on how long the match is held to the powder - it will either not burn or will
burn enough to char a little.)
• Light the tea candle and place it on the aluminum pan.
• Load the pipette with a small amount of dust powder (enough to fill the tip). DO NOT
turn the pipette upside down. There must be powder at the tip of the pipette for this to
work.
• Hold the pipette so the tip is near the flame and squeeze the pipette bulb to release the
lycopodium powder into the flame.
• There will be a flash of fire.
Ask students, Why was there a flash of fire?
When the powder is in a pile, it will not light. Oxygen cannot get inside the pile to react with enough of the particles of powder; it can only react with the particles on the outside of the pile. When the powder is suspended in the air, it has more surface area than when it was in a pile. This is because the particles are extremely small. When they are sprayed into the air near the
flame, the particles are spread out so the oxygen in the air reaches more particles at the same instant - hence more of the particles are burning at the same time and you see a big flash of flame. (Lycopodium powder is a dried-up moss. It is used for this type of demonstration because the powder has extremely small particles.)
SUMMARY: More of the surface of the particles is exposed to oxygen when the particles are sprayed into the flame. This causes a flash of fire that indicates more rapid burning (combustion) of the lycopodium powder.
B. DUST CAN EXPLOSION
Caution: This experiment is loud and sometimes propels the lid of the coffee can in the air.
Be sure the can is some distance away from the nearest students before you do
this experiment!
• Show students the "dust can”.
Light the tea light candle and place it in the coffee can.
• Load the pipette with a small amount of dust powder (enough to fill the tip). DO NOT
turn the pipette upside down. There must be powder at the tip of the pipette for this to
work.
• Show the pipette to the students. Tell students that there is a hole in the side of the can.
• Ask students to predict what will happen when you 'blow’ the dust into the can.
Accept logical responses.
• Holding the pipette at an angle (aiming down with about a 30° angle), place the pipette
in the hole (make sure the pipette is snug).
• Place the lid on the can.
• Squeeze firmly on the pipette and leave the pipette in the hole after squeezing.
• There will be a flash of fire, a loud explosion, and the lid will blow off the can.
Note: If the explosion does not happen on the first try, please try again. Some groups have to try this
three or four times to achieve the desired results. The students love to see this more than once and
it shows them that perseverance pays off.
Ask students, Why was the reaction quicker and bigger when the dust was blown around in the
can?
Accept student responses and use some of the following information to explain what happened.
EXPLANATION: The dust can explosion is a dramatic illustration of the effect of surface area on the rate of reaction. The chemical reaction is the same as any combustion reaction of any organic fuel - wood, coal, gasoline, natural gas. The contents of these fuels are carbon compounds which combine with oxygen to give carbon dioxide and water vapor. If these gases are confined, an explosion will occur because the gases take up much more volume than the solid fuel. Some explosions are useful. For example, the internal combustion engine in a car works by small explosions set off by sparks from the spark plugs in each cylinder which drives the pistons. Other explosions are a disaster - such as a flour mill explosion. The dust can explosion is a safe, small scale illustration of what happens in a flour mill explosion.
The dust can explosion illustrates why grain elevators, saw mills, and flour mills have to be very careful about sparks. A spark can set off burnable dust in the air to produce a large explosion.
Grain dust, flour, and saw dust are all potential fuels for combustion since they are made of organic matter.
Ask students, Since it is difficult to get a large log to burn, how do people use the concept of
surface area or particle size to start a camp fire or a fire in a fireplace?
(Small sticks, twigs or tinder are put under the larger pieces of wood. The small pieces
expose more surface area to the fire. The small pieces are burned first and that creates enough heat to ignite the larger pieces of wood.)
EXPLANATION: Burning requires a fuel and sufficient oxygen (air is 20% oxygen). Put something that burns easily - twigs and small sticks - under larger pieces of wood. Use a match to start the twigs. These ignite easily and create enough heat to ignite the larger pieces of wood. This illustrates the effect of surface area or particle size on the rate of reaction - the smaller the pieces of wood, the more surface area is exposed to the oxygen in the air.
III. EFFECT OF SURFACE AREA (PARTICLE SIZE)
MATERIALS – use test tubes “A” and “B”
2 test tubes with 10 ml regular vinegar at room temperature
2 pieces (1/8ths of a stick) of chalk
1 snack bag
1 textbook for crushing chalk (students provide)
1 pair of scissors
1 funnel
• Ask students to use what they learned about surface area in the last experiment to suggest
ways to increase the surface area of the tablets to speed up the rate of the reaction.
Accept logical responses and ask students to explain their answers.
Have each group do the following:
• Take one of the pieces of chalk, put it in the snack bag and crush to a fine powder by tapping on the bag with a textbook.
• Insert the funnel into test tube “B”.
• Ask students to observe the two pieces of chalk now and tell which piece has more surface area. (The crushed chalk - more of the inside surface of the chalk is now exposed.)
• Shake all the crushed chalk into one bottom corner of the bag and cut the other bottom corner off. Then pour the crushed chalk through the bottom cut corner into the test tube “B” through the funnel.
• Put the other piece of chalk into test tube “A”.
• Observe what happens and record observations.
Ask students, Which piece of chalk produced a faster reaction? Why?
(Bubbles of carbon dioxide come off more quickly from the crushed chalk than from the
whole piece.)
Ask students, How does this illustrate the effect of surface area or particle size on the rate of a
reaction?
(The crushed chalk reacts faster because the vinegar can reach more of the tablet when there is more exposed surface area. Therefore, increasing the surface area increases the rate of the reaction.)
IV. EFFECT OF TEMPERATURE
MATERIALS – use test tubes “C” and “D”.
1 test tube with 10 ml regular vinegar at room temperature
1 test tube with 10 ml warmed regular vinegar
2 pieces (1/8ths of a stick) of chalk
Ask students, What happens to food that is left out in the open on a hot day or in a hot room?
Accept logical responses. (Melts, spoils, molds, gets hard, ripens, stays the same and other responses – depending on the food item)
Ask, Since some foods spoil in heat, what do we do to slow down the rate of food spoilage?
Accept logical responses. (We refrigerate or freeze foods to delay the rate of food spoilage. The lower the temperature, the slower the reaction. Conversely, the higher the temperature, the faster the reaction. We know that food left out on a hot summer day can spoil fairly quickly. By cooling the food, we slow the chemical reaction of spoilage. Since food spoilage is a chemical reaction, this example illustrates the effect of temperature on the rate of a chemical reaction.)
Each group does the following (following the Instruction Sheet):
• Add one piece of chalk to test tube “C” and one to “D”.
• Tell the students to compare the rate of CO2 gas bubbling off (ie. the rate of bubbles being produced) and to write down their observations.
Ask students, Was the reaction faster in the warmer vinegar or the vinegar at room temperature? (Warmer temperature vinegar. Bubbles of carbon dioxide come off more slowly in room temperature vinegar.)
Ask students, How does this illustrate the effect of temperature on the rate of reaction?
(The rate of bubbles coming off in the cooler vinegar was slower so the lower the temperature the slower the reaction; and the higher the temperature the faster the reaction.)
Ask students, How could we change the temperature of the vinegar to make the reaction occur even faster? (Heat the vinegar to a higher temperature. The reaction rate doubles for every 10 degree increase in temperature.)
Ask students, Is the total amount of carbon dioxide given off in both the slow and fast reaction the same if you wait until the reaction is over? (Yes. It’s important for students to realize that since we started with the same AMOUNTS of vinegar and chalk, we will get the same amount of carbon dioxide gas produced, whether the reaction is fast or slow. The cooler vinegar/chalk test tube will continue to bubble long after the other one has stopped.
III. EFFECT OF CONCENTRATION IN A SOLUTION
MATERIALS – use test tubes “E” and “F”
1 test tube “E” with 10 mL regular vinegar (stronger concentration)
1 test tube “F” with the 50% vinegar (weaker concentration)
2 pieces (1/8ths of a stick each) of chalk
Tell students that concentration of a solution refers to how much of a substance is dissolved in water. A stronger (more concentrated) solution has more molecules of the reacting substance in water than a weaker (more dilute) solution does. The students prepared the weaker concentration of vinegar by diluting 5 mLs of vinegar with 5 mLs of water.
Have each group:
• Use test tubes “E” and “F”.
• Place a piece of chalk in the vinegar solutions and observe closely to see the results.
• Record observations.
Ask students to describe what happened.
Bubbles of carbon dioxide come off more slowly from the lower concentration of vinegar and faster from the stronger vinegar solution.
Ask, How does this illustrate the effect of concentration on the rate of reaction? (The rate of carbon dioxide bubble formation is slower for the weaker solution of vinegar. The stronger the solution, the more substance there is to react and the faster the reaction will occur.)
IV. EFFECT OF A CATALYST
MATERIALS – use test tubes “G, H and I”
1 vial of 3% hydrogen peroxide
1 small scoop
1 container of manganese dioxide
1 small piece of paper towel
1 sliver of potato
Ask students, What is a catalyst?
Accept logical responses.
(A catalyst is a substance that speeds up or slows down a chemical reaction but is not
changed itself by the reaction.)
Show the students the sample bottle of hydrogen peroxide.
Ask students what they know about hydrogen peroxide. Accept logical responses.
(Some students will know that hydrogen peroxide is often put on cuts and that it
bubbles up. Someone might know that it is H2O2.)
Tell students that hydrogen peroxide (H2O2) will chemically decompose (break down) into
oxygen gas (O2) (yes, the glowing splint test will work!) and water (H2O). Hydrogen peroxide is sold in brown bottles because it will decompose in the presence of light at room temperature. Hydrogen peroxide bottles are dated because even in a brown bottle, hydrogen peroxide will decompose over time.
Tell students that the decomposition of hydrogen peroxide will speed up if a catalyst is added to the hydrogen peroxide. The decomposition reaction is:
2H2O2 → 2H2O + O2
One GTF should use the tweezers to place a sliver of potato on each group’s small piece of towel.
Have each group do the following:
• Observe the hydrogen peroxide to see that nothing is happening. No bubbles are
being given off.
• Place a small scoop of manganese dioxide in test tube “G” and a sliver of potato in test tube “H”.
• Observe what happens and record observations. Compare with test tube “I”.
Ask students, What happened?
No bubbles are observed in test tube “I”. Bubbles are observed in both test tubes “G and H”. However, the manganese dioxide and the potato sliver still look the same and are not being used up. They are both catalysts that speed up the decomposition of hydrogen peroxide.
One set of test tubes could be left overnight to show students that the potato and manganese dioxide
will look the same the next morning even after the reaction has finished.
EXPLANATION: A catalyst speeds up a chemical reaction without being used up.. The catalyst does this without undergoing permanent change itself, so it can act over and over again. Many chemical reactions are slow because they involve several intermediate steps that are of high energy. A good analogy is the difference between driving over a mountain or driving through a tunnel in the mountain. It takes less time to drive through a tunnel than over the top of the mountain. The regular chemical reaction goes through an intermediate state of high energy like going over a mountain while the catalyst provides a new pathway of lower energy for the chemical reaction, similar to going through a tunnel in the mountain.
In the present case, the manganese dioxide catalyzes the decomposition of hydrogen peroxide by a series of fast reactions that change manganese dioxide to manganese oxide and then back to manganese dioxide. Since the manganese dioxide speeds up the decomposition of the hydrogen peroxide and is regenerated during the course of the reaction, it is a catalyst.
Enzymes are biological catalysts. The catalase in the potato slice is an example of an enzyme. Enzymes bind their substrates (in this case H2O2) into a cavity before catalyzing a reaction. Once the substrate is in the cavity, binding between the enzyme and the substrate weaken substrate bonds, making it easier for new bonds to form. The best analogy is the Venus flytrap. This plant traps an insect by folding around it. The folding is triggered by the presence of the insect, just as the folding of the enzyme is triggered by the presence of the substrate. After the catalyzed reaction is complete, the enzyme opens up again to let the products leave and to prepare for the next substrate molecule. Your body has thousands of enzymes that catalyze reactions, including catalase in the blood that catalyzes the breakdown of hydrogen peroxide that is formed in some biological reactions and must be decomposed to avoid harmful effects on the body.
Another example of an enzyme at work is the breakdown of starches to simple sugars. The next time you are chewing a cracker, see if you can tell whether the initial salty taste turns to a sweet taste. The saliva in your mouth causes a chemical breakdown of the starch in the cracker as the cracker is chewed by the mouth. The enzyme ptyalin begins breaking down the starch in the cracker to glucose so the cracker begins to taste sweet.
VII. REVIEW
In each activity one of the factors that influences the rate of chemical reactions was varied while
the others were held constant.
Ask students, What effect did surface area have on the rate of the reaction?
In this case, the crushed chalk reacted faster because of the higher surface area of the
particles as compared to the whole piece. (Part III)
Ask students, What effect did temperature have on the rate of reactions?
The lower the temperature, the slower the reaction. The higher the temperature,
the faster the reaction. (Part IV)
Ask students, What effect did concentration have on the rate of the reaction?
The higher the concentration, the faster the reaction occurred. (Part V)
Ask students, What effect did the catalysts have on the rate of reaction of hydrogen
peroxide?
Both manganese dioxide and catalase cause the decomposition of hydrogen peroxide to be faster as indicated by visible bubbles of gas being given off. (Part VI)
Lesson written by Dr. Melvin Joesten, Chemistry Department, Vanderbilt University
Pat Tellinghuisen, Coordinator of VSVS, Vanderbilt University
Reference: Journal Editorial Staff, J. Chem. Educ. 1998, 75, p. 1120A
Carolina Biological: Chemistry: Factors Affecting Rate of Reaction
of reaction.asp
Observation Sheet – Rates of Reaction
Name__________________________________________ Date____________
Instruction Sheet – Rates of Reactions
A. SET-UP:
1. Place the test tubes in order in the test tube rack (A-I)
2. Measure and pour 10 mL of vinegar into test tubes “A, B, C, D and E”.
3. Take test tube “D” and place in hot water until it is ready to use in Part IV.
4. Measure 5 mL of vinegar and pour into test tube “F”.
Measure 5 mL of water and pour into test tube “F”. This is your 50% vinegar solution.
5. Measure and pour 10 mL hydrogen peroxide into test tubes “G, H and I”.
B. EFFECT OF SURFACE AREA – USE TEST TUBES “A” AND “B”.
1. Demonstration with lycopodium powder.
2. Experiment:
• Take one of the pieces of chalk, put it in the snack bag and crush to a fine powder by tapping on the bag with the CAPPED vinegar bottle textbook.
• Insert the funnel into test tube “B”.
• Shake all the crushed chalk into one bottom corner of the bag and cut the other bottom corner off. Then pour the crushed chalk through the bottom cut corner into the test tube “B” through the funnel.
• Put the other piece of chalk into test tube “A”.
• Observe what happens and record observations.
C. EFFECT OF TEMPERATURE – USE TEST TUBES “C” AND “D”.
• Remove test tube “D” from the warm water and place in your rack.
• Add one piece of chalk to test tube “C” and one to “D”.
• Compare the rate of CO2 gas bubbling off (i.e., the rate of bubbles being produced) and write down your observations.
D. EFFECT OF CONCENTRATION OF A SOLUTION - USE TEST TUBES “E”AND “F”
• Place a piece of chalk in the 100% and 50% vinegar solutions and observe closely to see the results.
• Record observations.
E. EFFECT OF A CATALYST – USE TEST TUBES “G, H AND I”
• Observe the hydrogen peroxide (test tube “I”) to see that nothing is happening.
• Place a small scoop of manganese dioxide in test tube “G” and a sliver of potato in test tube “H”.
• Observe what happens and record observations. Compare with test tube “I”.
Chalk Rates
Chalk Rates
