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Quantity of Energy Contained in A Chemical Process
Lab 5: Chemical Processes
se types of cold packs u lize the chemical process of ammonium nitrate (NH4NO3 ) dissolving in water. The ammonium nitrate needs to absorb heat from the surrounding water to dissolve, so the overall temperature of the mixture de‐ creases as the reac on occurs.
In contrast, energy is released in an exothermic process. An example of an exothermic reac on is what occurs in common hand warmers. The increase in temperature is the result of the chemical reac on of rus ng iron:
4 Fe(s) + 3 O2(g) ® 2 Fe2O3(s) + energy
Iron usually rusts fairly slowly so that any heat transfer is not easily no ced. In the case of hand warmers, common table salt is added to iron filings as a catalyst to speed up the rate of the reac on. Hand warmers also have a permea‐ ble plas c bag that regulates the flow of air into the bag, which allows just the right amount of oxygen in so that the desired temperature is maintained for a long period of me. Other ingredients that are found in hand warmers include a cellulose filler, carbon to disperse the heat, and vermiculite to insulate and retain the heat.
Enthalpy is a quanty of energy contained in a chemical process. In the cases we will be dealing with, the energy re‐ leased or absorbed in a reac on is in the form of heat. Enthalpy by itself does not have an absolute quan ty, but changes in enthalpy can be observed and recorded. For example, if you s ck your finger into a glass of cold tap water, it probably feels pre y cold. However, a er being outside on a freezing winter day for a long period of me, the same glass of water might actually feel warm to touch. It would be difficult to measure the absolute quan ty of energy in the water in either case, but it is rela vely easy to no ce the movement of energy from one object to another. In exother‐ mic reac ons, heat energy is released and the change in enthalpy is nega ve, while in endothermic reac ons, energy is absorbed and the change in enthalpy is posi ve.
Pre‐lab Ques ons
Define enthalpy:
What is the rela onship between the enthalpy of a reac on and its classifica on as endothermic or exo‐ thermic?
With instant hot compresses, calcium chloride dissolves in water and the temperature of the mixture in‐ creases. Is this an endothermic or exothermic process?
Note: the energy term on the right side shows that the reac on is exothermic, but is not required.
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Lab 5: Chemical Processes
Experiment: Cold Packs vs. Hand Warmers
In this lab you will observe the temperature changes for cold packs and hand warmers. Since temperature is defined as the average kine c energy of the molecules, changes in temperature indicate changes in energy. You will use simply a Styrofoam cup as a calorimeter to capture the energy. The customary lid will not be placed on the cup since ample oxy‐ gen from the air is needed for the hand warmer ingredients to react within a reasonable amount of me.
Procedure
Part 1: Cold Pack
Measure 10 mL of dis lled water into a 10 mL graduated cylinder.
Place about 1/4 of the ammonium nitrate crystals found in the solid inner contents of a cold pack into a Styrofoam cup. The Styrofoam cup is used as a simple calorimeter.
Place a thermometer and a s rring rod into the calorimeter (Styrofoam cup). CAUTION: Hold or secure the calorimeter AND the thermometer to prevent breakage.
Pour the 10 mL of water into the calorimeter containing the ammonium nitrate, (NH4NO3) taken from the cold pack.
Immediately record the temperature and the me.
Quickly begin s rring the contents in the calorimeter.
Con nue s rring and record the temperature at thirty second intervals in Table 1. You will need to s r the reac on the en re me you are recording data.
Collect data for at least five minutes and un l a er the temperature reaches its minimum and then begins to rise. This should take approximately 5 to 7 minutes.
Record the overall minimum temperature in the appropriate place on the data table.
Materials Safety Equipment: Safety goggles, gloves En re contents of a hand warmer S r rod 1/4 contents of a cold pack Spatula Calorimeters (2 Styrofoam cups)
Stopwatch Thermometer (digital) *Dis lled water 10mL Graduated cylinder *You must provide
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Lab 5: Chemical Processes
Part 2: Hand Warmer
Wash and dry the thermometer. HINT: Remember to rinse it with dis lled water before drying.
Carefully place and hold the thermometer in another Styrofoam cup.
Cut open the inner package of a hand warmer and quickly transfer all of its contents into the calorimeter. Immediately record the ini al temperature of the contents and being ming the reac on. HINT: Data collec‐ on should start quickly a er the package is opened because the reac on will be ac vated as soon as it is
exposed to air.
Quickly insert the s rring rod into the cup and begin s rring the contents in the calorimeter.
Con nue s rring and record the temperature at thirty second intervals in Table 2. You will need to s r the reac on the en re me you are recording data.
Let the reac on con nue for at least five minutes and un l the temperature has reached its maximum and then fallen a few degree. This should take approximately 5 to 7 minutes.
Record the overall maximum temperature in the appropriate place in the data table.
Data
Please submit your table data and answers for this experiment on the Word document provided to you.
Table 1: Cold pack data
Time (sec) Temp. ( 0 C) Time (sec) Temp. in (
Ini al 240
30 270
60 * 300
90 330
120 360
150 390
180 420
210 450
Minimum Temperature (0C) : __________
63
Lab 5: Chemical Processes
Table 2: Hand warmer data
Time (sec) Temp. ( 0 C) Time (sec) Temp. in (
Ini al 240
30 270
60 * 300
90 330
120 360
150 390
180 420
210 450
Maximum Temperature (°C) : __________
64
Lab 5: Chemical Processes
Graph your data from the tables on the Word document provided to you. You may create the graph on any program, but make sure it can be integrated into the Word document.
Ques ons 1. Calculate the overall temperature change (referred to as ΔT) for the cold and hot pack substance. HINT:
This is the difference in the maximum temperature and minimum temperature of each.
Cold pack ΔT:
Hand warmer ΔT:
Which pack works by an exothermic process? Use experimental data to support your answer.
Which pack works by an endothermic process? Use experimental data to support your answer.
Which pack had the greatest change in enthalpy? How do you know?
Lab 6: Light
67
Lab 6: Light
For centuries, scien sts have used op cal equipment such as lenses and mirrors to study the nature of light. Telescopes and microscopes take advantage of the proper es of light to create images from stars across the galaxy and to magnify objects hardly visible to the naked eye. In the late 19th century, James Maxwell proposed a series of equa ons that unify what we know about electricity and magne sm—it turns out that what we see as light is really electromagne c waves in wavelengths ranging from radio waves to gamma rays. Whenever subatomic par cles interact, they release or absorb energy in the form of electromagne c radia on, which travels through space in the form of electromagne c waves! Many mes, this electromagne c radia on can be detected by the human eye as visible light, but other kinds of light such as infrared radia on require special equipment to view.
Figure 1: This camera uses a series of op cal lenses so that the user can adjust for the intended focal point (f‐stop) and magnifica on of the
desired image.
Concepts to explore: · Electromagne c waves
68
Lab 6: Light
Electromagne c waves travel fast—so fast that it took scien sts many years to confirm that light does not travel at an infinite speed. Over the past half century there have been a number of experiments con‐ ducted to measure the precise speed of light. Modern experiments confirm the speed of light to be about 2.998×10
8 m/s, usually rounded
off as: c = 3.00×10
8 m/s
Just as sound travels at different speeds through different materials, the speed of light also changes depending on the medium it travels in. You can calculate how fast light travels in a material by using the equa‐ on
where n is equal to the index of refrac on for the material. The value of n for all sorts of materials has been found experimentally; some of these materials are listed in Table 1. This number tells us a lot about how light will behave within a material or as it crosses from one medium to another. Because electromagne c waves are so small and fluctuate so quickly, we can divide the light up into idealized lines called rays. You can imagine a ray as a straight beam of light, but in reality, light is emi ed from a source in all direc ons. Reflec on occurs when a beam of light bounces off of a material. If the surface is smooth, the reflected beam leaves the surface at the same angle at which it approached. Thus, we say that the angle of inci‐ dence equals the angle of reflec on, or θi=θr. You can see your reflec on in a mirror because rays of light from different points on your body reflect in this uniform manner. When a beam of light transmits from one medium to another, refrac on occurs. The direc on of light bends one direc on or another depending on the refrac ve index of each material. In general, when light travels from a material with smaller n to larger n, the ray will bend toward the normal (θ1 > θ2); if it goes from larger n to smaller n, it bends away from the normal. See Figure 2 for a diagram.
Table 1: Sample indices of refrac‐ on for several materials.
Material n
Vacuum 1 (exact)
Air 1.00
Water 1.33
Glass (Crown) 1.52
Diamond 2.15
Figure 2: Reflec on (le ) and Refrac on (right). No ce the direc on the ray of light bends as it moves from a material with larger index of refrac on to a smaller one, and vice versa.
V = c n
69
Lab 6: Light
Mirrors and lenses are devices that u lize the phenomena of reflec on and refrac on to create a num‐ ber of useful results for scien sts and engineers. A mirror is usually a polished metal surface that re‐ flects almost all of the light that lands on it. While it is easy to predict how a ray will bounce off of a plane mirror, such as the one in your bathroom, curved mirrors can produce some very interes ng re‐ sults. The following diagrams show how incident rays will reflect off of different spherical mirrors.
Figure 3: Rays incident on a convex (le ) and concave (right) mirror reflect outward or inward as shown above. Images form where the rays converge (real image) or where they appear to emanate from (virtual image).
Figure 4: Rays incident on a convex (le ) and concave (right) lens reflect outward or inward as shown above. Convex lenses (le ) focus parallel incident rays through a single point, called the focus point. For this reason, they are some mes referred to as convergent lenses. Concave lenses (right) cause parallel incident rays to bend away from each other. In fact, they diverge away from each other as if they all began at the same focal
point (rather than converging at the same focal point, as with concave lenses )
70
Lab 6: Light
Parallel rays incident on a concave mirror all reflect toward the mirror’s focal point, which lies in front of the mirror. For a convex mirror, rays reflect outward in such a way that, if traced backward, they converge at a focal point behind the mirror (Figure 3). In each case, the focal point is halfway between the mirror surface and the center—the center of the imaginary sphere that the mirror surface shares: In the case of lenses, parallel rays refract through the lens material. For converging lenses, the rays converge at a focal point behind the lens. For diverging lenses, rays are refracted outward so that when traced backward they will intersect at a focal point in front of the lens. If the object is very far away from a concave mirror (we can say “at infinity”), rays hi ng the mirror surface will be pre y much parallel, and an image will form at the focal point in front of the mirror. In the case of a converging lens, rays refract through the lens and converge at the focal distance on the other side. A real image occurs when a mirror or lens focuses rays of light from all points on the object at a specific distance. If you know where all the light rays intersect, you could put a screen at that point and view the real image that forms there. The projec on screen at a movie theater, for instance, cre‐ ates a real image at the precise distance of the movie screen. Without a screen, you can view a real image by placing your eyes at just the right distance beyond where the image forms so that your eyes are focused at the image point—and an image will appear in the air in front of you! A virtual image occurs when rays coming off of a mirror or through a lens appear to originate from a specific spot, when really no actual object exists at that point. Virtual images are usually made with convex mirrors and diverging lenses. Your reflec on in a regular plane mirror is a virtual image—there is nothing really behind the mirror giving off light. With a concave mirror, the forma on of a virtual im‐ age depends on how close the object is to the mirror. An object closer than the mirror’s focal point is virtual and magnified, while an object placed outside the focal point creates a real image in front of the mirror that can only be seen clearly at the right distance (usually with a screen). When images form from spherical mirrors and lenses, o en mes the image appears to be larger or smaller than the original object. The magnifica on of a mirror or lens tells us how large or small the image is compared to the object. It turns out that the magnifica on (M) is also directly related to the image and object distances: Here the magnifica on is expressed as ra os of the image and object heights and distances. By conven‐ on, an inverted image has a nega ve image height, while an upright image is given a posi ve height.
Image distances are posi ve or nega ve depending on the conven ons listed in Figure 4. Consider a 3 cm tall object. If a lens forms an upright image that is 6 cm tall, the magnifica on of that lens is 2(or 2x, meaning “two mes”). On the contrary, an upside‐down image that is 1.5 cm tall yields a magnifica on of ‐0.5. As you can see, magnifica ons greater than 1 imply an image that appears larger than the origi‐ nal object, while magnifica ons less than one produce images that appear smaller than the original object.
f = c 2
M = h = ‐ si h0 so
71
Lab 6: Light
Mirrors: concave: convex: All image and object distances are posi ve on the re‐ flec ng side of the mirror (object side) and nega ve if “behind” the surface.
Lenses: convex: f > 0 concave: f < 0 so > 0 if object is on side of mirror that rays enter si > 0 if image is on side opposite where rays enter (real image) si < 0 if image is on same side as where rays enter (virtual image)
Figure 5: The Lens Equa on The most useful equa on when dealing with mirrors and lenses is called the lens equa on. This equa on works well, as long as the mirror you are working with is not too curved (meaning, small in size compared to the radius of its curvature) and if the lens is thin. It relates the focal length f, the object distance, so , and the image distance, si.
The following sign conven ons allow you to use this equa on with both mirrors and lenses. In gen‐ eral, real images are said to have posi ve distances, and virtual images are said to have nega ve dis‐ tances.
Example Lens Equa on Calcula on: What image is produced when placing an object 9 cm. away from a convex lens that is 3 cm. long. Given: f = 3 cm. so = 9 cm. We need to solve for si to determine the image length. To do this, plug in the known variables and iso‐ late si on one side of the equa on. 1. 1 = 1 + 1 3 si 9 2. 3 ‐ 1 = 2 = 1 9 9 9 si 3. 9 = si 2 1 Answer: Si = 4.5 cm
1 = 1 + 1 f si so
f = ‐ C 2
f = C 2
72
Lab 6: Light
A ray diagram is helpful for showing how to find where images will form. Generally, three rays can be used to locate the image formed by a mirror or a lens. The following examples in Figures 6‐8 will give you a be er picture of how mirrors and lenses affect rays of light from objects.
Figure 6: A real image formed by a concave mir‐ ror. Note the inverted
orienta on and the mag‐ nifica on.
Example Ray Diagrams
Figure 7: A virtual image is formed in a
convex mirror.
73
Lab 6: Light
Experiment 1: Ray Diagrams To complete this lab, you will need to draw three, separate ray diagrams. The start of each diagram has been provided for you in the beginning of Procedure 1, Procedure 2, and Procedure 3, respec vely. It is important that you use a ruler when drawing to ensure that each diagram reflects the correct dimensions (listed at the top of every diagram.) When drawing your diagrams, remember that the distances measured along the axis should begin at the center of each lens (convex or concave). For example, a focal point that is marked at 5 cm should be posi‐ oned 5 cm away from the center of the lens. The diagrams indicate if the focal point or object is placed
to the right or le of the lens.
Note: The size of your computer screen and the amount of “zoom” perspec ve you have applied to the manual will affect the scales of the diagrams. It is important for you to rely on the numbers provided at the top of each diagram, rather than measuring the dimensions of the images provided in the manual, to create your diagram. When you have completed your diagram, take a picture of it (using camera phone, digital camera, webcam, etc.) or scan the image onto your computer. These diagrams should be included in the final doc‐ ument you submit with your post‐lab ques ons.
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