Lesson 3: Chemical Reactions

In this section of the lesson we will consider chemical reactions which, of course,are the heart and soul of chemistry. Before you get started with the experiments in the lab, be sure to review the safety precautions and techniques that were covered in Lesson 1. (The summary of safety procedures is listed as Ex. 1 in your workbook.) Then we will consider a variety of chemical reactions.

Let's go on now to some chemical reactions. We will preview several of these, so that I can point out some things to look for in the reactions. The pictures here will give you an indication of what happens in the reaction, but they will not take the place of observing the reactions yourself. You will repeat them in the lab to get a first hand look at the reactions.

 

We can either use small test tubes or a spot plate. The test tubes hold more materials; the spot plate holds less, but the spot plate is a whole lot easier to wash. (The following examples correspond with Exercises 2-4 in your workbook.)

 

 

Reactions Creating Materials in New Phases | Reactions Creating Materials with New Colors

Reactions Creating or Using Heat Energy | Describing Chemical Reactions |

Chemical Decomposition of Water | Study Check

 

Reactions Creating Materials in New Phases

First Example

I'll add a few drops each of silver nitrate solution and hydrochloric acid solution to one another and see what happens. (Exercise 2.a.) Notice that as the two solutions come in contact you get a milky white color. This is called a precipitate. It is actually a solid. There are little tiny bits of solid particles that form as the two chemicals come in contact with one another. After a bit, you will be able to see that the solid material settles out to the bottom.

 

Second Example

Next we add a few drops each of sodium bicarbonate solution and hydrochloric acid solution. (Exercise 2.b.) Notice that bubbles are formed. If you don't see them when you mix the solutions together, try it again--little, tiny bubbles.

 

 

 

Third Example

Next we work with solid sodium bicarbonate and add some hydrochloric acid to it. (Exercise 2.c.) Again, notice the bubbles. Also, depending on how much of the solid sodium bicarbonate we use, it might all disappear. If we use quite a bit, then we would need more hydrochloric acid in order to have all of that disappear.

 

 

 

Comments

Notice that there is something common to all three of these reactions. In each case we ended up with a phase that wasn't there before. Actually, we ended up with a new material that was in a phase that wasn't there before. For example, the silver nitrate solution and the hydrochloric acid solutions are both liquids. Mixing them together forms a solid. There is still liquid there, but there is also a solid that wasn't there before. In the second and third examples, the new material was a gas. Specifically, it was carbon dioxide bubbles. So the new material is in a phase that wasn't there before, and consequently, we were able to observe its presence. On the other hand, if we were to mix two liquids together and get a new material that is also a liquid, we may not be able to see that. But if the new material is in a phase that is not already present, then we will be able to detect that it is being formed. Also, when you do Part C, you may note that the solid that you start with may disappear.

Now or when you are in the lab, do these exercises so that you can observe them first-hand; and be sure to clean up your glassware when you are finished.

Top of page

Reactions Creating Materials with New Colors

Fourth Example

The next example is to put a few drops of sodium hydroxide solution into a test tube and add a drop of phenolphthalein solution to it. (Exercise 3.a.) Notice that we get a change in color. We start off with two things that are colorless, and we end up with a material that has a pink color to it. So there is a change. You get a new material that has different properties than the materials that you started with.

Fifth Example

Next, we take that same solution that we just created and start adding, drop by drop, some hydrochloric acid solution to it. (Exercise 3.b.) Notice that after enough hydrochloric acid is added, the pink color disappears. Again, there was a chemical reaction. We started with something that was pink and something that was colorless, and we ended up with something that was colorless. Whatever it was that was pink has disappeared. It is no longer present. It must have been changed to something else.

Sixth Example

You cannot always count on a change in perceived color as being a chemical reaction. (Exercise 3.c.) These two dye solutions are just food colors. If we mix together a little bit of the yellow and a little bit of the blue in a test tube we end up with green. We started with yellow and blue, not green. We put them together and get green. So in a sense, there is a change. However, this change is not from a chemical reaction. There is not a new material that has green as one of its properties. Instead we have the mixing of the yellow and the blue colors.

Comments

Sometimes a color change is the result of a chemical reaction. Sometimes it is just the result of mixing colors. One way of telling which is the case is your own personal experience. If you are familiar enough with mixing colors, you know that yellow and blue together make green. Therefore, when you mix the two together and you get green, it is just mixing the colors. On the other hand, if you were to mix yellow and blue and get red, you would know you are not supposed to get that color from mixing; you must have created something new. Therefore, that would be a chemical reaction.

Another way of investigating things like this is to use a spectroscope to look at what colors are present. When you mix two colors together, you get a new color. Now look at the last column of this data sheet, "Colors in the Spectrum." Each solution has a visible color because it absorbs some colors and lets others pass through. The mixture of solutions 1 and 2 absorbs the colors that either solution 1 or 2 will absorb, and lets through only the colors that both solutions 1 and 2 will let through. The green and some of the red in this case. Thus, we get a different color (or color mix) without having a new and different chemical present.

Material	Color	Colors in Spectrum
white light	white	V B G Y O R
solution 1	blue	V B G (R)
solution 2	yellow	G Y O R
mix 1 & 2	green	G (R)

 

Now or when you are in the lab, do these exercises so that you can see the reactions first hand and, of course, clean up your glassware.

Top of page

Reactions Creating or Using Heat Energy

Seventh Example

Here (to the left) we have one test tube with some water in it and then another test tube (in the rack) with some solid ammonium nitrate in it. The amounts are specified in your workbook in Exercise 4. The thermometer is used to get the temperature of the water. (The reason we have you use 3 ml of water is to make sure you have enough to cover the bulb of the thermometer.) Note the temperature. (you may not be able to make it out here, but you will in the lab.) After the ammonium nitrate from the other test tube has been poured into the water (to the right), the thermometer is used to gently mix the solid and the liquid. The solid ammonium nitrate dissolves and as it does so the temperature goes down.

Comments

With some chemical reactions, heat is given off during the reaction and the temperature goes up. In this particular one, heat is used up; therefore, the temperature goes down.

Now, or when you are in the lab, do this exercise so that you can see the temperature change first hand and, of course, clean up your glassware.

Top of page

Describing Chemical Reactions

Essentially, what we have done in the previous pages is to look at one kind of change at a time: change in phase, change in color, change in temperature. What you will do in the lab when you do Exercise 5 is note a variety of different changes. Sometimes you'll have combinations of changes. For each part of Exercise 5, write down a description of the materials that you start with, what happens when they are put together, and what things are like afterward. Do that now or when you are in the lab.

Now let's expand on the distinction between chemical reactions and physical changes that was alluded to earlier.

Chemical reactions involve changing the materials that you're dealing with into new materials, and the changes that you see are the result of the appearance of the new materials. The new material might have a different phase than the starting materials. For example, if you create something new from two liquids and it happens to be a gas, then you will get gas bubbles. There will be a new material with a new phase that wasn't there before. Also, you can observe chemical reactions by seeing color changes, if the new material has a different color than the starting materials. When a chemical reaction takes place, usually the temperature will also change. It will either go up or down, sometimes imperceptibly. Those are the kinds of changes that you might see. In chemical reactions the changes that you observe are caused by the creation of or loss of different kinds of materials.

Physical changes are similar in some ways because you are looking at the same kinds of things--changes in the phase, the color, the temperature. But the physical changes are changes in the condition of the material or changes caused by mixing materials together or taking them apart--just mixing or separating, not the creation of something new. For example, temperature changes are physical changes if they are caused by heating or cooling. If you add heat to something and the temperature goes up, that is a physical change. If you cool it off and the temperature goes down, that is a physical change. Phase changes are physical changes also, if they are caused by heating or cooling. If you take some water and you heat it up and it changes to gas, that is a physical change. You still have water, only now it is steam instead of liquid. Also included in physical changes are color changes that are caused simply by mixing or by diluting a material or, in some cases, even by heating. For example, if you turn on an electric range and the element gets hot, you can see a color change. That is also a physical change.

Physical changes and chemical changes have much in common in that you are looking at similar kinds of results. The difference is whether the appearances or differences that you see are from a change in the condition of the material or from a change in what materials are present. That's the kind of thing you need to look for to distinguish between a chemical reaction and a physical change. It takes an inference to make that distinction. Sometimes it is very difficult to make that distinction and in some cases even chemists can get into arguments sometimes over what constitutes a physical or a chemical change. For example, when salt dissolves in water, some people say that is simply a physical change because if you evaporate away the water, you have the salt back again. Other people argue that there is a chemical change because the material present in solution has different properties than the separated pure water and pure salt. The solution will conduct electricity and the pure water and pure salt will not. So, there are cases where the distinction between physical and chemical changes gets kind of blurred. In a few weeks we will have more criteria we can use to distinguish between physical and chemical changes.

Top of page

Chemical Decomposition of Water

Electrolysis of Water

Pure water is a material which can go through distillation without a change in it properties. That is, it is water before and it is water afterwards. If it started as impure water, as it goes through the process it is separated from the impurities and becomes pure water. You can distill pure water over and over again; you can freeze it; you can thaw it; and it remains water throughout all those phase changes. It can, however, be separated into two components by a process known as electrolysis. This process will decompose water into two components, both are gases.

We have an electrolysis apparatus set up with instructions for your operation at the demonstration table. Electrolysis uses an electric current to decompose water into its elements: hydrogen and oxygen. At room temperature and normal pressure, these two elements exist as gases; in the photo at far right, you can see the stream of gas bubbles being created at the electrodes.
	After it has been operating for a while and a sufficient amount of each gas has been formed, you will be able to collect it in a test tube by inverting the test tube over the opening and turning the stopcock to release the gas. When almost all of the gas is released into the test tube, close the stopcock and put your thumb over the end of the test tube and you've got the gas. Try to avoid having any liquid come up into the stopcock. You will collect two gases, one at a time, in separate test tubes.
You should test these gases one at a time by two different methods. One of the gases will pop when a flaming splint (a small piece of wood) is held next to the mouth of the test tube. The gas that does this is hydrogen.
The second test is to put a glowing splint into the test tube and observe whether the glow gets brighter or goes out. Oxygen will make the splint glow brighter. Sometimes it will actually make the splint light up again.

You should do these tests in this order: Hold a burning splint by the mouth of the test tube (to test for hydrogen); then, if it doesn't pop, blow out the flame and put the still glowing splint into the test tube (to test for oxygen). If you are in the lab, take time now to observe the decomposition of water by electrolysis and test the products. It is Exercise 6. If not, you can do that later.

Top of page

Study Check: Chemical Reactions

Determine whether each of the following observations indicates a chemical reaction or a physical change. Check your answers below.

1.  "A homogeneous, transparent, yellow liquid is mixed with a homogeneous, transparent, colorless liquid and a homogeneous opaque yellow solid is formed that settles to the bottom of the container with a homogeneous, transparent, pale yellow liquid remaining above it."

2.  "A homogeneous, opaque, red solid is placed in sunlight and after a period of time it becomes a homogeneous, opaque, orange solid."

3.  "A homogeneous, colorless, transparent solid at -5^oC is placed in sunlight and after a period of time becomes a homogeneous colorless transparent liquid at +5^oC."

Answers to Quick Quiz on Chemical Reactions

1.  "A homogeneous, transparent, yellow liquid is mixed with a homogeneous, transparent, colorless liquid and a homogeneous opaque yellow solid is formed that settles to the bottom of the container with a homogeneous, transparent, pale yellow liquid remaining above it." Chemical reaction.

2.  "A homogeneous, opaque, red solid is placed in sunlight and after a period of time it becomes a homogeneous, opaque, orange solid." Chemical reaction.

3.  "A homogeneous, colorless, transparent solid at -5^oC is placed in sunlight an after a period of time becomes a homogeneous colorless transparent liquid at +5^oC." Physical change, not a chemical reaction.

 

Top of page