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.

To introduce you to chemical reactions, I will discuss several of the procedures you will be doing in the Tutorial Lab this week. While I am including some pictures of these processes, the best way to read through this section of the textbook is to actually go to the lab and do the procedures yourself as they are mentioned. This will give you the full experience of seeing and feeling them firsthand. If you decide to do the lesson this way (as is recommended), you should read through the lab packet before beginning and record observations you make where appropriate.

Indications Of a Chemical Reaction

Phases of Matter

To start, add a few drops each of silver nitrate solution and hydrochloric acid solution to one another and see what happens. 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.

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

Eventually the bubbles disappear, but that doesn't mean the gas formed is gone, it just means it has escaped from the test tube and mixed into the atmosphere.

Next we work with solid sodium bicarbonate and add some hydrochloric acid to it. 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.

As before, the gas that is formed is still present after the bubbling stops. It has mixed with the air in the room.

In all of these processes, a new phase of matter was formed. In the first example it is a solid (the precipitate) and in the last two it is a gas. Formation of a new phase (solid, liquid, or gas) is a common indicator of a chemical reaction. However, note that a new phase does not always indicate a chemical reaction. For example, ice melting from solid to liquid is not a chemical reaction. A chemical reaction is indicated when a new phase is formed without applying or removing heat.

Color Change

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. 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.

Next, we take that same solution that we just created and start adding, drop by drop, some hydrochloric acid solution to it. 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.

You cannot always count on a change in perceived color as being a chemical reaction. These two dye solutions in the image 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.

A color change is another common indicator of a chemical reaction. Sometimes it is just the result of mixing colors, however. 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.

Absorbing or Releasing Heat

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. The thermometer is used to get the temperature of the water. 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, 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.

With some chemical reactions, heat is given off during the reaction and the temperature goes up. You will do another similar procedure in this lab for such a reaction. In this particular one, heat is absorbed; therefore, the temperature goes down.

What is a Chemical Reaction?

These procedures have hopefully familiarized you with the signs of a chemical reaction: production of a new phase of matter, color change, and absorption or release of heat (or other forms of energy). But what is a chemical reaction?

Chemical reactions involve changing the substances that you're dealing with into new substances, 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.

Chemical reactions (also known as chemical changes) are contrasted with physical changes. 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 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.

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.

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

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.

Google Sites

Report abuse