Beacon Lesson Plan Library

How Stuff Is Put Together (Chemical Bonding)

Richard Angelini Sr.

Description

All compounds are made of combinations of elements held together by bonds in exact proportion. The demonstration of a simple experiment illustrates the ratio of the elements that make up the common chemical compound of water.

Objectives

The student knows that connections (bonds) form between substances when outer-shell electrons are either transferred or shared between their atoms, changing the properties of substances.

Materials

Per team of four students or one set for teacher demonstration:
-1 Glass beaker or glass measuring cup
-3 Feet of insulated wire
-3 Small 9-volt batteries (the kind used in radios)
-2 New pencils (You can substitute insulated wires, but pencils are more dramatic.)
-1 Piece of cardboard, 6” x 6” square
-Tape
-Salt
-Water
-Pre-Post Assessment, one per student (See Associated File)
-Directions for Experiment, one per student (See Associated File)

For the teacher:
-1 Two liter empty soda bottle
-2 One liter empty soda bottles

Preparations

1. Place two holes the size of a pencil diameter about 1 inch apart in the cardboard.
2. Break off the eraser and the metal from the end of both pencils and sharpen both ends of each pencil.
3. Hook up the 9-volt batteries in parallel (from minus pole to minus pole and from positive pole to positive pole). This will give you lots of energy and in only 9 volts for safety.
4. Mix lots of salt into some water.
5. Download and copy the Pre-Post Assessment and the Directions for Experiment for each student. (See Associated File)

Procedures

PRIOR KNOWLEDGE
1. It is helpful, but not required, that students understand the electron shell theory of atoms. (This experiment is also a good introduction into shell theory.)

2. It is valuable to the learning process that the student knows what he/she does not know and what they want to learn. Administer the Pre-Assessment (See Associated File) at this time for this purpose.

CLASS ORGANIZATION
If this experiment is to be done by the students, I recommend they be organized into teams of four students. Students should keep paper and pencil handy to jot down notes and answers to questions during the discussion and activity. (See Assessments for guide)

INTRODUCTION/HOOK
1. Hold up the two soda bottles. Ask the students, “If the 1-liter bottle was filled with oxygen and the 2-liter bottle was filled with hydrogen, then I added some energy and mixed them together, what would I get?” Some student will say water. If no student answers, tell them it makes water.

2. Next ask the students, “But how much water will be created? One, two or three liters?” (Answer: one liter, but don’t tell them yet. Wait to see if any student will ask later in the experiment and assess according to the formative guidelines. It is important to understand that one liter of water vapor will be created. The vapor will quickly cool and will shrink in volume greatly.)

PROCEDURE
1. Tell the students, “All compounds are made of exact ratios of elements. They are held together by the attraction to the electrons in their atoms. The ratio must be exact.”

2. Next perform the experiment:
a. Fill the beaker 2/3 full with salt water.
b. Place the cardboard over the beaker and insert the 2 pencils through the cardboard and 1 inch into the water.
c. Attach the batteries in parallel (all negatives together and all positives together) and attach one wire to each of the pencils.
d. Observe bubbles collecting around the points of the pencils in the salt water.
*** Oxygen will attract to the pencil hooked up to the positive terminal.
*** Hydrogen will attract to the pencil hooked up to the negative terminal.

3. Tell the students, “This is called electrolysis. It is the way electricity is used to break down water into its elements. It takes energy to form molecules and it takes energy to break down molecules.” Ask the students, “What do you observe? Is one pencil attracting more bubbles than the other? How much more?” (2 times as many bubbles of hydrogen)

4. Ask the students, “What is the chemical equation for water?” (H2O) What does the 2 represent in the equation? (2 hydrogen atoms for 1 oxygen atom) Here you can see the formula for water.”

5. Go to the board. Tell the students, “Hydrogen and oxygen are reactants and water is the product. The catalyst is usually heat when creating the molecule. They share electrons. This is called a chemical bond.” Write on the board: H + H + O = H2O. Tell the students, “This is the chemical equation for water.”

6. Ask the students to guess at the following questions, but they must explain their guess:
a. What happens to any extra oxygen or hydrogen atoms that cannot join into a water molecule? (They will join into a molecule that requires less energy.)
b. What would keep an atom from joining into a molecule? (No other atoms within the reach of available energy that match its electron needs.)
c. How do the hydrogen and oxygen atoms share electrons? (See #7)

7. Tell the students: “Chemical bonds are the sharing of electrons. In the hydrogen atom there is only one electron and it wants to have two. In the oxygen atom there are six electrons and it wants to have eight. The hydrogen atoms share their two electrons with the oxygen atom to form a chemical bond. Current theory holds that the two hydrogen electrons orbit both the hydrogen atom to which they belong and the oxygen atom in a kind of figure eight pattern.”

8. Ask the students, “What was the catalyst in this experiment?” (Electricity) “What is the reactant?” (Water) “What are the products?” (Hydrogen and oxygen) This is the electrolysis of water, the breaking down of water. In the creation of water, the reactants and product are reversed.

9. Line up the two 1-liter bottles and the 2-liter bottle and tell the students, “Let’s pretend that the two 1-liter bottles are filled with hydrogen and the 2-liter bottle is filled with oxygen. These will make one liter of water. The atoms will join closely together. The energy added will cause this to happen. When oxygen and hydrogen combine to form water, they use heat as the catalyst.”

10. Make sure that you have asked the questions, reviewed the answers and have covered the concepts on the Post-Assessment in the associated file prior to giving it to students.

Assessments

1. As a formative assessment, give points as follows:

-Hook (5 points): 2 liters of hydrogen and 1 liter of oxygen produce 1 liter of water
-Step 3 (3 points): observing two times as many bubbles being formed on the negative side
-Step 4 (2 points): knowledge of the chemical equation for water
-Step 6 (4 points): for each of two questions a and b (Question c is used only as in introduction to Step 7.)
-Step 8 (2 points): for each question--catalyst, reactant, products

Note: Use the points as you choose.

2. Use the Post-Assessment (See Associated File) when you feel that students understand the concept and are ready to be assessed.

Extensions

The logical progression in the study would be to learn Lewis diagrams and shell theory. Though not covered in this lesson, it is a logical next step in the study of the subject.

Web Links

Web supplement for How Stuff Is Put Together (Chemical Bonding)
Miniscience Projects

Web supplement for How Stuff Is Put Together (Chemical Bonding)
Water Experiments

Attached Files

The assessment of five questions.     File Extension: pdf

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