Key Concepts:

A physical change, such as a state change or dissolving, does not create a new substance, but a chemical change does.In a chemical reaction, the atoms and molecules that interact with each other are called reactants.In a chemical reaction, the atoms and molecules produced by the reaction are called products. In a chemical reaction, only the atoms present in the reactants can end up in the products. No new atoms are created, and no atoms are destroyed.In a chemical reaction, reactants contact each other, bonds between atoms in the reactants are broken, and atoms rearrange and form new bonds to make the products.

You are watching: What happens to chemical bonds during chemical reactions?

Summary

The teacher will use a small candle flame to demonstrate a chemical reaction between the candle wax and oxygen in the air. Students will see a molecular animation of the combustion of methane and oxygen as a model of a similar reaction. Students will use atom model cut-outs to model the reaction and see that all the atoms in the reactants show up in the products.

Objective

Students will be able to explain that for a chemical reaction to take place, the bonds between atoms in the reactants are broken, the atoms rearrange, and new bonds between the atoms are formed to make the products. Students will also be able to explain that in a chemical reaction, no atoms are created or destroyed.

Evaluation

Download the student activity sheet, and distribute one per student when specified in the activity. The activity sheet will serve as the “Evaluate” component of each 5-E lesson plan.

Safety

Be sure you and the students wear properly fitting goggles. Be careful when lighting the candle. Be sure that the match and candle are completely extinguished when you are finished with the demonstration.

Materials for the Demonstration

Tea light candle or other small stable candle MatchesGlass jar, large enough to be placed over the candle

Materials for Each Student

Atom cut-outs from the activity sheetSheet of colored paper or construction paperColored pencilsScissorsGlue or tape
Engage

Review what happens during a physical change and introduce the idea of chemical change.

Tell students that in previous chapters they have studied different aspects of physical change. When atoms and molecules speed up or slow down, that is a physical change. When they change state from liquid to solid or from gas to liquid, that is a physical change. When a substance is dissolved by water or some other solvent, a new substance has not really been formed. The ions or molecules can still come back together to form the original substance.

Let students know that in this chapter they will explore what happens during a chemical change. In a chemical change, the atoms in the reactants rearrange themselves and bond together differently to form one or more new products with different characteristics than the reactants. When a new substance is formed, the change is called a chemical change.

As a demonstration, light a candle and explain what is happening using the terms reactants, products, and chemical reaction.

Explain that in most chemical reactions, two or more substances, called reactants, interact to create different substances called products. Tell students that burning a candle is an example of a chemical reaction.

Materials for the Demonstration

Tea light candle or other small stable candle MatchesGlass jar, large enough to be placed over the candle

Procedure

Carefully light a tea light candle or other small candle.Keep the candle burning as you ask students the questions below. You will put the candle out in the second part of the demonstration.

Expected Results

The wick will catch on fire and the flame will be sustained by the chemical reaction.

The following question is not easy and students are not expected to know the answer at this point. However, thinking about a candle burning in terms of a chemical reaction is a good place to start developing what it means when substances react chemically.

Ask students:

What do you think are the reactants in this chemical reaction? Wax and oxygen from the air are the reactants.

Students often say that the string or wick is burning. It is true that the string of the wick does burn but it’s the wax on the string and not so much the string itself that burns and keeps the candle burning. Explain that the molecules that make up the wax combine with oxygen from the air to make the products carbon dioxide and water vapor.

Point out to students that this is one of the major characteristics of a chemical reaction: In a chemical reaction, atoms in the reactants combine in new and different ways to form the molecules of the products.

Students may be surprised that water can be produced from combustion. Since we use water to extinguish a fire, it may seem strange that water is actually produced by combustion. You may want to let students know that when they “burn” food in their bodies, they also produce carbon dioxide and water.

Place a jar over the candle to help students realize that oxygen is a reactant in the burning of a candle.

Remind students that air is a mixture of gases. Explain that when something burns, it reacts with the oxygen in the air.

Ask students to make a prediction:

Will the candle still burn if one of the reactants (wax or oxygen) is no longer available? Students may guess that the candle will not burn because both reactants are required for the chemical reaction to continue.

Procedure

Carefully place a glass jar over the lit candle.

*

Expected Results

The flame goes out.

Ask students:

Why do you think the flame goes out when we put a jar over the candle? Placing a jar over the candle limits the amount of oxygen in the air around the candle. Without enough oxygen to react with the wax, the chemical reaction cannot take place and the candle cannot burn. When a candle burns for a while, it eventually gets smaller and smaller. Where does the candle wax go? When a candle burns, the candle wax seems to “disappear.” It doesn’t really disappear, though: It reacts chemically, and the new products go into the air.

Note: Some curious students may ask what the flame is made of. This is a great question and not trivial to answer. The flame is burning wax vapor. The light of the flame is caused by a process called chemiluminescence. Energy released in the chemical reaction makes electrons from different molecules move to a higher energy state. When the electrons come back down, energy is released in the form of light.

Explore

Have students make a model to show that in a chemical reaction the atoms of the reactants rearrange to form the products.

Question to Investigate

Where do the atoms in the products of a chemical reaction come from?

Materials for Each Student

Atom model cut-outs (carbon, oxygen, and hydrogen)Sheet of colored paper or construction paperColored pencilsScissorsGlue or tape

Procedure

Prepare the AtomsColor the carbon atoms black, the oxygen atoms red, and leave the hydrogen atoms white. Use scissors to carefully cut out the atoms.Build the Reactants

On a sheet of paper, place the atoms together to make the molecules of the reactants on the left side of the chemical equation for the combustion of methane.

*
Write the chemical formula under each molecule of the reactants. Also draw a + sign between the reactants.

After you are sure that students have made and written the formula for the reactant molecules, tell students that they will rearrange the atoms in the reactants to form the products.

Build the ProductsDraw an arrow after the second oxygen molecule to show that a chemical reaction is taking place.Rearrange the atoms in the reactants to make the molecules in the products on the right side of the arrow. Write the chemical formula under each molecule of the products. Also draw a + sign between the products.

Tell students that in a chemical reaction, the atoms in the reactants come apart, rearrange, and make new bonds to form the products.

Represent the Chemical EquationHave students use their remaining atoms to make the reactants again to represent the chemical reaction as a complete chemical equation. Glue or tape the atoms to the paper to make a more permanent chemical equation of the combustion of methane.Extend

Introduce two other combustion reactions and have students check to see whether or not they are balanced.

Tell students that, in addition to the wax and methane, some other common hydrocarbons are propane (the fuel in outdoor gas grills), and butane (the fuel in disposable lighters). Have students count the number of carbon, hydrogen, and oxygen atoms in the reactants and products of each equation to see if the equation is balanced. They should record the number of each type of atom in the chart on their activity sheet.

See more: How To Play Pal Games On Ntsc Ps2 Games To Ntsc, Can You Play Pal Games On An American Ps2

*

Lighting an outdoor gas grill—Combustion of propane

C3H8 + 5O2 → 3CO2 + 4H20

Using a disposable lighter—Combustion of butane

2C4H10 + 13O2 → 8CO2 + 10H2O

After students have counted up each type of atom, review their answers to make sure they know how to interpret subscripts and coefficients.