3.02 Chemical Reactions

Two new gases, carbon dioxide and water vapor, have formed.

In the burning propane example, how do you know that a chemical reaction has taken place?

Exothermic. An exothermic reaction releases energy, and a combustion reaction produces light and heat, which are forms of energy.

Is a combustion reaction an endothermic or exothermic reaction? Why?

It is an exothermic reaction because it releases heat.

Is burning propane an endothermic or exothermic reaction?

b. A chemical reaction has not occurred. The color mixing does not indicate a color change because red and yellow make orange. Mixing two primary colors to form another color is not a chemical reaction.

1. A student mixes a red liquid and a yellow liquid. The solution turns orange. Which of these is correct? a. A chemical reaction has occurred. A color change has occurred. b. A chemical reaction has not occurred. The color mixing does not indicate a color change because red and yellow make orange. c. There is not enough information to tell if a reaction has occurred.

a. A chemical reaction has occurred. Formation of a precipitate indicates a chemical reaction has occurred. A precipitate is a solid that has formed when two liquids are mixed together.

2. Doc Brown is trying to perform a reaction in the lab. After mixing two clear liquids together, a white solid forms. Which of these is true? a. A chemical reaction has occurred. Formation of a precipitate indicates a chemical reaction has occurred. b. A chemical reaction has not occurred. A precipitate is not an indicator of a chemical reaction. c. There is not enough information to tell if a reaction has occurred.

a. A chemical reaction has occurred. A gas is forming to inflate the air bag. The chemicals in the air bag are combining to produce a new gas. This is evidence of a chemical reaction.

3. Three solid ingredients are mixed together to make nitrogen gas to fill an air bag during deployment. Which of these is true? a. A chemical reaction has occurred. A gas is forming to inflate the air bag. b. A chemical reaction has not occurred. Nitrogen gas is found in the air. c. There is not enough information to tell if a reaction has occurred.

b. A chemical reaction has not occurred. The gas that is bubbling is still water. It is just changing state. . The gas is still water; it's just in a different phase.

4. When you heat a pot of water on the stove, it bubbles. Which of these is true? a. A chemical reaction has occurred. A gas is forming. b. A chemical reaction has not occurred. The gas that is bubbling is still water. It is just changing state. c. There is not enough information to tell if a reaction has occurred.

1. formation of a precipitate, 2. formation of a gas, 3. a change in color. 4. release of heat or light

How can you know when a chemical reaction has occurred?

Propane molecules and oxygen molecules combine. Existing bonds are broken and new bonds are made, forming carbon dioxide and water molecules.

How would you describe the chemical reaction of burning propane?

The reaction is endothermic. It absorbs more energy than it releases.

If a chemical reaction occurs in a flask, and the flask becomes colder, was the reaction endothermic or exothermic? Explain.

a chemical reaction in which energy is released to its surroundings

exothermic reaction

A solid that forms as a result of a chemical reaction. A white solid precipitate may form when aluminum chloride is added to a substance that contains aluminum.

precipitate (prih-SIH-puh-tayt)

the molecules that result from a chemical reaction

products

the starting molecules in a chemical reaction

reactants

a. A chemical reaction has taken place. Emission of light is an indicator that a chemical reaction has occurred. Light is a form of energy that is released during an exothermic chemical reaction.

5. At a celebration, fireworks light up the night sky. Which of these is true? a. A chemical reaction has taken place. Emission of light is an indicator that a chemical reaction has occurred. b. A chemical reaction has not occurred. Light does not indicate that a chemical reaction has occurred. c. There is not enough information to tell if a reaction has occurred.

a. A chemical reaction has taken place. Color change is an indicator that a chemical reaction has occurred. A change in color can indicate evidence of a chemical reaction.

6. In autumn the leaves change from green to bright red and yellow. Which of these is true? a. A chemical reaction has taken place. Color change is an indicator that a chemical reaction has occurred. b. A chemical reaction has not occurred. The color change is not an indicator of a chemical reaction. c. There is not enough information to tell if a reaction has occurred.

c. Both examples indicate that a chemical reaction has occurred. Formation of a gas and emission of light are two indicators that a chemical reaction has occurred.

7. Which of these indicates that a chemical reaction has occurred? a. Light is released from burning a piece of wood. b. Carbon dioxide gas is released from adding an antacid tablet to water. c. Both examples indicate that a chemical reaction has occurred.

a. A silver car changes to brown because of rusting. Rusting causes iron to change from silver to reddish brown. A color change can be an indicator that a chemical reaction has occurred.

8. Which of these indicates that a chemical reaction has occurred? a. A silver car changes to brown because of rusting. b. Red and blue paint are mixed to form purple paint. c. Both examples indicate that a chemical reaction has occurred.

process of breaking and forming bonds

Chemical Reaction

Bonds break between reactants and then form again to create products. This process of breaking and forming bonds occurs in a chemical reaction.

Describe how chemical reactions involve breaking and reforming bonds (either ionic or covalent).

a chemical reaction in which energy is absorbed from its surroundings

Endothermic

a chemical reaction in which energy is released to its surroundings

Exothermic

Energy is always involved in chemical reactions. Exothermic reactions release heat and light energy. Endothermic reactions absorb more energy from its surroundings than it releases.

Explain that energy is always involved in chemical reactions either as absorption or release of heat.

the final substances that result from a chemical reaction

Products

the starting substances in a chemical reaction

Reactants

A substance that separates in the form of solid particles from a liquid is called a precipitate. When potassium iodide is added to lead nitrate, a bright yellow precipitate, lead iodide, is formed.

Recognize that chemical reactions may involve the formation of a precipitate, the generation of gas, or a change in color.

Exothermic reactions release energy and Endothermic reactions absorb energy.

Understand the difference between an exothermic reaction and an endothermic reaction.

Burning propane is a combustion reaction.

What type of chemical reaction is burning propane?

In the case of a rotting tree, the tree is being broken down into simpler substances which are absorbed into the soil. The tree is more complex than its products.

Why is a rotting tree considered a decomposition reaction?

If a chemical reaction occurs in a flask, and the flask becomes warm, was the reaction exothermic? Why?

Yes. The reaction is exothermic because it releases energy as heat. This is why the flask feels warm.

an exothermic reaction in which oxides are usually formed

combustion reaction

a kind of chemical reaction in which a single compound is broken down into two or more simpler compounds

decomposition reaction

a chemical reaction in which energy is absorbed from its surroundings

endothermic reaction