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Chemical Reactions

Iron is abundant, easy to shape when heated, and relatively strong, especially when mixed with carbon in steel. Over time, objects made of iron will rust if they are left exposed to air. You will learn to recognize chemical changes and to distinguish them from physical changes.

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Chemical Reactions >

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Chemical Changes

Chemical Changes

What always happens during a chemical change?

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Chemical Reactions > Chemical Changes

The ability of a substance to undergo a specific chemical change is called a chemical property.

Chemical properties (a type of intensive property) can be used to identify a substance. But chemical properties can be observed only when a substance undergoes a chemical change.

EX: “reacts with oxygen” or “does not react with sodium”

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Chemical Reactions > Chemical Changes

During a chemical change, the composition of matter always changes.

Recall that during a physical change, the composition of matter never changes.

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Chemical Reactions > Chemical Changes

A magnet separates iron from sulfur. This is an example of a physical change.

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Chemical Reactions > Chemical Changes

A mixture of iron and sulfur is heated. The iron and sulfur react and form iron sulfide. This is an example of a chemical change.

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Chemical Reactions > Chemical Changes

• A chemical change is also called a chemical reaction.

• One or more substances change into one or more new substances during a chemical reaction.

• A substance present at the start of the reaction is a reactant.

• A substance produced in the reaction is a product.

• The product(s) has different intensive properties than the reactant(s) because there is a change in composition (arrangement of atoms)

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Chemical Reactions >

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Recognizing Chemical Changes2.4

Recognizing Chemical Changes

What are four possible clues that a chemical change has taken place?

What is the only true method to verify that chemical change has taken place?

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Chemical Reactions > Recognizing Chemical Changes

Possible clues to chemical change include:

•a transfer of energy (such as light or a change in temp)

•a change in color

•the production of a gas*

•the formation of a precipitate*

*if these phases were not present in the reactants

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Chemical Reactions > Chemical Changes

A precipitate is a solid that forms and settles out of a liquid mixture.

Clues to chemical changes have practical applications.

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Chemical Reactions > Recognizing Chemical Changes2.4Recognizing Chemical Changes

What is the only true method to verify that chemical change has taken place?

Verify a change in intensive properties

Ex: Sulfur is odorous and Iron is magnetic, but the compound iron sulfide is not magnetic and does not have an odor

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Chemical Reactions >

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Conservation of Mass2.4

Conservation of Mass

How are the mass of the reactants and the mass of the products of a chemical reaction related?

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Chemical Reactions > Conservation of Mass2.4

During any chemical reaction, the mass of the products is always equal to the mass of the reactants.

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Chemical Reactions > Conservation of Mass

The law of conservation of mass states that in any physical change or chemical reaction, mass is conserved.

The conservation of mass is easily observed when a change occurs in a closed container.

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Chemical Reactions > Conservation of Mass2.4

Example MME question:

7 g of iron (Fe) reacts completely with 32 g of phosphorus (P) to form iron phosphide (Fe3P).

a)Write the chemical equation:

3Fe + P Fe3P

b) Circle the reactant, underline the product

C) What is the mass of the product? Show the reasoning:

Mr = Mp 7g + 32 g = Mp Mp = 39 g

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Section Quiz

-or-Continue to: Launch:

Assess students’ understanding of the concepts in Section

2.4 Section Quiz.

2.4.

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2.4 Section Quiz.

1. Which of the following is a chemical reaction?

a. melting of lead

b. dissolving sugar in water

c. rusting of iron

d. crushing of stone

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2. Which of the following is NOT a possible clue that a chemical change is taking place?

a. a change of state

b. a change in color

c. production of a gas

d. formation of a precipitate

2.4 Section Quiz.

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3. During any chemical change, the mass of the products is

a. always equal to the mass of the reactants.

b. always greater than the mass of the reactants.

c. always less than the mass of the reactants.

d. sometimes different than the mass of the reactants.

2.4 Section Quiz.

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Chemical Reactions > Concept Map 2

Concept Map 2 Solve the Concept Map with the help of an interactive guided tutorial.

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