Chemical Reactions

Chapter 20

Physical Changes

A physical change is a change that affects only the physical properties of a substance.

These properties include:•Size•Shape•State (liquid vs solid)

Chemical Changes

A chemical change is a change in substance that involves breaking and reforming bonds to make one or more different substances.

Four indicators of chemical change are:

Chemical Changes

1. Formation of a new gas

2. Formation of a new solid3. Release of energy (heat or light)4. Color Change

• In chemical reactions, you start with reactants that are combined to make products.

Chemical Reactions

-The reactants are the starting substances

-The products are the new substances which result from the chemical reaction

Chemical Reactions

Reactants Products

Chemical Reactions

• In this reaction, methane (natural gas) is burned or combusted.

• Some energy is added to get the reaction started.

Chemical Reactions

Reaction Symbols

• The small symbols in the parentheses (s, l, g, aq) next to each chemical formula indicate the phase of each substance in the reaction.

• An arrow is always included between reactants and products.

• It means “to produce” or “to yield.”

Reactants

to produce

Products

“Methane combines with oxygen gas to produce carbon dioxide gas and water vapor.”

Reaction Symbols

• Antoine Laurent Lavoisier, established an important principal based on his experiments with chemical reactions.

• He stated that the total mass of the products of a reaction is equal to the total mass of the reactants.

• The law of conservation of mass holds true for even a burning mass of wood.

Conservation of Mass (atoms)

• The combined mass of the burning wood and oxygen is converted into carbon dioxide and water.

Conservation of Mass (atoms)

• Lavoisier showed that a closed system must be used when studying chemical reactions.

• When chemicals are reacted in a closed container, you can show that the mass before and after the reaction is the same.

Conservation of Mass (atoms)

• The law conservation of mass is applied by balancing the number and type of atoms on either side of the equation.

Balancing Reactions

Balancing Reactions

• Counting atoms is necessary to balance an equation.

How many carbon atoms?

How many hydrogen atoms?

How many oxygen atoms?

Balancing Reactions• A balanced chemical equation has the

same number of each type of atom on the product side and the reactant side.

• To balance the equation, we add another water molecule to the product side and add another oxygen molecule to the reactant side.

Type of atom in methane reaction

Total on Reactants side

Total on Products side

Balanced?

C 1 1 Yes

H 4 4 Yes

O 4 4 Yes

Balancing Reactions

1. If not provided, write the word form of the equation.

2. If not provided, write the chemical equation from the word form.

3. Count the number of each type of atom on both sides.

4. Add coefficients to balance the equation.

Steps for Balancing:

Balancing Reactions

Calcium carbonate reacts with hydrochloric acid to produce calcium chloride, carbon dioxide and water.

Step 2:

Step 1:

Balancing ReactionsStep 3:

Step 4:

Types of Reactions

There are 6 types of chemical reactions.

1. Addition/Synthesis Reactions

2. Decomposition Reactions3. Single Displacement Reactions

4. Double Displacement Reactions

6. Combustion Reactions

5. Precipitation Reactions

Addition/Synthesis Reactions

• In an addition reaction, two or more substances combine to form a new compound.

A + B -----> AB

Fe (s) + O2 (g) -----> Fe2O3 (s)

Remember to balance!

Fe + O2 -----> Fe2O34 3 2

Addition/Synthesis Reactions

• A chemical reaction in which a single compound is broken down to produce two or more smaller compounds is called a decomposition reaction.

Decomposition Reactions

Decomposition Reactions

AB -energy-> A + B

H2O (l)

-electricity-> H2 (g) + O2 (g)2 2

• In a single-displacement reaction, one element replaces a similar element in a compound.

Single Displacement Reactions

Double Displacement Reactions

• In a double-displacement reaction, ions from two compounds in solution exchange places to produce two new compounds.

• One of the compounds formed is usually a precipitate that settles out of the solution, a gas that bubbles out of the solution, or a molecular compound such as water.

AB + CD ---> AC + BD

Pb(NO3)2 + KI ---> PbI2 + KNO32 2

Double Displacement Reactions

• A precipitate is a new solid product that comes out of solution in a chemical reaction.

• The formation of a cloudy precipitate is evidence that a double-displacement reaction has occurred.

Precipitation Reactions

• The limewater test for carbon dioxide is a precipitation reaction.

Precipitation Reactions

• A combustion reaction, also called burning, occurs when a substance such as wood, natural gas, or propane combines with oxygen and releases a large amount of energy in the form of light and heat.

Combustion Reactions

CxHy + O2 ---> CO2 + H2O + energy

C6H12O6 + O2 ---> CO2 + H2O66 6

+ energy

Combustion Reactions

Types of Reactions