Chapter 17 – Chemical Change

17.2 – Balancing Equationspp. 388-397

17.2 Balancing Equations

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.

17.2 Balancing Equations

17.2 Balancing Equations

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

17.2 Conservation of mass 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.

17.2 Formula mass

The sum of the atomic mass values of the atoms in a chemical formula is called the formula mass.

17.2 Avogadro’s Number The Avogadro number was named in honor

of Amedeo Avogadro who discovered that a mole of any gas under the same conditions has the same number of molecules.

Johann Josef Loschmidt, a German physicist, named and discovered the Avogadro number.

Loschmidt realized that a mole of any substance—be it a gas, liquid, or solid—contains 6.02 x 1023 atoms or molecules.

17.2 Molar Mass

The mass (in grams) of one mole of a compound is called its molar mass.

What is the molar mass of one mole of CaCO3?

1. Looking for: … molar mass of CaCO3

2. Given … chemical formula

3. Relationships: no. amu in formula = molar mass in

grams

Solving Problems

4. Solution

Solving Problems

Formula mass CaC03 = 100.09 g

1 mole CaC03 = 100.09 g CaCO3

17.2 Chemical Equations When a chemical reaction is written

using chemical formulas and symbols, it is called a chemical equation.

17.2 Chemical equations

An arrow is always included between reactants and products.

It means “to produce” or “to yield.”

Reactants Products

to produce

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

17.2 Numbers in equations

17.2 Balancing equations The law conservation of mass is applied by

balancing the number and type of atoms on either side of the equation.

17.2 Balancing equations Counting atoms is necessary to

balance an equation.

How many hydrogen atoms?

How many oxygen atoms?

How many carbon atoms?

17.2 Steps for balancing

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

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

17.2 Steps for balancing

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

17.2 Steps for balancing3. Count the number of each type of

atom on both sides.

17.2 Steps for balancing4. Add coefficients to balance the

equation.

In this reaction, chalcocite (a mineral) reacts with oxygen in the presence of heat. The products are a type of copper oxide and sulfur dioxide. Balance this equation:

Cu2S + O2 → Cu2O + SO2

Solving Problems

1. Looking for: …the coefficients for each molecule

2. Given … chemical formulas which show

types and no. of atoms

Solving Problems

3. Relationships Coefficients can be added in front of any

chemical formula in a chemical equation. When a coefficient is added in front of a

chemical formula, all atoms in that formula are multiplied by that number.

Use common denominators to help choose coefficients to try.

Solving Problems

4. Solution- Trial and error

Solving Problems