CHEMICAL CHANGE 17.2

Chapter Seventeen: Chemical
Change
 17.1 Chemical Reactions
 17.2 Balancing Equations
 17.3 Classifying Reactions

Chapter 17.2 Learning Goals
 Relate a balanced chemical equation to the law of conservation of mass.
 Determine the formula and molar masses of chemical compounds.
 Write and balance chemical equations.

Investigation 17B
Conservation of Mass
 Key Question:
How do scientists describe what happens in a chemical reaction?

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
 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 10 23 atoms or molecules .

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

Solving Problems
What is the molar mass of one mole of
CaCO
3
?
1.
Looking for:
 … molar mass of CaCO
3
2.
Given
 … chemical formula
3.
Relationships:
 no. amu in formula = molar mass in grams

Solving Problems
4.
Solution
 Use periodic table and round values as needed.
Formula mass CaC0
3
= 100.19 g
1 mole CaC0
3
= 100.19 g CaCO
3

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.” to produce
Reactants Products
“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 carbon atoms?
How many hydrogen atoms?
How many oxygen atoms?

17.2 Balancing chemical equations
 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.
 We can practice balancing equations using
CPO periodic table tiles and pencil and paper .

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

17.2 Steps for balancing
4.
Add coefficients to balance the equation.

Solving Problems
 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:
Cu
2
S + O
2
→ Cu
2
O + SO
2

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