Chapter 17 – Chemical Change 17.2 – Balancing Equations pp. 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? Looking for: Given Solving Problems What is the molar mass of one mole of CaCO3? Looking for: … molar mass of CaCO3 Given … chemical formula Relationships: no. amu in formula = molar mass in grams

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.” 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 Steps for balancing 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 If not provided, write the chemical equation from the word form.

17.2 Steps for balancing Count the number of each type of atom on both sides.

17.2 Steps for balancing 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: Cu2S + O2 → Cu2O + SO2

Looking for: Given …the coefficients for each molecule Solving Problems Looking for: …the coefficients for each molecule Given … chemical formulas which show types and no. of atoms

Solving Problems 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.

Solution- Trial and error Solving Problems Solution- Trial and error