Chapter 11: Stoichiometry Section 11.1: Defining Stoichiometry

Objectives: Identify the quantitative relationships in a balanced chemical equation Determine the mole ratios from a balanced chemical equation

What is stoichiometry? From observing the demonstration, what do you conclude is the reason the reaction stops? How would you be able to determine the amount of oxygen that was used up? How would you be able to determine the amounts of products formed? Answers to both questions depends on the change in mass of the _________ .

What is stoichiometry? A chemist knows that a chemical reaction stops when one of the reactants is used up. Therefore, the amounts of other reactants used or products formed depends on the amount of reactant that is used up. Stoichiometric calculations allow us to use this information to determine the amounts of other reactants used or products formed in a reaction. So in planning a chemical reaction the chemist needs to know how many grams of potassium permanganate are needed to react completely with a known mass of sodium hydrogen sulfite.

What is stoichiometry? Stoichiometry is the study of quantitative relationships between amounts of reactants used and the products formed by a chemical reaction. It is based on the Law of Conservation of Mass: the amount of matter present at the end of a reaction is the same as was present at the beginning. The total mass of the reactants equals the mass of the products. Stoichiometry is based on the law of conservation of mass which was introduced by Antoine Lavoisier in the 18th century.

Interpretation of Chemical Equations The coefficients in a chemical equation can be interpreted several ways. We have learned to interpret them in terms of representative particles.

The coefficients represent not only numbers of individual particles but also numbers of moles of particles.

What happens is you multiply each coefficient by 6.02 x 1023 particles?

Each coefficient is now “number of Does the equation tell you anything about the masses of the reactants and products? Not directly. But the mass of any substance can be determined by multiplying the number of moles of the substance by the conversion factor that relates mass and number of moles, which is the molar mass. Do mass calculations: Each coefficient is now “number of moles”! We can, therefore, interpret coefficients as numbers of moles!

Interpret this equation: C3H8(g) + 5O2(g) 3CO2(g) + 4H2O(g) The equation in terms of molecules = ? The equation in terms of moles = ? The equation in terms of mass = ? Coefficients indicate number of molecules Coefficients indicate the number of moles Calculate the mass of each reactant and product by multiplying the number of moles by the conversion factor molar mass.

Interpret this equation: C3H8(g) + 5O2(g) 3CO2(g) + 4H2O(g) The equation in terms of molecules = ? The equation in terms of moles = ? The equation in terms of mass = ? 44 g + 160 g = 132 g + 72 g 204 g = 204 g Coefficients indicate number of molecules Coefficients indicate the number of moles Calculate the mass of each reactant and product by multiplying the number of moles by the conversion factor molar mass.

Practice Problems Interpret each equation in terms of particles, moles, and mass. Show that the Law of Conservation of Mass is obeyed. 4Fe + 3O2 --> 2Fe2O3 N2 + 3H2 --> 2NH3

Mole Ratios In the following reaction, how many moles of Al will react with 3 moles of Br2? How many moles of AlBr3 are formed? 2Al(s) + 3Br2(l) → 2AlBr3(s) The relationships above can be expressed as MOLE RATIOS. A Mole Ratio is a ratio between the numbers of moles of any two substances in a balanced chemical equation. 2 mol Al/3 mol Br2 2 mol Al/2 mol AlBr3 3 mol Br2/2 mol Al 2 mol AlBr3/3 mol Br2 2 mol AlBr3/2 mol Al

Mole Ratios 2Al(s) + 3Br2(l) → 2AlBr3(s) Mole ratios are: 2 mol Al and 3 mol Br2 3 mol Br2 2 mol Al 2 mol Al and 2 mol AlBr3 2 mol AlBr3 2 mol Al 3 mol Br2 and 2 mol AlBr3 2 mol AlBr3 3 mol Br2

Mole Ratios Mole ratios are used as conversion factors in stoichiometric calculations With the chemical equation and the mole ratios, you can calculate the amount used of any reactant in the equation and the maximum amount of product you can obtain.

Practice Problems Balance the equation and determine all the possible mole ratios. ZnO + HCl --> ZnCl2 + H2O Al + O2 --> Al2O3