SCH3U 5.2 Introduction to Stoichiometry

What is Stoichiometry? Stoichiometry is the study of the quantities involved in chemical reactions. The word stoichiometry means “measuring the elements” Stoichiometry allows a chemist to predict how much of a reactant reacts or how much of a product forms in a chemical reaction.

Balanced Reaction Equations and the Mole Ratio Consider this balanced reaction equation: 4 Al (s) + 3 O 2(g) → 2 Al 2 O 3(s) The coefficients of this reaction represent: - The number of reacting PARTICLES or - The number of reacting MOLES These numbers are FIXED, the ratio of reacting substances NEVER changes.

Balanced Reaction Equations and the Mole Ratio We can interpret this reaction in 2 ways: 4 Al (s) + 3 O 2(g) → 2 Al 2 O 3(s) 1) Every time 4 atoms of Al react with 3 molecules of O 2, 2 molecules of Al 2 O 3 form. OR 2) Every time 4 moles of Al atoms react with 3 moles of O 2 molecules, 2 moles of Al 2 O 3 molecules form.

Using the Mole Ratio Remember that the coefficients from a balanced reaction represent the ratio of the moles of substances that react and form during a chemical reaction. These numbers are fixed - they do not change We can use these ratios to predict the amounts of substances that react and form in a reaction when any amount of a substance is reacted. The conversion factor we use to make these predictions is called a MOLE RATIO.

Using the Mole Ratio In this reaction: 4 Al (s) + 3 O 2(g) → 2 Al 2 O 3(s) What is the minimum amount of O 2 moles that react with 1.20 moles of Al? To solve this, create a mole ratio:

Using the Mole Ratio To solve, substitute in the values: - moles given = 1.20 moles Al - moles of unknown = ? moles O 2 - coefficient of given = 4 Al - coefficient of unknown = 3 O 2 The mole ratio becomes: We predict that mol of O 2 react with 1.20 mol of Al.

Using the Mole Ratio We can also predict the “theoretical yield” - which is defined as the amount of a product forms if all the reactants are converted to products. What it the theoretical yield of Al 2 O 3 when 1.20 moles of Al reacts? Using the mole ratio: We predict that mol of Al 2 O 3 should form, if all the reactants are converted into products (the theoretical yield)

Terminology of Stoichiometry Coefficients - Numbers that represent the ratios of reacting moles in a balanced chemical reaction. Mole Ratio -A conversion factor that allows you to determine how much of one substance reacts or forms in a chemical reaction. Theoretical Yield -The amount of a product that should form in a chemical reaction, if all of the reactants are converted into products.