CHAPTER 7: THE MOLE CONCEPT & STOICHIOMETRY

ADVANCE CHEMISTY CHAPTER 7: THE MOLE CONCEPT & STOICHIOMETRY

Purpose • Know what is stoichiometry. • Learn how to use mole. • Know the type of chemical reactions. • Learn more about chemical reaction. • Learn how to find percent yield. • Know to find the limiting reactant.

What is stoichiometry? • Stoichiometry is the area of study that examines quantities of substances consumed and produced in chemical reactions.

Avogadro’s Number & The Mole • We define the mole (mol) as the number of a substance or entities (atoms/ molecules/ particles/ ions etc.) as there are 12C atoms in 12 g of 12C. This number is Avogadro’s number (NA) and is 6.022.

Average Atomic Mass • The elements occur in proportions of different isotopes on the earth. • The atomic mass is the average atomic mass of the naturally occurring isotopes. This is calculated by the sum of the products of the fraction of the natural abundance and the atomic masses of each component. • Example: Gallium has two natural isotopes: 69Ga and 71Ga. 0.60468.9257 + 0.39670.9249 = 69.72

Formula Weight (F.W.) • A formula weight is the sum of the atomic weights for the atoms in a chemical formula. • So, the formula weight of calcium chloride, CaCl2, would be Ca: 1(40.08 amu) + Cl: 2(35.453 amu) = 110.99 amu • Molecular weight/mass (Mᴡ) is the sum of the atomic weights of the atoms in a molecule. • For the molecule ethane, C2H6, the molecular weight would be C: 2(12.011 amu) + H: 6(1.00794 amu) = 30.070 amu

Mole, Molar Mass, and Avogadro’s Number • Using Moles: One mole of atoms, ions, or molecules contains Avogadro’s number of those particles. One mole of molecules or formula units contains Avogadro’s number times the number of atoms or ions of each element in the compound. • Use the molar mass of an element of a compound to convert between the mass of a substance and the moles of the substance. Molar Mass Avogadro’s Number

The Mole-Volume Relationship The volume of a gas changes with a change in temperature or a change in pressure. Due to these variations, the volumes of a gas are usually measured at a standard temperature and pressure. Standard temperature and pressure (STP) means a temperature of 0⁰C at a pressure of 101.3 kPa or 1 atmosphere (atm). At STP, 1 mol of a gas occupies a volume of 22.4L. The quantity 22.4 L is called molar volume of a gas. In calculating the volume and moles of a gas at STP, the molar volume is used to convert between the number of moles of a gas and the volume of a gas.

Percent Composition and Chemical Formula • The relative amounts of the elements in a compound are expressed as percent composition. • One can find the percentage composition from a mass data of the mass of a compound that comes from each of the elements in the compound by using this equation: Percent composition of an element = • To find the percent composition from the chemical formula uses the expression below: Percent composition of an element =

Empirical Formulas • Empirical formula is one which gives the lowest whole-number ratio of atoms or moles of each element in a compound. The percent composition is used to calculate the empirical formula of that compound.

Molecular Formulas • The molecular formula of a compound is either the same as its experimentally determined empirical formula, or it is a simple whole-number multiple of its empirical formula. • We can obtain the molecular formula for any compound having known the molecular weight or the molar mass of a compound. • Whole-number multiple= molecular weight / empirical formula weight

Combustion Analysis • Combustion Analysis is the technique for determining empirical formula in the laboratory.

Concentration of Solution • Concentration of a solution is the amount of solute in a given volume of solvent or solution. • Dilution of Solutions: Quantitative analysis is the determination of the concentration of a substance in a solution. When diluting solutions we make use of the following equation: • C₁V₁ = C₂V₂

Chemical Reaction • A chemical reaction is the process by which one substance is changed to one or more other substances. • Achemical equation is a conventional method by which chemical reactions are formulated.

Writing Chemical Equations • Reactantsare the starting materials before the reaction. • Productsare the resultant materials after the reaction.  • The states of the reactants and products are written in parentheses to the right of each compound. [solid (s), liquid (l), gas (g), aqueous solution (aq)]

Balancing Chemical Equations • 1) Identify all the reactants and products on the respective sides of the equation. • 2) Experiment with different coefficients in an effort to balance the elements on both sides • 3) Start with elements that appear once only on both sides, then those that appear in one place but in different numbers, and finally those that appear in more than one reactant or product. • 4) Check that all the elements are present in equal amounts both sides of the equation. • CH₄ + O₂ → CO₂ + H₂O unbalanced • CH4 + 2O₂ → CO₂ + 2H₂O balanced

Types of Chemical Reactions • In combination reactions (Synthesis) two or more substances react to form one product. • 2Mg(s) + O2(g) → 2MgO(s) • In a decomposition reaction one substance breaks down into two or more substances. • CaCO3(s) → CaO(s) + CO2(g) • Combustion reactions are generally rapid reactions that produce a flame. Combustion reactions most often involve hydrocarbons reacting with oxygen in the air. • CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)

In a substitution or single displacement reactions, a single free element replaces or is substituted for one of the elements in a compound. • Zn + 2HCl →H₂ + ZnCl₂ • In a metathesis or double displacement reaction, exchange of atoms occurs. • HBr + NaOH → NaBr + H2O (Acid-Base Rxn)

Stoichiometric Calculations • Stoichiometry is the quantitative study of reactants and products in a chemical reaction. • This can be considered per molecule, but more often per mole, known as the mole method. • Steps to be taken to calculate theoretical yields: • Write a balanced equation. • Convert gram of reactant to moles. • Use molar ratio from balanced equation to determine the number of moles of product. • Convert moles of product to grams.

Limiting Reactants • The limiting reactant is the reactant present in the smallest stoichiometric amount. In other words, it’s the reactant you’ll run out of first. It is the reagent that is consumed first in a chemical reaction. • Excess reagents are those that are not all consumed when the limiting reagent is.

Reactant Yield • Theoretical yield is the yield obtained if all the limiting reagent reacts. • The actual yield is invariably less than the theoretical yield because of incomplete reaction, by-products and product loss in experimental procedure.

References • Lecture: Theodore E. B., Eugene, H. L. H., Bruce E. B., Catherine M., Patrick W., (2011). Chemistry: The Central Science (12 Ed). Prentice Hall. USA. • Laboratory: Theodore E. B., John H. N., Kenneth C. K., Matthew S. (2011). Laboratory Experiments for Chemistry: The Central Science (12 Ed). Prentice Hall. USA. • Theodore E. B., (2011). Solutions to Exercises for Chemistry: The Central Science. Prentice Hall. USA. • John M., Robert C. F. (2010). Chemistry (4 Ed): Prentice Hall Companion Website. http://wps.prenhall.com/esm_mcmurry_chemistry_4/9/2408/616516.cw/index.html • Chemistry Online at http://preparatorychemistry.com/Bishop_Chemistry_First.htm • Chemistry and You at http://www.saskschools.ca/curr_content/science9/chemistry/index.html 7. Teachers Notes

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CHAPTER 7: THE MOLE CONCEPT & STOICHIOMETRY