Stoichiometry Predicting amounts of reagents needed or amounts

Stoichiometry Predicting amounts of reagents needed or amounts of products made

Stoichiometry • Composition stoichiometry – Mass relationships of elements in compounds • Reaction stoichiometry – Mass relationships between participants in a chemical reaction • Must begin all stoichiometry problems with a BALANCED chemical equation.

Conservation of Charge • Balanced chemical equation must be balanced for both mass and charge. • Total charge on reactant side must equal total charge on product side. • So far, most of the equations you have seen are neutral on each side, but this is not always the case.

Coefficients in Balanced Equations • Give relative number of particles in rxn. • Give relative number of moles in rxn. • Coefficients give relationship between moles of each component in equation. • You use these relationships in MOLE-MOLE problems to convert from moles of 1 substance to moles of another substance.

Stoichiometry Problems • Come in several flavors. • Start with the simplest: mole-mole • Given: amount of one substance in moles • Unknown: amount of some other substance in moles

Mole-mole stoichiometry problem • Stoichiometry problems are factor-label problems or conversion problems. • Use the balanced chemical equation to generate the conversion factors.

2 C 2 H 6 + 7 O 2 4 CO 2 + 6 H 2 O • How much CO 2 will be produced if 4 moles of C 2 H 6 are consumed? Identify a stoichiometry problem: 1) You always get a balanced chemical equation. 2) The problem will ask how many moles or how many liters of one species given a certain amount of another species. 3) The equation has a arrow.

2 C 2 H 6 + 7 O 2 4 CO 2 + 6 H 2 O • How much CO 2 will be produced if 4 moles of C 2 H 6 are consumed? • Identify given: 4 moles of C 2 H 6. • Identify conversion factor from equation: 4 moles CO 2 2 moles C 2 H 6

Conversion 4 moles C 2 H 6 4 moles CO 2 = 2 moles C 2 H 6 4 X 4 2 moles CO 2 = 8 moles CO 2

2 C 2 H 6 + 7 O 2 4 CO 2 + 6 H 2 O • How much H 2 O will be produced in the combustion of 5 moles of C 2 H 6? • Given: 5 mol C 2 H 6 • Conversion factor: 6 mol H 2 O 2 mol C 2 H 6

Conversion 5 mol C 2 H 6 6 mol H 2 O = 2 mol C 2 H 6 5 X 6 2 mol H 2 O = 15 mol H 2 O

2 C 2 H 6 + 7 O 2 4 CO 2 + 6 H 2 O • How much oxygen will react with 5 moles of C 2 H 6? • Given: 5 moles C 2 H 6 • Conversion factor: 7 mol O 2 2 mol C 2 H 6

Conversion 5 moles C 2 H 6 7 mol O 2 2 mol C 2 H 6 5 X 7 mol O 2 = 17. 5 mol O 2 2 =

Other flavors of problems • Other types of stoichiometry problems include: • Volume-Volume (for gas phase only) • Mole-mass or mass-mole • Mass-Mass

Gas-Phase Equations • 1 mole of any gas occupies the same volume as any other gas. At STP, 22. 4 L. • So coefficients in equations represent the ratio of the volumes of gases involved in the reaction. • For rxns where all reactants & products are gases, volume-volume problems. • Unit doesn’t matter as long as it’s constant throughout.

N 2(g) + 3 H 2(g) 2 NH 3(g) • How much hydrogen gas will react with 15 liters of nitrogen gas? • Given: 15 L of N 2(g) • Conversion factor: 3 L H 2 1 L N 2

Conversion 15 L N 2(g) 3 L H 2 = 1 L N 2 15 X 3 L H 2 1 = 45 L H 2

Other types of stoich problems • General strategy: 1. Convert given into moles 2. Perform stoichiometric calculation using mole ratios from balanced equation 3. Convert to desired unit

A word about …

Reactions in aqueous solution • Many reactions, esp. many double replacement reactions, occur in water. • What happens when substances dissolve in water? • Depends on if they are ionic or covalent.

Dissolving • Covalent substance – sugar or C 6 H 12 O 6 • C 6 H 12 O 6(s) C 6 H 12 O 6(aq) • The sugar molecules are spread out among the water molecules.

Dissolving • Ionic substance – table salt or Na. Cl • Na. Cl(s) Na+(aq) + Cl-(aq) • The ions are spread out among the water molecules.

Double Replacement Reactions • Often occur when you mix 2 solutions of ionic compounds. – 1 product may be water or – 1 product may be a gas or – 1 product may be a solid • We say these reactions “go to completion. ”

Reactions producing Solids • • Precipitation: the opposite of dissolving! What do you see in the following clips: S 1043. mov S 1045. mov S 1046. mov S 1050. mov S 1057. mov S 1058. mov and S 1060. mov

Reactions producing Solids 2 Na. OH(aq) + Cu. Cl 2(aq) 2 Na. Cl(aq) + Cu(OH)2(s) Complete Chemical Equation

Complete Ionic Equations 2 Na+(aq) + 2 OH-(aq) + Cu 2+(aq) + 2 Cl-(aq) 2 Na+(aq) + 2 Cl-(aq) + Cu(OH)2(s) Substances that are ions in solution are written as ions in solution.

Net Ionic Equations 2 Na+(aq) + 2 OH-(aq) + Cu 2+(aq) + 2 Cl-(aq) 2 Na+(aq) + 2 Cl-(aq) + Cu(OH)2(s) Notice that some ions do not participate in the reaction. They are spectator ions. Cross out all the spectator ions & you get the net ionic equation. 2 OH-(aq) + Cu 2+(aq) Cu(OH)2(s)

Reactions that form Water HBr(aq) + Na. OH(aq) H 2 O(l) + Na. Br(aq) H+(aq) + Br-(aq) + Na+(aq) + OH-(aq) H 2 O(l) + Na+(aq) + Br-(aq) H+(aq) + OH-(aq) H 2 O(l)

Reactions that form Gases HCl(aq) + Na. HCO 3(aq) H 2 CO 3(aq) + Na. Cl(aq) H 2 CO 3(aq) H 2 O(l) + CO 2(g) ___________________ HCl(aq) + Na. HCO 3(aq) H 2 O(l) + CO 2(g) + Na. Cl(aq)

Reactions that form Gases HCl(aq) + Na. HCO 3(aq) H 2 O(l) + CO 2(g) + Na. Cl(aq) H+(aq) + Cl-(aq) + Na+(aq) + HCO 3 -(aq) H 2 O(l) + CO 2(g) + Na+(aq) + Cl-(aq) H+(aq) + HCO 3 -(aq) H 2 O(l) + CO 2(g)