Chapter 4 Chemical Quantities and Aqueous Reactions

Reaction Stoichiometry the numerical relationships between chemical amounts in a reaction is called stoichiometry the coefficients in a balanced chemical equation specify the relative amounts in moles of each of the substances involved in the reaction 2 2 C 8 H 18 (l) + 25 O 2 (g)  16 CO 2 (g) + 18 H 2 O(g) ◦ 2 molecules of C 8 H 18 react with 25 molecules of O 2 to form 16 molecules of CO 2 and 18 molecules of H 2 O ◦ 2 moles of C 8 H 18 react with 25 moles of O 2 to form 16 moles of CO 2 and 18 moles of H 2 O Write the molar stoichiometry ratio for the reaction

Stoichiometry: Chemical Arithmetic Stoichiometry: The relative proportions in which elements form compounds or in which substances react. aA + bB cC + dD Moles of A Grams of A Moles of B Grams of B Mole Ratio Between A and B (Coefficients) Molar Mass of BMolar Mass of A

Stoichiometry: Chemical Arithmetic How many grams of NaOH are needed to react with 25.0 g Cl 2 ? 2NaOH(aq) + Cl 2 (g) NaOCl(aq) + NaCl(aq) + H 2 O(l) Aqueous solutions of sodium hypochlorite (NaOCl), best known as household bleach, are prepared by reaction of sodium hydroxide with chlorine gas: Moles of Cl 2 Grams of Cl 2 Moles of NaOH Grams of NaOH Mole Ratio Molar Mass

Example Magnesium hydroxide, the active ingredient in milk of masnesia, neutralizes stomach acid, primarily HCl, according to the following reaction: Mg(OH) 2 (aq) + 2 HCl(aq)  2 H 2 O(l) + MgCl 2 (aq) What mass of HCl, in grams, can be neutralized by a dose of milk of magnesia containg 3.26g Mg(OH) 2 ?

Practice According to the following equation, how many milliliters of water are made in the combustion of 9.0 g of glucose? C 6 H 12 O 6 (s) + 6 O 2 (g)  6 CO 2 (g) + 6 H 2 O(l) 6 density of water = 1.00 g/mL

Limiting Reactant when this reactant is used up, the reaction stops and no more product is made the reactant that limits the amount of product is called the limiting reactant ◦ sometimes called the limiting reagent ◦ the limiting reactant gets completely consumed reactants not completely consumed are called excess reactants the amount of product that can be made from the limiting reactant is called the theoretical yield 7

Reactions with Limiting Amounts of Reactants At a high temperature, ethylene oxide reacts with water to form ethylene glycol which is an automobile antifreeze and a starting material in the preparation of polyester polymers: C 2 H 4 O(aq) + H 2 O(l) C2H6O2(l)C2H6O2(l)

Examples How many sandwiches can be made from 10 bread slices and 8 cheese slices? Which one is the limiting reactant? The balanced chemical equation is 2Bd + Ch  Bd 2 Ch

Limiting and Excess Reactants in the Combustion of Methane If we have 5 molecules of CH 4 and 8 molecules of O 2, which is the limiting reactant? 10 H H C H H + OO OO OO OO OO OO OO OO ? H H C H H H H C H H H H C H H H H C H H CH 4 (g) + 2 O 2 (g)  CO 2 (g) + 2 H 2 O(g)

Things Don’t Always Go as Planned! many things can happen during the course of an experiment that cause the loss of product the amount of product that is made in a reaction is called the actual yield ◦ generally less than the theoretical yield, never more! the efficiency of product recovery is generally given as the percent yield 11

Example The reaction between aluminum and iron (III) oxide can generate temperatures approaching 3000 o C and is used in welding metals: 2 Al(s) + Fe 2 O 3 (s)  Al 2 O 3 (s) + 2Fe(s) In one process, 124 g Al are reacted with 601 g of Fe 2 O 3. a.Which reactant is limited? b.What is the theoretical yield of Al 2 O 3 (s)?

Example Example: The following reaction reaction is used to obtain iron from iron ore Fe 2 O 3 (s) + 3 CO(g)  2 Fe(s) + 3 CO 2 (g) The reaction of 167 g Fe 2 O 3 with 85.8 g CO produces 72.3 g Fe. Find the limiting reaction, theoretical yield and percent yield. 13

Reactions with Limiting Amounts of Reactants Li 2 O(s) + H 2 O(g) 2LiOH(s) Lithium oxide is used aboard the space shuttle to remove water from the air supply according to the equation: If 80.0 g of water are to be removed and 65.0 g of Li 2 O are available, which reactant is limiting? How many grams of excess reactant remain?

Concentrations of Reactants in Solution: Molarity Molarity: The number of moles of a substance dissolved in each liter of solution. In practice, a solution of known molarity is prepared by weighing an appropriate amount of solute, placing it in a container called a volumetric flask, and adding enough solvent until an accurately calibrated final volume is reached. Solution: A homogeneous mixture. Solute: The dissolved substance in a solution. Solvent: The major component in a solution.

Concentrations of Reactants in Solution: Molarity Molarity converts between mole of solute and liters of solution: molarity = moles of solute liters of solution L mol or 1.00 M 1.00 L 1.00 mol = mol of sodium chloride placed in enough water to make 1.00 L of solution would have a concentration equal to:

Example Find the molarity of a solution that has 25.5 g KBr dissolved in 1.75 L of solution Calculate the concentration of a solution made by adding 45.4g of NaNO 3 to a flask and dissolving with water to a total volume 250.0mL

Using molarity in solution molarity shows the relationship between the moles of solute and liters of solution If a sugar solution concentration is 2.0 M, then 1 liter of solution contains 2.0 moles of sugar Express 2.0 M as a conversion factor Examples: How many liters of M NaOH contains mol NaOH? How many grams of solute would you use to prepare 1.50 L of M glucose, C 6 H 12 O 6 ?

Dilution often, solutions are stored as concentrated stock solutions to make solutions of lower concentrations from these stock solutions, more solvent is added ◦ the amount of solute doesn’t change, just the volume of solution moles solute in solution 1 = moles solute in solution 2 the concentrations and volumes of the stock and new solutions are inversely proportional M 1 ∙V 1 = M 2 ∙V 2 19

Example To what volume should you dilute L of 15.0 M NaOH to make 3.00 M NaOH? Sulfuric acid is normally purchased at a concentration of 18.0 M. How would you prepare mL of M aqueous H 2 SO 4 ?

Solution Stoichiometry aA + bB cC + dD Moles of A Volume of Solution of A Moles of B Volume of Solution of B Mole Ratio Between A and B (Coefficients) Molar Mass of B Molarity of A

Solution Stoichiometry H 2 SO 4 (aq) + 2NaOH(aq) Na 2 SO 4 (aq) + 2H 2 O(l) What volume of M H 2 SO 4 is needed to react with 50.0 mL of M NaOH? Moles of H 2 SO 4 Volume of Solution of H 2 SO 4 Moles of NaOH Volume of Solution of NaOH Mole Ratio Between H 2 SO 4 and NaOH Molarity of NaOH Molarity of H 2 SO 4

Example What volume of M KCl is required to completely react with L of M Pb(NO 3 ) 2 in the reaction 2 KCl(aq) + Pb(NO 3 ) 2 (aq)  PbCl 2 (s) + 2 KNO 3 (aq) What mass (in grams) of carbon dioxide is formed when 35.7 ml of a 0.108M Na 2 CO 3 react with 0.150M HNO 3 solution. Na 2 CO 3 (aq) + 2 HNO 3 (aq)  2NaNO 3 (aq) + CO 2 (g) + H 2 O(l) What is the minimum amount (L) of 6.0 M H 2 SO 4 necessary to produce 25.0 g. Assumed H 2 SO 4 is limited 2 Al(s) + 3 H 2 SO 4 (aq)  Al2(SO 4 ) 3 (aq) + 3 H 2 (g)