Chapter 4 Chemical Quantities and Aqueous Reactions

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1 Chapter 4 Chemical Quantities and Aqueous Reactions

2 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 C 8 18 (l) (g) 16 C 2 (g) (g) 2 molecules of C 8 18 react with 25 molecules of 2 to form 16 molecules of C 2 and 18 molecules of 2 2 moles of C 8 18 react with 25 moles of 2 to form 16 moles of C 2 and 18 moles of 2 Write the molar stoichiometry ratio for the reaction 2

3 Stoichiometry: Chemical Arithmetic Stoichiometry: The relative proportions in which elements form compounds or in which substances react. aa + bb cc + dd Grams of A Moles of A Moles of B Grams of B Molar Mass of A Mole Ratio Between A and B (Coefficients) Molar Mass of B

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

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

6 Practice According to the following equation, how many milliliters of water are made in the combustion of 9.0 g of glucose? C (s) (g) 6 C 2 (g) (l) density of water = 1.00 g/ml 6

7 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

8 Examples ow 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

9 Limiting and Excess Reactants in the Combustion of Methane If we have 5 molecules of C 4 and 8 molecules of 2, which is the limiting reactant? 9 C +? C C C C C 4 (g) (g) C 2 (g) (g)

10 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 Percent Yield actual yield theoretica l yield 100% 10

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 3 (s) Al 2 3 (s) + 2Fe(s) In one process, 124 g Al are reacted with 601 g of Fe 2 3. a. Which reactant is limited? b. What is the theoretical yield of Al 2 3 (s)?

12 Example Example: The following reaction reaction is used to obtain iron from iron ore Fe 2 3 (s) + 3 C(g) 2 Fe(s) + 3 C 2 (g) The reaction of 167 g Fe 2 3 with 85.8 g C produces 72.3 g Fe. Find the limiting reaction, theoretical yield and percent yield. 12

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

14 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.

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

16 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 NaN 3 to a flask and dissolving with water to a total volume 250.0mL

17 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: ow many liters of M Na contains mol Na? ow many grams of solute would you use to prepare 1.50 L of M glucose, C ?

18 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 18

19 Example To what volume should you dilute L of 15.0 M Na to make 3.00 M Na? Sulfuric acid is normally purchased at a concentration of 18.0 M. ow would you prepare ml of M aqueous 2 S 4?

20 Solution Stoichiometry aa + bb cc + dd Volume of Solution of A Moles of A Moles of B Volume of Solution of B Molarity of A Mole Ratio Between A and B (Coefficients) Molarity of B

21 Solution Stoichiometry What volume of M 2 S 4 is needed to react with 50.0 ml of M Na? 2 S 4 (aq) + 2Na(aq) Na 2 S 4 (aq) (l) Volume of Solution of Na Moles of Na Moles of 2 S 4 Volume of Solution of 2 S 4 Molarity of Na Mole Ratio Between Na and 2 S 4 Molarity of 2 S 4

22 Example What volume of M KCl is required to completely react with L of M Pb(N 3 ) 2 in the reaction 2 KCl(aq) + Pb(N 3 ) 2 (aq) PbCl 2 (s) + 2 KN 3 (aq) What is the minimum amount (L) of 6.0 M 2 S 4 necessary to produce 25.0 g of 2. Assumed 2 S 4 is limited 2 Al(s) S 4 (aq) Al 2 (S 4 ) 3 (aq) (g)

23 Titration A procedure for determining the concentration of a solution by allowing a carefully measured volume to react with a solution of another substance (the standard solution) whose concentration is known. Titrant: solution that is added from the buret Equivalence Point: The point at which stoichiometrically equivalent quantities of acid and base have been mixed together. Endpoint: is the point at which the titration is complete (usually by a sudden color change), as determined by an indicator, Types of acid-base titration Strong acid-strong acid Strong acid-weak acid Weak acid-strong base Weak acid weak base 23

24 Titration The base solution is the titrant in the burette. As the base is added to the acid, the + reacts with the to form water. But there is still excess acid present so the color does not change. At the titration s endpoint, just enough base has been added to neutralize all the acid. At this point the indicator changes color. 24

25 Example The titration of ml of Cl solution of unknown concentration requires ml of M Na solution to reach the end point. What is the concentration of the unknown Cl solution? The titration of a 20.0 ml sample of an 2 S 4 solution of an unknown concentration requires ml of a 0.158M K solution to reach the equivalent point. What is the concentration of the unknown 2 S 4 solution?

Solution Concentration

Solution Concentration solution: homogeneous mixture of substances present as atoms, ions, and/or molecules solute: component present in smaller amount solvent: component present in greater amount Note: