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Solution Stoichiometry

Solution Stoichiometry . Electrolytes, Acid/Base Reactions, and Stoichiometry . Electrolytes . Defining and Identifying . Electrolytes . Where or in what context have you heard this term used? Define Electrolyte A substance whose aqueous solutions contain ions

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Solution Stoichiometry

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  1. Solution Stoichiometry Electrolytes, Acid/Base Reactions, and Stoichiometry

  2. Electrolytes Defining and Identifying

  3. Electrolytes • Where or in what context have you heard this term used? • Define Electrolyte • A substance whose aqueous solutions contain ions • Substances can be defined as strong or weak electrolytes depending on the % ionization of the substance. • Solutions of electrolytes conduct electricity due to the presence of ions • (Nonelectrolyte: a substance whose aqueous solutions do not contain ions)

  4. Electrolytes Strong Weak Less than 100% ionization when dissolved in water Weak electrolytes establish an equilibrium between the ionized state and the molecular state • Completely Ionize in aqueous solutions • Essentially 100%* of the substance breaks down into ions when dissolved in water

  5. Strong Electrolytes • The following substances are strong electrolytes: • 1) Soluble ionic compounds • 2) Strong Acids (7) • HCl • HBr • HI • HNO3 • HClO4 • HClO3 • H2SO4

  6. Weak Electrolytes • These substances only partially ionize in aqueous solutions • Molecular compounds are the only type of substances that are weak electrolytes • When these molecular compounds dissolve in water, they mostly exist as neutral molecules, but a small portion of the molecules breakdown into ions. An Equilibrium results from this partial breakdown.

  7. Electrolytes as reaction equations • Strong Electrolytes: • HCl(aq)  H+(aq) + Cl-(aq) • Ca(NO3)2(aq)  Ca2+(aq) + 2 NO3-(aq) • HNO3(aq)  • AlCl3(aq)  • Weak Electrolytes: • HC2H3O2(aq)  H+(aq) + C2H3O2-(aq)

  8. Practice Problem • The picture to the right represents an aqueous solution of either MgCl2, KCl, or K2SO4. • Which solution does this picture best represent?

  9. Practice Problem • If 2.50 grams of K2SO4 is dissolved in enough water to make 500 ml of solution, what would be the molarity of each ion in this solution?

  10. Acids, Bases, and Neutralization Reactions

  11. Acids & Bases Acids Bases Are substances that accept hydrogen ions “Proton Acceptors” Strong Bases: soluble metal hydroxides Group I hydroxides Heavy Group II (Ca, Sr, and Ba) hydroxides Weak Bases: usually contain nitrogen (NH3) • Substances that ionize in aqueous solutions to from hydrogen ions (H+) • “Proton Donors” • Strong Acids (7) fully ionize • Weak Acids partially ionize, form equilibrium • Monoprotic, diprotic, and polyprotic

  12. Neutralization Reactions • Neutralization Reactions occur between an Acid and a Base • They are named neutralization reactions, since the remove the acidic or basic character of the solution and produce a “neutral” solution • The H+ from the acid reacts with the base to form water and either a salt, or a gas • Salt: any ionic compound that is formed from the cation of a base and an anion of an acid • Not always NaCl !

  13. Neutralization Reactions • When determining the products of a neutralization reaction, treat them like double replacement reactions. • HCl(aq) + NaOH(aq)  • 2 HCl(aq) + Na2S(aq) 

  14. Neutralization Reactions • Reactions involving carbonate compounds are a special type of neutralization reaction. The formation of carbonic acid in a reaction leads to a secondary reaction: • Carbonic acid is unstable and decomposes to produce water and carbon dioxide • HCl(aq) + NaHCO3(aq)

  15. Solution Stoichiometry Quantitative analysis involving aqueous reactions

  16. Example problem • How many grams of Ca(OH)2 are needed to neutralize 25.0 ml of 0.100 M HNO3?

  17. Critical Skills • In order to be effective with aqueous stoichiometry problems you need to be comfortable with the following: • Writing and Balancing Reactions • Using Molarity and volume to find moles (v x M=Moles) • Many steps in these problem are very similar to our previously encountered stoichiometry problems

  18. Example problem • How many liters of 0.500 M HCl(aq) are needed to react completely with 0.100 moles of Pb(NO3)2(aq), forming a precipitate of PbCl2?

  19. Titrations • A fundamental technique in analyzing aqueous reactions is the process of carrying out a titration. • Titrations involve combining a solution where the solute concentration is unknown with a reagent solution of a known concentration. • The solution of known concentration is referred to as a standard solution.

  20. Titrations • One of the solutions is added to the reaction through a buret. • A buret allows for controlled additions of a liquid and an accurate volume readings.

  21. Titrations • The solution in the buret is added to the reaction until an endpoint is reached. • This endpoint is known as an equivalence point • The point at which stoichiometrically equivalent quantities are brought together.

  22. Equivalence Point • The equivalence point is what a titration is trying to find. You are trying to find the volume of a known molarity solution that is required to completely react with the unknown solution. • Equivalence points are often signaled by a change in color. • When the reaction involved is complete, there is a change in the properties of the resulting solution. These new properties cause chemicals known as indicators to change color. • At the equivalence point the volume of solution added through the buret is recorded.

  23. Titrations • When a titration is complete you will have the following data: • Volume of unknown solution • Molarity of the standard solution • Volume of the standard solution • These 3 pieces of data will allow you to calculate the number of moles in the unknown solution and then the molarity of the unknown solution.

  24. Example Problem • 48.0 ml of an unknown concentration NaOH solution is titrated with 0.144 M H2SO4. The solution goes from pink to clear after the addition of 35.0 ml of the H2SO4 solution from a buret. Determine the concentration of the unknown NaOH solution.

  25. Example Problem • The quantity of Cl- in a municipal water supply is determined by titrating the sample with Ag+. The precipitation reaction that occurs is: • Ag+(aq) + Cl-(aq)  AgCl(s) • An indicator is used to determine the equivalence point of the titration. • 1) How many grams of chloride ion are in a sample of water if 20.2 ml of 0.100 M Ag+ is needed to react with all the chloride in the sample? • 2) If the mass of the water sample was 10.0 grams, what is the % mass of the Cl- ion in the sample?

  26. Chapter 4 Homework • Complete these problems from the end of chapter 4: • 2, 6, 14, 15, 18, 31, 37, 39, 41, 79, 81, 83, 86, 90

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