Chemistry 1210: Introduction to General Chemistry

1. Chemistry 1210: Introduction to General Chemistry Molecular View of Reactions in Aqueous Solution Dr. Gina M. Florio 20 Sept. 2012 Jespersen, Brady, Hyslop Chapter 5A

2. Definitions A solution is a homogeneous mixture in which the two or more components mix freely. The solvent is taken as the component present in the largest amount. A solute is any substance dissolved in the solvent. CH. 5.1

3. Example of a Solution Formation of a solution of iodine molecules in ethyl alcohol where ethyl alcohol is the solventand iodine the solute: Crystal of I2 placed in ethanol: A solution of I2 in ethanol: CH. 5.1

4. Properties of Solutions: Qualitative Composition Solutions have variable composition and they may be characterized using a solute-to-solvent ratio called the concentration. The relative amounts of solute and solvent are often given without specifying the actual quantities. Example: The dilute solution (left) has less solute per unit volume than the (more) concentrated solution (right). CH. 5.1

5. Properties of Solutions: Qualitative & Quantitative There is usually a limit to the amount of solute that can dissolve in a given amount of solvent. The solubility of a solute is the number of grams of solute that can dissolve in 100 grams of solvent at a given temperature, T. • Example: 36.0 g of NaCl can dissolve in 100 g of H2O at 20°C Unsaturatedsolution – able to dissolve more solute at a given T. Saturatedsolution – no more solute can be dissolved at a given T. Supersaturated solution – contains more solute than required for saturation at a given T. CH. 5.1

6. Properties of Solutions: Qualitative Supersaturated solutions – can be formed by careful cooling of saturated solutions. – are unstable and often result in the formation of a precipitate. Precipitate – a solid substance that separates from solution. Precipitates can also form from reactions (precipitation reactions). CH. 5.1

7. Molecular Compounds in Solutions Most solutions of molecular compounds do not conduct electricity and are called nonelectrolytes (no ions are formed). CH. 5.2

8. Ionic Compounds in Solutions Ionic compounds dissociate when they dissolve in water. Ions separate from the solid and become hydrated. Ionic compounds are electrolytes (hydrated ions conduct electricity). The dissociation reaction of an ionic compound: The hydrated ions, with the symbol (aq), have been written separately. Polyatomic ions remain intact. CH. 5.2

9. Reactions of Ions in Solution Example: When a solution of Pb(NO3)2 is mixed with a solution of KI the yellow precipitate PbI2 rapidly forms: CH. 5.2

10. Reactions of Ions in Solution This reaction may be represented with a molecular, ionic, or net ionic equation: Molecular Ionic Net ionic The most compact notation is the net ionic equation which eliminates all the non-reacting spectator ions from the equation. CH. 5.2

11. Balancing Ionic Reactions Criteria for balanced ionic and net ionic equations: • Mass balance – the same number of each type of atom on each side of the arrow • Charge balance – the net electrical charge on the left side of the arrow must equal the net electrical charge on the right side of the arrow CH. 5.2

12. Acids & Bases Arrhenius Definitions: – a substance that reacts with water to produce the hydronium ion Acid HA + H2O  A– + H3O+ – a substance that reacts with water to produce the hydroxide ion Base B + H2O  B+ + OH− The characteristic reaction between acids and bases is neutralization: • HCl(aq) + NaOH(aq)  NaCl(aq) + H2O(l) In general, the reaction of an acid and a base produces a salt and water. CH. 5.3

13. Deprotonation Reaction: Ionization of acids in water Ionization of an acid in water: HA + H2O  A– + H3O+ CH. 5.3

14. Acids Acids are molecular compounds that undergo ionization reactions with water: proton Acids can be classified as monoprotic or polyprotic: 1 hydronium ion Monoprotic: 2 hydronium ions Diprotic: 3 hydronium ions Triprotic: CH. 5.3

15. Acid and Basic Anhydrides Some nonmetal oxides react with water to produce acids. They are called acidic anhydrides(anhydride = without water). Soluble metal oxides are basic anhydrides. CH. 5.3

16. Molecular Bases Ammonia gas ionizes in water producing hydroxide ions: This is an example of a molecular base. CH. 5.3

17. Classification of Acids and Bases Acids and bases can be classified as strong or weak and so as strong or weak electrolytes. Strong electrolytes undergo complete ionization in water. Common Strong Acids: Common Strong Bases: CH. 5.3

18. Weak Acids and Bases Most acids and molecular bases are not completely ionized in water. They are classified as weak electrolytes. Weak acids and bases are weak electrolytes because less than 100% of the molecules ionize. HCl (strong) HC2H3O2 (weak) NH3 (weak) CH. 5.3

19. Reactions: Strong Acid and Strong Base The reaction of a strong acid with strong base gives a salt and water: This is called aneutralization reaction. The driving force is the formation of water. This net ionic equation applies only to strong acids and bases. CH. 5.3

20. Reactions: Weak Acid and Strong Base The neutralization of a weak acid with a strong base involves a strong and weak electrolyte. Consider the reaction of acetic acid with sodium hydroxide: The driving force is the formation of the weak electrolyte. Note that in ionic equations the formulas of weak electrolytes are written in “molecular” form. CH. 5.3

21. Reactions: Strong Acid and Weak Base The situation is similar when a strong acid reacts with a strong base. For example, the reaction between hydrochloric acid and ammonia: For ammonia and HCl the net ionic equation is: Note that water only appears as a product if the hydronium ion is used. CH. 5.3

22. Weak Acids and Bases Weak acids and bases are in dynamic equilibrium in solution. Example: Acetic Acid Two opposing reactions occur in solution: 1. Ionization of the acid (the forward reaction) 2. Recombination of ions into molecules (the reverse reaction). Chemical or dynamic equilibrium results when the rate of the forward and reverse reaction are equal. CH. 5.3

23. Naming Binary Acids Binary compounds of many nonmetals and hydrogen are acidic (binary acids). They are named by adding the prefix hydro- and the suffix –ic to the stem of the nonmetal name, followed by the word acid. Example: CH. 5.4

24. Naming Oxoacids Acids that contain hydrogen, oxygen, plus another element are called oxoacids. They are named according to the number of oxygen atoms in the molecule and do not take the prefix hydro-. When there are two oxoacids, the one with the larger number of oxygens takes the suffix –ic and the one with the fewer oxygen atoms takes the suffix –ous. Example: Halogens can form up to four different oxoacids. The oxoacid with the most oxygens has the prefix per- the one with the least has the prefix hypo-. Example: CH. 5.4

25. Ions of Oxoacids Anions are produced when oxoacids are neutralized. • Relationship between the name of the polyatomic ion and the parent acid: • –ic acids give –ate ions • -ous acids give –ite ions In naming polyatomic anions, the prefixes per- and hypo- carry over from the parent acid: perchlorate ion (ClO4-) makes perchloric acid (HClO4) CH. 5.4

26. Polyprotic Acid Salts An acidic salt contains an anion that is capable of furnishing additional hydrogen ions. The number of hydrogens that can still be neutralized is also indicated. CH. 5.4

27. Naming Bases Ionic compounds containing metal ions are named like any other ionic compound. Molecular bases are specified by giving the name of the molecule CH. 5.4

28. Reactions: Insoluble Hydroxide and Oxide Bases Magnesium hydroxide has a low solubility in water, but can react with a strong acid. The net ionic equation is: Magnesium hydroxide is written as a solid because it is insoluble. A number of metal oxides also dissolve in acids. For example, iron(III) oxide reacts with hydrochloric acid: The driving force is the formation of water. CH. 5

29. Reactions: Formation of Insoluble Gases Some reactions with acids or bases produce a gas. The driving force is the formation of the gas. CH. 5