1. Chapter 5: Introduction to Reactions in Aqueous Solutions

2. Electrolyte. A substance, such as sodium chloride, that dissolved in water and produces ions to give an electrically conducting solution is called an electrolyte. A substance, such as sucrose, or table sugar (C 12 H 22 O 11 ), that is not ionized and does not conduct electric current when dissolved in water is called a Nonelectrolyte.

3. Strong electrolyte: is a substance that is almost completely ionized in aqueous solution, and the solution is a good electrical conductor. Weak electrolyte: is partially ionized in aqueous solution, and the solution is only a fair conductor of electricity. (is an electrolyte that dissolves in water to give an equilibrium between a molecular substance and a relatively small quantity of ions.) ~ Essentially all soluble ionic compounds and only a relatively few molecular compounds are strong electrolytes. ~ Most molecular compounds are either nonelectrolytes or weak electrolytes.

4. Table 1 Electrolyte Classification of Some Common Substances Strong ElectrolytesWeak ElectrolytesNonelectrolytes HCl, HBr, HICH 3 COOHH 2 O HClO 4 HFCH 3 OH HNO 3 C 2 H 5 OH H 2 SO 4 C 12 H 22 O 11 (sucrose) KBrMost organic compounds NaCl NaOH, KOH Other soluble ionic compounds

5. Aqueous Reactions Aqueous reactions can be grouped into three general categories, each with its own kind of driving force: precipitation reactions, acid base neutralization reactions, and oxidation-reduction reactions.

6. Precipitation Reactions Precipitation reactions are process in which soluble reactants yield an insoluble solid product that falls out of solution. Most precipitations take place when certain cations and anions combined to produce an insoluble ionic solid called a precipitate. E.g. the reaction of silver nitrate and sodium iodide in an aqueous water solution yields sodium nitrate in solution and a yellow precipitate of silver iodide. We can write the equation for the reaction as follow: AgNO 3 (aq) + NaI(aq)  AgI(s) + NaNO 3 (aq) we called this a molecular equation, an equation in which the substances are written as if they were molecular substances, even though they may exist as ions in the solution. Molecular equation is useful because it is explicit about what solutions have been added and what products are obtained; and to calculate the amount of reactants or products.

7. Ionic Equations The molecular equation does not tell us that the reaction actual involves ions in solution. However, we know that soluble ionic substances in solution should be represented by their separate ions. To represent this, the above reaction as an ionic equation, in which all the ions are explicitly shown: An ionic equation: Ag + (aq) + NO 3 - (aq) + Na + (aq) + I - (aq)  AgI(s) + Na + (aq)+NO 3 - (aq) This is an example of ionic equation, which is a chemical equation for a reaction involving ions in solution in which soluble substances are represented by the formulas of the predominant species in that solution.

8. Net Ionic Equations Note that some ions appear on both side of equation. These ions go through the reaction unchanged- does not take part in the reaction. We called them spectator ions. We can cancel them from the equation. The resulting equation is a net ionic equation. Ag + (aq) + I-(aq)  AgI(s) Net ionic Equation A net ionic equation is an equation that includes only the actual participants in a reaction, with each participant denoted by the symbol or formula that best represent it.

9. rules for converting molecular equations to ionic equations The rules for converting molecular equations to ionic equations follow: 1) Make sure the molecular is balanced 2) Ionic substances indicated in the molecular equation as dissolved in solution, such as NaCl(aq), are normally written as ions. 3) Ionic substances that are insoluble (do not dissolve) either as reactants or products (such as precipitate) are represented by formulas of the compounds 4) Molecular substances that are strong electrolytes, such as strong acids, are written as ions. Thus, HCl(aq) is written as H 3 O + (aq) + Cl - (aq) or as H + (aq) + Cl - (aq). 5) Molecular substances that are weak electrolytes or nonelectrolytes are represented by their molecular formulas.

10. Acid-Base Reactions Consider the production of ions in pure water. It produces a small percentage of ions (about 2x10 -7 % of the molecules react to gives ions). H 2 O(l)+ H 2 O(l)  H 3 O + (aq) + OH - (aq) Arrhenius definition An acid is a substance that provides hydrogen ions (H + ) (increase the concentration of H + ) in aqueous solution. The symbol H + (aq) does not really represent the structure of the ion present in aqueous solution. H + is too reactive to exit by itself, it attaches to water to give the more stable hydronium ion, H 3 O +. A base is a substance that produces hydroxide ions (OH - ) (increase the conc. of hydroxide ions) in aqueous solution. HA (aq)  H + (aq) + A - (aq) an acidHA is a general formula for an acid MOH(aq)  M + (aq) + OH - (aq) an baseMOH is a general formula for a base

11. Writing Formula, Total and Net Ionic Equations We must be able to represent a reaction in three different ways: Formula Equation Total or Complete Ionic Equation Net Ionic Equation

12. Formula Equation: A balanced chemical equation in which all the reactants and products are given by their chemical formula. For Example: AgNO 3 (aq) + NaCl (aq) → AgCl (s) + NaNO 3 (aq) Soluble species are indicated by placing “ (aq) ” after the formula. Precipitates are indicated by placing “ (s) ” after the formula. This way of expressing the reaction is used whenever we want to indicate the chemical formula of the reactants and products.

13. Total or Complete Ionic Equation Shows all the soluble ionic species broken up into their respective ions. For Example: Ag+ (aq) + NO 3 - (aq) + Na + (aq) + Cl - (aq) → AgCl (s) + Na + (aq) + NO 3- (aq) This method is not frequently used because it is cumbersome to write out. It is used to emphasize the situation which exists before and after the reaction. Spectator ions: Ions that remain unchanged during a reaction and do not participate in the reaction. The spectator ions in the above equation are NO 3 - (aq) and Na + (aq) NEVER write a total ionic equation until you have first written a balanced formula equation. If you proceed directly to a total ionic equation the result may be balanced but still may not represent the actual situation.

14. Net Ionic Equation : Shows only the reacting species (species which are actively involved in the reaction…the one making the PPT) in the equation. The net ionic equation is formed by omitting the spectator ions from the total ionic equation. For Example: We had: AgNO 3 (aq) + NaCl (aq) → AgCl (s) + NaNO 3 (aq) then: Ag + (aq) + NO 3- (aq) + Na + (aq) + Cl - (aq) → AgCl (s) + Na + (aq) + NO 3- (aq) Canceling out spectator ions, we get: Ag + (aq) + Cl - (aq) → AgCl (s)

15. Let’s try an example putting all these ideas together Example: Write a formula equation, a total ionic equation, and a net ionic equation for the reaction which occurs when 0.2 M solutions of Al(NO 3 ) 3 (aq) and MgS (aq) are mixed.

16. 1. Write the complete formula equation. Determine if there is a precipitate using“ (s) ” to identify the precipitate. The balanced formula equation is: 2 Al(NO 3 ) 3 (aq) + 3 MgS (aq) → Al 2 S 3 (s) + 3 Mg(NO 3 ) 2 (aq) 2. Write the complete ionic equation by breaking up the balanced formula equation into ions. Remember: The precipitate remains in molecular form. The number of ions is found by multiplying the subscript after the ion, by the coefficient in front of the molecule. The final equation must still be balanced. We get: 2Al 3+ (aq) +6NO 3- (aq) +3Mg 2+ (aq) +3S 2- (aq)  Al 2 S 3 (s) +3Mg 2+ (aq) +6NO 3- (aq) 3. Write the net ionic equation by deleting the spectator ions and simplifying the coefficients if necessary. 2 Al 3+ (aq) + 3 S 2- (aq) → Al 2 S 3 (s)

17. Let’s Try Some More  Write the formula, complete ionic, and net ionic equation for each of the following reactions: 1)Aqueous nickel (II) chloride reacts with aqueous sodium hydroxide to give nickel (II) hydroxide and sodium chloride. 2)Solid potassium metal reacts with water to give potassium hydroxide and hydrogen gas. 3)Aqueous sodium hydroxide reacts with phosphoric acid to give water and sodium phosphate.

18. Predicting Precipitation Reaction

19. E.g. Predict whether a reaction will occur in each of the following case. If so, write a net ionic equation for the reaction. If no reaction occurs, write NR after arrow. (a) Al 2 (SO 4 ) 3 + NaOH  i) write down the reactants and interchange of anions to get product Al 2 (SO 4 ) 3 + 6NaOH  2Al(OH) 3 + 3Na 2 SO 4 All common Na compounds are water soluble Na + remain in solution. The combination of Al 3+ and OH - produce insoluble Al(OH) 3. Then the ionic equation is 2Al 3+ +3SO 4 2- + 6Na + + 6OH -  2Al(OH) 3 (s)+ 6Na + + 3SO 4 2- The net ionic equation is : Al 3+ + 3OH -  Al(OH) 3 (s)

20. (b) K 2 SO 4 (aq)+FeBr 3 (aq)  (c) CdCl 2 (aq) + (NH 4 ) 2 S(aq)