Chem. 1A Week 4 Discussion Notes Dr. Mack/S12 Net Ionic Equations: Reactions in Aqueous Solutions One of the most important components of this course is the ability to predict and write the products of reactions in aqueous solutions. These are often called exchange or double displacement reactions where the cations and anions of aqueous species are swapped. AX + BY → AY + BX There are several types of these reactions that you will encounter, they are classified as follows: • Precipitation Reactions: Reaction where and insoluble solid forms. • Acid Base Reactions: Neutralization reactions where the products are water and a salt (usually) • Gas−Forming Reactions: Formation of an insoluble (or slightly soluble) gas. • Reduction and Oxidation (RedOx) Reactions: Electron transfer reactions. Precipitation Reactions: A precipitation reaction produces a water‐insoluble solid product known as a precipitate (ppt). The reactants in such reactions are generally strong electrolytes. When dissolved in water strong electrolytes dissociate into cations and anions. If the cation from one reactant can form an insoluble compound with the anion from the other reactant, precipitation occurs. The solubility of ionic compounds is governed by the solubility rules. It is your responsibility to learn these rules, there is a chart available to download on the Lab web page for the course. Example: Aqueous solutions of silver nitrate and sodium chloride are mixed. A white precipitate forms. Write the reaction for this process. AgNO3(aq) + NaCl(aq) → NaNO3(aq) + AgCl(s) Since all sodium and nitrate containing salts are strong electrolytes, silver chloride must be the ppt. Acid Base Reactions: For the purposes of this course, we will consider and acid as any substance that produces (or donates) H+(aq) in solution. A base is a substance that produces OH−(aq) or accepts a proton (H+) in solution. Please be sure to familiarize yourself with the strong and weak acids/bases. This information is in your nomenclature handout and in the text. Example: Solutions of hydrochloric acid and barium hydroxide are mixed. 2HCl(aq) + Ba(OH)2 (aq) → BaCl2 (aq) + 2H2 0(l) Gas−Forming reactions: When the exchange of cations and anions produce a compound that is an insoluble gas, often bubbles or an odorous gas will ensue. Example: 2HCl (aq) + Na2S (aq) → H2S (g) + 2 NaCl (aq) In this example, H2S(g) (rotten egg smell) is produced. Other gases that my form are CO2, SO2 and NH3 for instance. See your text for more examples. Page 1 of 2 Chem. 1A Week 4 Discussion Notes Dr. Mack/S12 Net Ionic Equations: Writing the molecular, ionic, and net ionic equations for a chemical reaction in aqueous solution. The steps for writing a net ionic equation for the exchange reaction of aqueous reactants are listed as follows: 1. Write the balanced molecular equation which gives the complete chemical formula and the phase (s, l, g or aq) and products. Swap the reactant cation /anion pair to form the products and use the solubility rules to determine the phase of the products. 2. Write the balanced ionic equation which shows all strong electrolytes as ions. 3. Cancel out the spectator ions. These are the ions that appear as both reactants and products in the ionic equation. Never break up (s), (l), (g) or weak electrolyte & molecular (aq) species. What remains is the net ionic equation. Example: The reaction of hydrochloric acid and a solution of barium hydroxide. Molecular Equation: 2HCl(aq) + Ba(OH)2 (aq) → BaCl2 (aq) + 2H2 0(l)
Ionic Equation: 2H + (aq) + 2Cl − (aq) + Ba2+ (aq) + 2OH − (aq) → Ba2+ (aq) + 2Cl − (aq) + 2H2O(l)
Result: 2H + (aq) + 2OH − (aq) → 2H2O(l)
Net Ionic Equatoin: H + (aq) + OH − (aq) → H2O(l)
Reduction and Oxidation Reactions (RedOx): When a chemical species undergoes oxidation, the change involves an atom or compound losing electrons. Conversely, reduction involves an atom or substance gaining electrons. Neither process can occur alone… that is, there must be an exchange of electrons in the process. The substance that is oxidized is the reducing agent, the substance that is reduced is the oxidizing agent. When a substance loses electrons its charge or oxidation number becomes more positive. The opposite holds for species undergoing reduction. Please refer to your text and class notes for the details on assigning oxidation numbers. Example: What are the oxidation numbers for the elements in the compound potassium permanganate? Recall that the sum of the oxidation numbers must be equal to the overall charge of the compound… zero in this case. KMnO4 K = +1, O = −2 Since the compound must have a charge of zero overall we can write: (+1) + (?) + 4×(−2) = 0 Mn = +7 Example: When solid copper is placed in a solution of silver ions, solid silver forms at the surface of the copper. The solution develops a blue color indicating copper (II) ions. Reduced Oxidized + Cu(s) → 2Ag(s) + Cu2+(aq)
2Ag+(aq) Oxidizing Reducing Agent Agent Copper is oxidized since its oxidation number increases from 0 to +2 (loss of 2 electrons) Page 2 of 2