Acid-Base, Gas, and Redox Reactions

Acid-Base Reactions Neutralization reactions: Acid reacts with Base to neutralize each other producing water (or a weak electrolyte) and an ionic compound (a salt). Arrhenius’s definition of acids and bases: – Acid: substances that produce H + ions in aqueous solution. Protons combine with water molecules to form H 3 O + (aq). – Bases: substances that produce OH - ions in aqueous solution

Types of Acids Polyprotic acids: Contains more than one ionizable hydrogen. – Example: H 2 SO 4 (aq) is considered diprotic (2 ionizable protons). H 3 PO 4 (aq) is considered triprotic (3 ionizable proton). How many ionizable protons does HCl(aq) release? 1 (monoprotic) How about acetic acid (HC 2 H 3 O 2 )? 1 (monoprotic)

Sample Problem Write the molecular and a net ionic equation for the reaction that occurs between aqueous H 2 SO 4 and aqueous LiOH. H 2 SO 4 (aq) + 2LiOH(aq)  2H 2 O(l) + Li 2 SO 4 (aq) 2H + (aq) + 2OH - (aq)  2H 2 O(l) (# of moles reduce)

Acid-Base Titrations Titration: a solution of known concentration is reacted with a solution of unknown concentration. Equivalence Point: the # of moles of OH - = # of moles of H +. In a titration this is usually signaled by an indicator. Titration formula: M A V A = M B V B

Sample Problem The titration of 20.0 mL of an H 2 SO 4 solution of unknown concentration requires mL of a M KOH solution to reach the equivalence point. What is the concentration of the unknown H 2 SO 4 solution? H 2 SO 4 (aq) + 2KOH(aq)  K 2 SO 4 (aq) + 2H 2 O(l) MAVA=MBVBMAVA=MBVB 2M A (20.0 mL H 2 SO 4 ) = (0.158 M KOH)(22.87 mL KOH) (0.158 M)(22.87 mL) M A = = 9.03X10 -2 M H 2 SO 4 (aq) (2)(20.0 mL)

Gas Evolution Reaction Two aqueous solutions are mixed and a gas is produced. Some intermediate products decompose further to gain stability. – Be familiar with the fact that: H 2 CO 3 breaks down to H 2 0(l) and CO 2 (g) H 2 SO 4 breaks down to H 2 O(l) and SO 2 (g) NH 4 OH breaks down to H 2 O(l) and NH 3 (g)

Sample Problem Write a molecular equation for the gas-evolution reaction that occurs when you mix aqueous hydrobromic acid and aqueous potassium sulfite. HBr(aq) + K 2 SO 3 (aq)  KBr(aq) + H 2 SO 3 (aq)  KBr(aq) + H 2 O(l) + SO 2 (g) 2HBr(aq) + K 2 SO 3 (aq)  2KBr(aq) + H 2 O(l) + SO 2 (g)

Oxidation-Reduction Reactions (Redox) In a redox reaction, electrons are transferred (or shared unevenly) between one reactant and the other. Oxidation: Loss of electrons or gain of oxygen Reduction: gain of electrons or loss of oxygen Oxidizing agent: the substance that is reduced Reducing agent: the substance that is oxidized Great mnemonic device: LEO the lion goes GER. Loss Electrons Oxidation Gain Electrons Reduction

Use The Rules for Signing Oxidation # Assign an oxidation state to each of atom in each element, ion, or compound. a.) Cr b.) Cr 3+ c.) CCl 4 d.) SrBr 2 e.) SO 3 f.) NO 3 - a.) Cr = 0 b.) Cr 3+ = +3 c.) C = +4 Cl = -1 d.) Sr = +2 Br = -1 e.) S = +6 O = -2 f.) N = +5 O = -2

Another Sample Problem Use oxidation states to identify the element that is oxidized and the element that is reduced in the following redox reaction. Also indicate the oxidizing agent and the reducing agent. Sn(s) + 4HNO 3 (aq)  SnO 2 (s) + 4NO 2 (g) + 2H 2 O(l) Sn is oxidized and N is reduced N is the oxidizing agent and Sn in the reducing agent.

Chapter 4 pg. 189 #’s 80, 84, 88, 90, 92, & 96 (only the a’s) Study for Exam