Chapter 4 Chemical Quantities and Aqueous Reactions

What Happens When a Solute Dissolves? there are attractive forces between the solute particles holding them together there are also attractive forces between the solvent molecules when we mix the solute with the solvent, there are attractive forces between the solute particles and the solvent molecules if the attractions between solute and solvent are strong enough, the solute will dissolve 2

Salt vs. Sugar Dissolved in Water 3 ionic compounds dissociate into ions when they dissolve molecular compounds do not dissociate when they dissolve

Electrolytes and Nonelectrolytes materials that dissolve in water to form a solution that will conduct electricity are called electrolytes materials that dissolve in water to form a solution that will not conduct electricity are called nonelectrolytes 4

Electrolytes in Aqueous Solution Weak Electrolytes: Compounds that dissociate to a small extent into ions when dissolved in water. H 1+ (aq) + CH 3 CO 2 1- (aq)CH 3 CO 2 H(aq) Strong Electrolytes: Compounds that dissociate to a large extent into ions when dissolved in water. K 1+ (aq) + Cl 1- (aq)KCl(aq) Nonelectrolytes: Substances which do not produce ions in aqueous solutions. H2OH2O C 12 H 22 O 11 (aq)C 12 H 22 O 11 (s)

Acids acids are molecular compounds that ionize when they dissolve in water the molecules are pulled apart by their attraction for the water when acids ionize, they form H + cations and anions the percentage of molecules that ionize varies from one acid to another acids that ionize virtually 100% are called strong acids HCl(aq)  H + (aq) + Cl - (aq) acids that only ionize a small percentage are called weak acids HF(aq) H + (aq) + F - (aq) 6

Acids, Bases, and Neutralization Reactions Weak acids and weak bases are weak electrolytes. Strong acids and strong bases are strong electrolytes.

Solubility of Ionic Compounds compounds that dissolve in a solvent are said to be soluble, while those that do not are said to be insoluble ◦ NaCl is soluble in water, AgCl is insoluble in water ◦ the degree of solubility depends on the temperature ◦ even insoluble compounds dissolve, just not enough to be meaningful 8

Precipitation Reactions and Solubility Guidelines

Precipitation Reaction Precipitation Reactions: Processes in which soluble reactants yield an insoluble solid product that falls out of solution. NaNO 3 (aq) + AgCl(s)AgNO 3 (aq) + NaCl(aq)

Aqueous Reactions and Net Ionic Equations Molecular Equation: All substances in the chemical equation are written using their complete formulas as if they were molecules. strong electrolytes indicated as (aq) soluble Precipitate insoluble NaNO 3 (aq) + AgCl(s)AgNO 3 (aq) + NaCl(aq) Ionic Equation: All of the strong electrolytes are written as ions. Ag 1+ (aq) + NO 3 1- (aq) + Na 1+ (aq) + Cl 1- (aq) Na 1+ (aq) + NO 3 1- (aq) + AgCl(s)

Aqueous Reactions and Net Ionic Equations Spectator Ions: Ions that undergo no change during the reaction and appear on both sides of the reaction arrow. Na 1+ (aq) + NO 3 1- (aq) + AgCl(s)Ag 1+ (aq) + NO 3 1- (aq) + Na 1+ (aq) + Cl 1- (aq) Net Ionic Equation: Only the ions undergoing change are shown. AgCl(s) Ag + (aq) + Cl 1- (aq)

Example Predict the products and write a balanced molecular equation, ionic equation and net ionic equation for the following reactions: ◦ K 2 S(aq) + Fe(NO 3 ) 2 (aq)  ◦ NaCl(aq) +(NH 4 ) 2 CO 3 (aq ) 

Acid-Base Reaction Acid-Base Neutralization Reactions: Processes in which an acid reacts with a base to yield water plus a salt. H 2 O(l) + NaCl(aq)HCl(aq) + NaOH(aq) Acid + Base  Water + Salt

Acids, Bases, and Neutralization Reactions H 3 O 1+ (aq) + Cl 1- (aq) HCl(aq) + H 2 O(aq) Acid (Arrhenius): A substance that dissociates in water to produce hydrogen ions, H 1+ : H 1+ (aq) + A 1- (aq)HA(aq) In water, acids produce hydronium ions, H 3 O 1+ : H 1+ (aq) + Cl 1- (aq)HCl(aq)

Acids, Bases, and Neutralization Reactions Ammonia, commonly called “ammonium hydroxide” is a base: M 1+ (aq) + OH 1- (aq)MOH(aq) Na 1+ (aq) + OH 1- (aq)NaOH(aq) NH 4 1+ (aq) + OH 1- (aq)NH 3 (aq) + H 2 O(aq) Base (Arrhenius): A substance that dissociates in water to produce hydroxide ions, OH 1- :

Acids, Bases, and Neutralization Reactions Write the molecular, ionic, and net ionic equations for the reaction of aqueous H 2 SO 4 and aqueous KOH Write the molecular, ionic, and net ionic equations for the reaction of aqueous NaOH and aqueous HF.

Titration A procedure for determining the concentration of a solution by allowing a carefully measured volume to react with a solution of another substance (the standard solution) whose concentration is known. Titrant: solution that is added from the buret Equivalence Point: The point at which stoichiometrically equivalent quantities of acid and base have been mixed together. Endpoint: is the point at which the titration is complete (usually by a sudden color change), as determined by an indicator, Types of acid-base titration ◦ Strong acid-strong acid ◦ Strong acid-weak acid ◦ Weak acid-strong base ◦ Weak acid – weak base 18

Gas Evolving Reactions Some reactions form a gas directly from the ion exchange K 2 S(aq) + H 2 SO 4 (aq)  K 2 SO 4 (aq) + H 2 S(g) Other reactions form a gas by the decomposition of one of the ion exchange products into a gas and water K 2 SO 3 (aq) + H 2 SO 4 (aq)  K 2 SO 4 (aq) + H 2 SO 3 (aq) H 2 SO 3  H 2 O(l) + SO 2 (g) 19

Compounds that Undergo Gas Evolving Reactions 20 Reactant Type Reacting With Ion Exchange Product Decom- pose? Gas Formed Example metal n S, metal HS acidH2SH2SnoH2SH2S K 2 S(aq) + 2HCl(aq)  2KCl(aq) + H 2 S(g) metal n CO 3, metal HCO 3 acidH 2 CO 3 yesCO 2 K 2 CO 3 (aq) + 2HCl(aq)  2KCl(aq) + CO 2 (g) + H 2 O(l) metal n SO 3 metal HSO 3 acidH 2 SO 3 yesSO 2 K 2 SO 3 (aq) + 2HCl(aq)  2KCl(aq) + SO 2 (g) + H 2 O(l) (NH 4 ) n anionbaseNH 4 OHyesNH 3 KOH(aq) + NH 4 Cl(aq)  KCl(aq) + NH 3 (g) + H 2 O(l)

Examples When an aqueous solution of sodium carbonate is added to an aqueous solution of nitric acid, a gas evolves Write a molecular equation for the gas-evolution that occurs when you mix aqueous hydrobromic acid and aqueous potassium sulfite

Oxidation-Reduction (Redox) Reactions 2Fe 2 O 3 (s)4Fe(s) + 3O 2 (g) Rusting of iron: an oxidation of Fe 4Fe(s) + 3CO 2 (g)2Fe 2 O 3 (s) + 3C(s) Manufacture of iron: a reduction of Fe

Oxidation-Reduction (Redox) Reactions 1.An atom in its elemental state has an oxidation number of 0. Rules for Assigning Oxidation Numbers Oxidation Number (State): A value which indicates whether an atom is neutral, electron-rich, or electron-poor. NaH2H2 Br 2 SNe Oxidation number 0 2.A monatomic ion has an oxidation number identical to its charge. Na Ca Al Cl 1- O 2- -2

Oxidation-Reduction (Redox) Reactions b)Oxygen usually has an oxidation number of -2. HO HHCa +2 3.An atom in a polyatomic ion or in a molecular compound usually has the same oxidation number it would have if it were a monatomic ion. a)Hydrogen can be either +1 or -1. OHO +1 HHO H +1

Oxidation-Reduction (Redox) Reactions HCl +1 c)Halogens usually have an oxidation number of Cl O Cr 2 O x 4.The sum of the oxidation numbers is 0 for a neutral compound and is equal to the net charge for a polyatomic ion. x = +6 2(x) + 7(-2) = -2 (net charge) H2SO3H2SO3 x-2 x = +4 2(+1) + x + 3(-2) = 0 (net charge)

Example Determine the oxidation number for each atom in the following compounds/molecules ◦ CO 2 CCl 4 ◦ CoSO 4 K 2 O 2

Types of Redox reaction Combustion Reaction of nonmetal and metal 2 H 2 (g) + O 2 (g)  2 H 2 O(g) 2 Na(s) + Cl 2 (g)  2 NaCl(s)

Identifying Redox Reactions Oxidation: losing one or more electrons decreasing in oxidation number  Reducing agent 2Fe 2 3O2(g)3O2(g)+4Fe(s)O3O3 (s)(s) +30 oxidation reduction -2 0 Reduction: gaining one or more electron increasing in oxidation number  Oxidizing agent

Oxidation and Reduction Oxidation and Reduction Another Definition Oxidation and Reduction oxidation occurs when an atom’s oxidation state increases during a reaction reduction occurs when an atom’s oxidation state decreases during a reaction 29 CH O 2 → CO H 2 O – oxidation reduction

Example Identify each of half reactions as 1) oxidation or 2) reduction. __A. Sn(s) Sn 4+ (aq) + 4e − __B. Fe 3+ (aq) + 1e − Fe 2+ (aq) __C. Cl 2 (g) + 2e − 2Cl - (aq) 30

Writing Oxidation and Reduction Reactions Write the separate half oxidation and reduction reactions for the following equation. 2Cs(s) + F 2 (g) 2CsF(s) 3 Na(l) + AlCl 3 (l)  3 NaCl(l) + Al(l) 31

Identify the Oxidizing and Reducing Agents in Each of the Following 3 H 2 S(aq) + 2 NO 3 – (aq) + 2 H + (aq)  3 S(s) + 2 NO(g) + 4 H 2 O(l) MnO 2 (s) + 4 HBr(aq)  MnBr 2 (aq)+ Br 2 (g)+ 2 H 2 O(l) 32

Combustion Reactions Reactions in which O 2 (g) is a reactant are called combustion reactions Combustion reactions release lots of energy Combustion reactions are a subclass of oxidation-reduction reactions Tro, Chemistry: A Molecular Approach 33 2 C 8 H 18 (g) + 25 O 2 (g)  16 CO 2 (g) + 18 H 2 O(g)

Combustion Products Reactions in which O 2 (g) is a reactant are called combustion reactions Combustion reactions release lots of energy Combustion reactions are a subclass of oxidation-reduction reactions to predict the products of a combustion reaction, combine each element in the other reactant with oxygen 34 ReactantCombustion Product contains CCO 2 (g) contains HH 2 O(g) contains SSO 2 (g) contains NNO(g) or NO 2 (g) contains metalM2On(s)M2On(s)

Practice – Complete the Reactions combustion of C 3 H 7 OH(l) combustion of CH 3 NH 2 (g) 35