Announcements Print worksheet #6 prior to your Tuesday discussion section LON-CAPA assignment 4 is now available Don t forget to bring your Clicker to class EVERY day The points from last week have been uploaded to the online gradebook. Check to make sure your points are shown! Exam 1 is on Thursday, September 23 from 7-8:10pm; Conflict exam is from 5:35-6:45pm
Aqueous Reactions Lead(II) nitrate is a solid that can be dissolved in water Lead(II) nitrate is a strong electrolyte, so it dissociates completely into ions when dissolved in water Pb(NO 3 ) 2 (s) + H 2 O (l) Pb 2+ (aq) + 2 NO 3- (aq) Potassium iodide is a solid that can be dissolved in water Potassium iodide is a strong electrolyte, so it dissociates completely into ions when dissolved in water KI (s) + H 2 O (l) K + (aq) + I - (aq)
Aqueous Reactions When solutions of lead(ii) nitrate and potassium iodide are mixed together, a yellow solid forms in the solution The yellow solid is known as a precipitate Pb 2+ (aq) + 2 NO 3- (aq) + K + (aq) + I - (aq) What s the identity of the yellow solid? yellow solid + solution Pb(NO 3 ) 2 PbI 2 KNO 3 KI displacement reaction the ions switch partners
Solubility rules 1. Most NO 3- salts are soluble 2. Most salts of alkali metals (IA) and NH 4+ are soluble 3. Most Cl -, Br - and I - salts are soluble Exceptions: Ag +, Pb 2+ and Hg 2+ 2 insoluble 4. Most SO 2-4 are soluble Exceptions: Ca 2+, Ba 2+, Pb 2+, Hg 2+ 2 insoluble 5. Most OH - are insoluble Exceptions: IA, Ca 2+, Ba 2+, Sr 2+ soluble 6. Most S 2-, CO 2-3, CrO 2-4, PO 3-4 insoluble Exceptions: IA, NH 4+ soluble
Pb 2+ (aq) + 2 NO 3- (aq) + K + (aq) + I - (aq) PbI 2 (s) + K + (aq) + NO 3- (aq) yellow solid + solution 1. Most NO 3- salts are soluble 2. Most IA salts are soluble PbI 2 KNO 3 3. Most Cl -, Br - and I - salts are soluble Exceptions: Ag +, Pb 2+ and Hg 2+ 2 insoluble This equation still needs to be balanced...
balance molecular equation Pb(NO 3 ) 2 (aq) + 2KI (aq) PbI 2 (s) + 2KNO 3 (aq) balance ionic equation 1. Most NO 3 2- salts are soluble 2. Most Group I salts are soluble 3. Most Cl -, Br - and I - salts are soluble except Ag +, Pb 2+ and Hg 2+ 2 Pb 2+ (aq)+ 2NO 3- (aq) + 2K + (aq) + 2I - (aq) PbI 2 (s) +K 2 + (aq) + 2NO 3- (aq)
The ionic equation contains several ions that appear as both reactants and products these are known as spectator ions Pb 2+ (aq)+ 2NO 3- (aq) + 2K + (aq) + 2I - (aq) PbI 2 (s) + 2K + (aq) + 2NO 3- (aq) A net ionic equation eliminates all the spectator ions: Pb 2+ (aq) + 2I - (aq) PbI 2 (s)
Acid/Base reactions HCl dissociates completely H + (aq) Cl - (aq) strong NaOH dissociates completely strong Na + (aq) OH - (aq) acid base molecular eqn HCl(aq)+ NaOH(aq) H 2 O(l) + NaCl(aq) ionic eqn net ionic eqn H + +Cl - + Na + + OH - H 2 O + Na + + Cl - H + + OH - H 2 O
Acid/Base reactions HCl dissociates completely H + (aq) Cl - (aq) strong acid Mg(OH) 2 does not dissociate completely weak base Mg(OH) 2 (aq) Mg 2+ (aq) + 2OH - (aq) H + + Cl - + Mg(OH) 2 + Mg 2+ + 2OH - H 2 O+ Mg 2+ + Cl - molecular eqn Mg(OH) 2 + 2 HCl 2 H 2 O+ MgCl 2 ionic eqn Mg(OH) 2 + 2H + + 2Cl - 2 H 2 O+ Mg 2+ +2Cl - net ionic eqn Mg(OH) 2 (s) +2 H + (aq) 2 H 2 O(l) + Mg 2+ (aq)
Acid/Base reactions neutralization reactions produce water from an acid and a base At the equivalence point, [H + ] = [OH - ] mol H + L = mol OH - L What volume of 0.100 M HCl is needed to neutralize 25.00 ml of 0.350 M NaOH? 25.00 x 10-3 L 0.350 mol NaOH = 8.75 x 10-3 mol NaOH L 8.75 x 10-3 mol HCl 1 L = 0.0875 L= 87.5 ml HCl 0.100 mol HCl
A 2.2 g sample of an unknown acid, HA, is dissolved in 1.0 L of water. A titration required 25.0 ml of 0.500 M NaOH to react completely. What is the molecular weight of the unknown? molecular weight = g HA mol HA = mol HA = mol H + = mol OH - 2.2 g HA? mol HA 0.025 L 0.500 mol NaOH L = 0.0125 mol OH - 2.2 g HA 0.0125 mol HA = 180 g/mol
Enthalpy *The textbook goes into too much detail in sections 6.1-6.2 ΔH = enthalpy change = heat gained/released by a reaction ΔH > 0 means the reaction uses heat as a reactant. The reaction needs the heat to generate the products. (Endothermic) ΔH < 0 means heat is a product of the reaction. Heat is given off during the course of the reaction. (Exothermic) See problems 6.41-6.42 for practice identifying endothermic and exothermic reactions
Enthalpy The amount of heat required/released by a reaction can be determined using stoichiometry: CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O(l) ΔH = -891 kj The stoichiometry implies that 891 kj of heat are released for every 1 mole of CH 4 reacted... or that 891 kj of heat are released for every 2 moles of H 2 O produced or that 891 kj of heat are released for every 1 mole of CO 2 produced, etc. How much heat is released if 1.00 g of CH 4 reacts with an excess of oxygen? 1.00 g CH 4 1 mol CH 4-891 kj 16.042 g CH4 1 mol CH 4 = -55.5 kj heat See problems 6.43-6.44 for more stoichiometry practice