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Transcript
3/16/2009 The Mid-Term Test • COVERS ALL MATERIAL IN CHAPTERS 4, 5, and 6, PLUS NECESSARY SKILLS FROM CHAPTERS 1-3 • The following outline of the test indicates the most important topics and skills, but is not meant to indicate that items not listed here will not be covered • Section A: 8 multiple choice questions (16 marks) – Chapters 4-6 – Includes 5 calculations Section B: longer questions (34 marks) • Writing balanced equations – Learn Reaction Types – Learn Solubility Rules – Learn Activity Series (Na, Zn, H2, Cu, Ag) • Writing ionic equations and identifying spectator ions – Learn weak and strong electrolytes and acids/bases • • • • • Oxidation, reduction and oxidation number Stoichiometry – solutions, gases and heat Gas Laws and calculations Heat transfer and Calorimetry Hess’s Law and Heats of Formation 1 3/16/2009 The Mid-Term Test • UNITS AND SIGNIFICANT FIGURES • SHOW WORKING • The periodic table on the back of your lab manual will be provided, as well as the list of constants inside the back cover. Review of Chapter 6 • Checklist, Key Equations, Summary and Operational Skills on pages 194-195 • Types of energy (kinetic, potential, internal, heat) • ENTHALPY (H) is internal energy that can be released as heat • Law of conservation of mass • Identify the SYSTEM (reaction) and SURROUNDINGS • qsurroundings = -qsystem • qsystem is negative if heat is transferred from the system to the surroundings (qsurroundings is positive) • Calculate heat transferred to/from surroundings from: • Change in the temperature of the surroundings • A thermochemical equation – Exothermic vs. Enothermic reactions (sign of q and ? H; temperature change) 2 3/16/2009 Review of Chapter 6 • Calorimetry ? H = qreaction/nreaction or Heat of combustion = qreaction/norganic compound • Constant pressure: qreaction = -sm? T • Bomb: qreaction = -Ccalorimeter? T or qreaction = -(Ccalorimeter + mwater swater)? T • Use thermochemical equation stoichiometry to convert to ? H. E.g. for: 2C2H6(g) + 7O2(g) → 4CO2(g) + 6H2O(l) ? H = ? kJ/mol nreaction = mol C2H6 x (1 mol reaction/2 mol C2H6) Review of Chapter 6 • Hess’s Law • For a chemical reaction that can be written as the sum of two or more steps, the enthalpy change is the sum of the changes for the individual steps • You do not need to draw enthalpy diagrams • Standard Enthalpy of Formation • ? Hfo is the enthalpy change for formation of 1 mole of a substance in its standard state from its elements in their standard states ? Ho = ? n? Hfo(products) - ? n? Hfo(reactants) Applies only when all enthalpies of formation are given 3 3/16/2009 Applying Stoichiometry • The thermochemical equation provides a conversion factor to convert between moles of a substance and heat released or absorbed. Moles of substance ⇔ heat release or adsorbed – Note that the heat is determined by the limiting reagent • Example How much ethane would need to be burned to heat 100 g of water by 10 K? 2C2H6(g) + 7O2(g) → 4CO2(g) + 6H2O(l) ? H = -3122 kJ/mol Answer: 2.68x10-3 mol Calorimetry Example Problem • Sodium metal reacts with water to produce hydrogen gas and sodium hydroxide according to the equation: 2 Na(s) + 2 H2O(l) → 2 NaOH(aq) + H2(g) ?H = ? When 0.0300 mol of Na was added to 100.0 g of water, the temperature of the resulting solution rose from 25.0 oC to 37.9 oC. If the specific heat of the solution was 4.18 J g-1 K-1, calculate ? H, in kJ, for the reaction as written. • Answer: -359 kJ 4 3/16/2009 Gas Example Problem • A 1.00 L flask at 30.0 oC and 215 kPa contains a mixture of CO2(g) and Ar(g) in a 1:1 molar ratio (XAr = 0.500). Calculate the mass of Ar in the flask. • ANSWER: 1.70 g Gases Example Problems • Calculate the change in pressure (in torr) when 1 mL of water is added to dry air in a 10.0 L flask at STP (the vapour pressure of water is 4.6 torr at O oC). – ANSWER: pressure change = 4.6 torr • If Xoxygen = 0.20 in the dry air, calculate the partial pressure of O2 (in atm) after addition of the water. – ANSWER: Poxygen = 0.20 atm 5 3/16/2009 Titration Problem • 20.0 mL of a saturated solution of Ca(OH)2 was neutralized by 8.85 mL of 0.0510 M HCl. Calculate the solubility of Ca(OH)2 in mol/L. – Answer: 0.0113 mol/L Gases Example Problem • If 5.0 cm3 of gas at 25oC are heated to 250oC at constant pressure. What is the final volume of the gas in L? • Answer: 8.8 x 10-3 L 6