Key to Review Worksheet for Exam 2 Chem 101, Summer 2006 1. Balance the following chemical equations: + 2ZnO 3CaCl2 a) 2ZnS b) 2Na3PO4 + c) XeF6 3H2O + 3O2 + 2SO2 Ca3(PO4)2 XeO3 + + 6NaCl 6HF 2. Identify the type (or types) of reaction for each of the following: a) PbO(s) + C(s) Pb(s) + CO(g) single replacement, redox b) 2K(s) + Cl2(g) 2KCl(s) combination, redox c) 2AgBr(s) 2Ag(s) + Br2(l) decomposition, redox d) 2C6H6(l) + 15O2(g) 12CO2(g) + 6H2O(l) + heat combustion, exothermic e) AgNO3(aq) + H2S(aq) Ag2S(s) + HNO3(aq) double replacement, redox 3. For each of the following re-dox reactions, identify what is being oxidized and what is being reduced: a) 2Al + 3I2 2AlI3 Oxidized __Al___ Reduced ___I___ b) 2KBr + Cl2 2KCl + Br2 Oxidized __Br___ Reduced __Cl___ c) 2Ag + 2HNO3 2AgNO3 + H2 ___H___ Oxidized __Ag___ Reduced 4. Complete and balance the following chemical equations: a) 2K(s) + b) H2CO3(aq) c) CuCl2(aq) d) C3H8(g) e) Ca(s) 2KBr(s) CO2(g) + + + + Br2(l) Na2S(aq) H2O(l) CuS(s) 5O2(g) 4H2O(l) 2HCl(aq) CaCl2(aq) + + 2NaCl(aq) 3CO2(g) + + heat H2(g) 5. Draw an energy diagram for a chemical reaction that has the following characteristics: exothermic and fast reaction. Label the axes, reactants, transition state, products, activation energy and heat of reaction on your diagram. Transition State EA Energy Reactants Heat of Reaction Products Reaction Progress 6. How would each of the following change the rate of the following reaction? 2Mg + O2 2MgO + Heat a) The reaction is cooled to 0C Faster Slower b) The reaction is done in an atmosphere of pure O2 instead of in air (air is a mixture of gases that include O2) Faster Slower 7. Which way will the equilibrium of the following reversible reaction be shifted under each of the following conditions: N2 + O2 + heat 2NO a) Heat is added to the reaction Towards Reactants Towards Products b) NO is added to the reaction Towards Reactants Towards Products c) O2 is added to the reaction Towards Reactants Towards Products 8. Consider the compound Mg(OH)2 a) How many atoms of oxygen are in 2.0 moles of Mg(OH)2? mol Mg(OH)2 mol O atoms O 2.0 mol Mg(OH)2 x (2 mol O/1 mol Mg(OH)2) x (6.02 x 1023 atoms O/1 mol O) = 2.4 x 1024 atoms O b) How many moles of Mg(OH)2 contain 1.5 x 1023 atoms of Mg? atoms Mg mol Mg mol Mg(OH)2 1.5 x 1023 atoms Mg x (1 mol/6.02 x 1023 atoms) x (1 mol/1 mol) = 0.25 mol Mg(OH)2 c) How many grams of hydrogen are in 5.0 g of Mg(OH)2? g Mg(OH)2 mol Mg(OH)2 mol H g H 5.0 g Mg(OH)2 x (1 mol Mg(OH)2/58.33 g Mg(OH)2) x (2 mol H/1 mol Mg(OH)2) x (1.008 g H/1 mol H) = 0.17 g H 9. Consider the following reaction: 2Al + Fe2O3 Al2O3 + 2Fe + 204 kcal a) What type of reaction is this (besides redox)? Combination Decomposition Replacement Combustion b) Identify what in the reaction is oxidized and what is reduced: Oxidized __Al___ Reduced __Fe___ c) Is the reaction exothermic or endothermic ? d) How many grams of Fe2O3 are required to react with 5.00 g of Al? g Al mol Al mol Fe2O3 g Fe2O3 5.00 g Al x (1 mol/26.98 g) = 0.1853 mol Al 0.1853 mol Al x (1 mol Fe2O3/2 mol Al) = 0.9266 mol Fe2O3 0.9266 mol Fe2O3 x (159.7 g/1 mol) = 148 g Fe2O3 e) If 1.0 mol Al is reacted with 1.0 mol Fe2O3, which is the limiting reactant? (Circle your answer) Al or Fe2O3 10. Calculate the % composition by mass for Na2S Molar mass of Na2S = 78.04 g % Na = (45.98 g/78.04 g) x 100% = 58.92% Na % S = (32.06 g/78.04 g) x 100% = 41.08% S (58.92% + 41.08% = 100.00%) 11. Consider the following reaction: 2H2 + O2 2H2O a) If 10.0 g of H2 is reacted with 10.0 g of O2, which is the limiting reactant? g H2 mol H2 mol H2O g O2 mol O2 mol H2O Choose the reactant that gives the lower yield: 10.0 g H2 x (1 mol/2.016 g) x (2 mol H2O/2 mol H2) = 4.96 mol H2O 10.0 g O2 x (1 mol/32.00 g) x (2 mol H2O/1 mol O2) = 0.625 mol H2O b) What is the theoretical yield? g O2 mol O2 mol H2O g H2O 10.0 g O2 x (1 mol/32.00 g) x (2 mol H2O/1 mol O2) = 0.625 mol H2O (From 15a) 0.625 mol H2O x (18.016 g/1 mol) = 11.3 g H2O O2 c) If 10.0 g of H2O is produced, what is the % yield? (10.0 g/11.3 g) x 100% = 88.5% 12. Consider the following reaction: PCl5 PCl3 + Cl2 a) Write the equilibrium expression: Keq = [PCl3][Cl2]/[PCl5] b) If Keq = 2 x 10-2, does the equilibrium favor reactants (or) products (or) neither ? c) If Cl2 is removed from the reaction, does the Keq get larger (or) smaller (or) neither ? 13. Complete the following statements regarding gases: a) At constant T and n, the volume b) At STP, the volume increases c) At constant V and n, the temperature d) At constant P and n, the density 14. increases when P is decreased. when n is increased. increases increases when P is increased. when T is decreased. A steel cylinder with a volume of 25.0 L is filled with 125 g of O2 gas at 20.0C (R = 0.0821 L atm/mol K). a) What is the pressure (in atm) of O2 gas in the cylinder? PV = nRT P = nRT/V n = 125 g O2 x 1 mol/32.00 g = 3.906 mol P = (3.906 mol)(0.0821 L x atm/mol x K)(293 K)/25.0 L = 3.76 atm b) If 500.0 g of N2 gas is added to the cylinder, what is the total pressure in the cylinder? n = 500.0g x 1 mol/28.02 g = 17.84 mol P = nRT/V = (17.84 mol)(0.0821 L x atm/mol x K)(293 K)/25.0 L = 17.17 atm Ptotal = P1 + P2 = 3.76 atm + 17.17 atm = 20.9 atm 15. What is the density (in g/L) of H2S gas at STP? 1 mol H2S = 34.016 g = 22.4 L d = m/V = 34.016 g/22.4 L = 1.52 g/L 16. A balloon is filled with He gas to a volume of 1.0 L. If the pressure and the moles of He are held constant, and the temperature goes from 20.0C to 40.0C, what is the new volume? P1V1/T1 = P2V2/T2 Pressure is constant: P1 = P2 V1/T1 = V2/T2 V1 = 1.0 L T1 = 20.0C + 273 = 293 K T2 = 40.0C + 273 = 313 K V2 = V1T2/T1 = (1.0 L x 313 K)/293 K = 1.1 L 17. Consider the following reaction: 2H2(g) + O2(g) 2H2O(g) a) How many liters of H2 gas are required to react with 1.0 L of O2 gas at STP? L O2 mol O2 mol H2 L H2 1.0 L O2 x (1 mol/22.4 L) x (2 mol H2/1 mol O2) x (22.4 L/1 mol) = 2.0 L b) How many liters of H2O gas are produced when 10.0 g of H2 gas are reacted with sufficient O2 at STP? g H2 mol H2 mol H2O L H2O 10.0 g H2 x (1 mol/2.016 g) x (2 mol H2O/2 mol H2) x (22.4 L/1 mol) = 111 L H2O 18. Indicate whether each of the following substances are soluble in water: a) CaOH No b) K2S Yes c) H2S Yes d) Ba(NO3)2 Yes e) PbSO4 Yes f) C8H18 No 19. Write a balanced equation for the dissociation of the following electrolytes in water: a) NaNO3 (strong) NaNO3 Na+ + NO3- b) Cu(NO3)2 (strong) Cu(NO3)2 Cu2+ + 2NO3- c) HF (weak) HF + H2O F- + H3O+ 20. KCl has a solubility of 42.6 g/100 g H2O at 50C. How many grams of KCl are required to make a saturated solution with 50.0 mL of H2O at 50.0C? (density of H2O = 1.00 g/mL) mL H2O g H2O g KCl 50.0 mL H2O x (1.00 g/mL) x (42.6 g KCl/100 g H2O) = 21.3 g KCl 21. KCl has a solubility of 42.6 g/100 g H2O at 50C and 34.0 g/100 g H2O at 20C. If a saturated solution of KCl is made with 50.0 mL of H2O at 50C and then cooled to 20C, how many grams of KCl will precipitate out of the solution? (density of H 2O = 1.00 g/mL) g KCl dissolved at 50C = 21.3 g (see above, question 3) g KCl dissolved at 20C = 50.0 mL H2O x (1.00 g/mL) x (34.0 g KCl/100 g H2O) = 17.0 g g KCl that precipitate = 21.3 g – 17.0 g = 4.3 g 22. If ale is 7.5% (v/v) alcohol, how many ounces of alcohol will you consume if you drink 4.0 pints? (1 pint = 16 oz) pints ale oz ale oz alcohol 4.0 pints x (16 oz/1 pint) x (7.5 oz alcohol/100 oz ale) = 4.8 oz alcohol 23. If you were in the laboratory, how would you prepare each of the following solutions? a) 1.0 L of 5.0% (m/v) glucose L solution mL solution g glucose 1.0 L x (1000 mL/1 L) x (5.0 g glucose/100 mL) = 50. g glucose Weigh out 50. g glucose and place it in a 1.0 L volumetric flask. Add some water to dissolve glucose, then add water to 1.0 L total volume. b) 5.00 L of 0.100 M KCl 5.00 L x (0.100 mol KCl/1 L) x (74.55 g/1 mol) = 37.3 g KCl Weigh out 37.3 g KCl and place it in a 5.00 L volumetric flask. Add some water to dissolve KCl, then add water to 5.00 L total volume. 24. Consider the following reaction: Na2CO3(aq) + 2HCl(aq) CO2(g) + H2O(l) + 2NaCl(aq) How many mL of a 2.00 M HCl solution are required to react with 3.00 g of Na2CO3? 3.00 g Na2CO3 x (1 mol/105.99 g) = 0.0283 mol Na2CO3 0.0283 mol Na2CO3 x (2 mol HCl/1 mol Na2CO3) = 0.0566 mol HCl 0.0566 mol HCl x (1 L/2.00 mol) x (1000 mL/1 L) = 28.3 mL HCl solution 25. State whether each of the following substances would form a solution, colloid or suspension when mixed with water: a) starch colloid b) NaCl solution c) oil suspension d) glucose solution 26. Equal volumes of two solutions, A and B, are placed into a beaker where they are separated by a semi-permeable membrane that allows only water to pass through. A = 1.0 M NaCl and B = 1.0 M CaCl2. a) Which solution has the higher osmotic pressure? B (CaCl2 has more particles per mole than NaCl) b) On which side (A or B) will the volume increase over time? B (water flows from A to B due to osmosis) 27. What happens to a red blood cell when it is placed into each of the following: a) sea water It shrivels (water goes out of the cell due to osmosis). b) distilled water It bursts (water goes into the cell due to osmosis). 28. If a mixture of protein, starch, glucose and NaCl in water is placed into a dialysis bag, and the bag is put into a beaker of distilled water, which components will flow out of the bag? Glucose and NaCl will flow out of the bag.