Calorimetry, Heat and ΔH Problems

Transcription

1 Calorimetry, Heat and ΔH Problems 1. Calculate the quantity of heat involved when a 70.0g sample of calcium is heated from C to C. c Ca= J/g C q = 2.91 kj 2. Determine the temperature change involved when 452g of nickel absorbs 102.5kJ of heat. C Ni= J/g C T = 511 C 3. Calculate the amount of heat required for the fusion of 375g of ethanol. The molar heat of fusion for ethanol is kj/mol. H = kj 4. A student carefully heated a sample of arsenic tribromide until the compound melted. The molten compound was then poured into a calorimeter containing 300. g cold water. When the last bit of the compound had solidified, the temperature of the water had increased 1.63 C. Calculate the enthalpy change involved for this process. H = - q = kj 5. A sample of molten gallium at its melting point is added to 200.g water in a calorimeter. The temperature of the water increased by 1.20 C once the gallium had solidified. Calculate the enthalpy change involved in the solidification of gallium. H = - q = kj 6. Determine the total energy change when 250. g of snow at C is heated to liquid water at 35.6 C. You should always use a heating curve to ID all parts of this type of calculation. E total = q ice + H fus + q water = 127 kj 7. Calculate ΔH for the reaction CH 4 (g) + NH 3 (g) HCN (g) + 3 H 2 (g), given: N 2(g) + 3 H 2(g) 2 NH 3(g) ΔH = kj REV /2 C(s) + 2 H 2(g) CH 4(g) H 2(g) + 2 C(s) + N 2(g) 2 HCN(g) CH 4 (g) + NH 3 (g) HCN (g) + 3 H 2 (g) ΔH = kj REV ΔH = kj /2 ΔH = kj 8. Consider the combustion of octane, express this process using each of the three methods studied (as a balanced equation with the heat of reaction, thermochemical equation, and enthalpy diagram). heat of reaction : 2 C 8H 18 (l) + 25 O 2 (g) 16 CO 2 (g) + 18 H 2O (g) ΔH = kj thermochemical equation : 2 C 8H 18 (l) + 25 O 2 (g) 16 CO 2 (g) + 18 H 2O (g) kj enthalpy diagram: Combustion of Octane 2 C 8H O 2 Enthalpy Change (kj) ΔH = kj 16 CO H 2O

2 9. Determine the ΔH using formation values for the reaction: 2Al + Fe 2 O 3 2Fe + Al 2 O 3. H = H fp - H fr = ( 0 kj kj) ( 0 kj kj) = kj Le Chatelier Principle Problems provide valid reasons for each shift 1. For the reaction H 2(g) + Cl 2(g) 2HCl (g) + Heat, predict the shift in the equilibrium position that will occur when each of the following changes is imposed: A) HCl is added to reactant B) H 2 is added to products C) The volume is decreased no change equal mol on each side D) The temperature is increased to reactant 2. For the reaction CaCO 3(s) + heat CO 2(g) + CaO (s), predict the shift in the equilibrium position that will occur when each of the following changes is imposed: A) More CaCO 3 is added to products B) Temperature is increased to products C) a leak in the container gases to escape to products 3. For the reaction 2SO 3(g) kj 2SO 2(g) + O 2(g), predict the shift in the equilibrium position that will occur when each of the following changes is imposed: A) Decrease the volume to reactant B) Decrease the temperature to reactant 4. For the reaction CH 4(g) + H 2 O (g) + Heat CO (g) + 3H 2(g), predict the shift in the equilibrium position that will occur when each of the following changes is imposed: A) Add heat to products B) Decrease volume to reactant C) Remove CO to products 5. For each of the previous reactions, indicate what conditions of temperature and pressure (high or low for each) will maximize product. Explain. Temperature: Qu 1 requires lower temperature, 2, 3, 4 require increase T Pressure: Qu 1 & 2 are unaffected by pressure, 3,4 require decrease P Equilibrium Problems 1. Write the equilibrium constant expression for each of the following. a) CH 4 + H 2 O CO + 3H 2 b) N 2 H 4 + 2O 2 2NO + 2H 2 O [ CO ] [ H K eq = ] 3 [ NO ] K eq = [ H 2O] 2 [CH 4 ] [ H 2 O] [N 2 H 4 ] [O 2 ]2 2. An equilibrium: H 2 + I 2 2HI (all gases) exists in a container. The concentrations are: [H 2 ]= 0.200M, [I 2 ]= 0.100M, [HI]= 0.003M. Calculate K eq = N 2 decomposes to produce NO 2 according to the reaction: N 2 2NO 2. Keq=1.50 x 10-2 under certain conditions. If the concentration of N 2 under these conditions is 0.500M, what is the NO 2 concentration? [ NO 2] = M

3 4. At a certain temperature, the following equilibrium exists: H 2 + 2O 2 2H 2 O. At equilibrium, the concentrations of H 2, O 2 and H 2 O are M, M and 1.20 x 10-4 M respectively. Calculate K eq = Given the following K eq values, in which case are products most favoured? Explain. a) 2.30 x 10 4 b) 2.30 x 10-4 c) Write the solubility product expression for each of the following. a) copper(ii) hydroxide K sp = [Cu 2+ ] [OH 1- ] 2 b) silver sulfide K sp = [Ag 1+ ] 2 [S 2- ] 7. Calculate the silver ion concentration in a saturated solution of silver iodide, K sp = 8.3 x :1 ion ratio [Ag 1+ ] = 9.1 x 10-9 M 8. Calculate the calcium ion concentration in a saturated solution of calcium fluoride, K sp = 3.9 x :1 ion ratio Let x = [Ca 2+ ] and 2x = [F 1- ].. [Ca 2+ ] = 2.2 x 10-4 M 9. Consider the concept of the common ion effect. What is the sulfate ion concentration of a 1.00L saturated solution of barium sulfate (K sp = 1.1 x ) that has had mol of barium nitrate added? [S 2- ] = 1.44 x 10-9 M Acid-Base Concepts 1. Write an equation for the ionization of the following acids in water: a) HClO4 H + + ClO4 - b) H2SO4 H + + HSO4 - c) HC2H3O2 H + + C2H3O2 - d) H2S H + + HS - e) HCl H + + Cl - f) HNO3 H + + NO3-2. Write an equation for the dissociation of the following bases in water: a) Mg(OH)2 Mg OH - b) NaOH Na + + OH - c) Ba(OH)2 Ba OH - d) KOH K + + OH - e) LiOH Li + + OH - 3. What is the conjugate base of HSO4 -? SO Write the formula and name for the conjugate acid for the following bases: a) NH3 conjugate acid = NH4 + _ name = ammonium ion b) PO4 3- conjugate acid = _HPO4 2- name = _hydrogen phosphate ion_ c) CH3COO - conjugate acid = CH3COOH name = acetic acid d) F - conjugate acid = HF name = hydrofluoric acid_

4 5. Write the formulas of the conjugate acids for the following bases: a) CN - conjugate acid is HCN b) HCO3 - conjugate acid is H2CO3 c) NH3 conjugate acid is NH4 + d) PO4 3- conjugate acid is HPO Write a balanced equation for the Bronsted-Lowry acid HPO4 2- in water. HPO H2O(l) H3O + (aq) + PO4 3- (aq) ph 7. What is the equation used for finding ph? ph = -log ([H + ]) 8. What is the equation that relates ph and poh? ph + poh = Calculate the [OH - ] for a solution that has a hydrogen ion concentration of 4.9 x 10-6 M. [OH - ] = 2.0 x 10-9 M 10. Complete the following table: (first one is done as an example) [H3O + ] [OH - ] ph poh Acidic, Basic, or Neutral? a 1.0 x 10-3 M 1.0 x M Acidic b 2.19 x Basic c 4.5 x x Acidic d 1.6 x x Acidic e 3.2 x x Acidic 11. Determine the ph and poh of the following 4 solutions: a) A 4.5 x 10-3 M HBr solution. Strong acid: ph = -log (H3O + ) = 2.35, poh = = b) A 3.67 x 10-5 M KOH solution. Strong base: poh = -log (OH - ) = 4.435; ph = = c) A solution made by diluting 25 ml of 6.0 M HCl until the final volume of the solution is 1.75 L. M1V1 = M2V2 (6.0M)(0.025 L) = (M2)(1.75 L) M2 = M; ph = 1.07; poh = d) 2.35 L of an aqueous solution that contains 51.6 grams of barium hydroxide. Ba(OH)2 Ba OH - [Ba(OH)2] =51.6g/2.35L x (1 mol/171.35g) = M M x 2 = M; poh = -log (0.256 M) = ph = = Determine the ph and poh of a M sulfuric acid (H2SO4) solution. ph = -log[h + ] = -log( x 2) = 2.17 poh = = 11.83

5 13. Determine the ph and poh of a 4.3 x 10-4 M NaOH solution. poh = -log[oh - ] = -log(4.3 x 10-4 ) = 3.37 ph = 14 poh = = Equilibrium Constants of Weak Acids/Bases 14. What is the equilibrium constant expression (Ka) for the acid dissociation of formic acid (HCOOH)? HCOOH + H2O H3O + + COOH - Ka = [H3O + ][COOH - ] [HCOOH] 15. What is the ph of a M formic acid solution with a Ka = 1.8 x 10-4? [HA] comparison value > 500 quadratic formula not needed Ka [H + ] = M ph = - log ( ) = Calculate the ph of a M CH3NH3Cl solution. Kb for methylamine, CH3NH2, is 3.7 x [WB] comparison value > 500 quadratic formula not needed Kb [OH - ] = M poh = - log ( ) = ph = =