Chemistry: Principles and Reactions H Chemistry Masterton/Hurley NPHS
Chapter 8 p. 241
16) Calcium carbide, CaC2, is the raw material for the production of acetylene (used in welding torches). Calcium carbide is produced by reacting calcium oxide with carbon, producing carbon monoxide as a by-product. When one mole of calcium carbide is formed, 464.8 kJ is absorbed. a) Write the thermochemical equation for this reaction.
CaO + 3C CaC2 + CO H = +464.8 kJ b) Is the reaction exothermic or endothermic? Endothermic (H = positive) c) Draw and energy diagram showing the path of this reaction. (see ex – Fig. 8.4)
products
enthalpy H > 0 reactants
rxn path
d) What is H when 1.00 g of CaC2(g) is formed? 464.8kJ 1mol 1.00g 7.263kJ mol CaC2 64.0g e) How many grams of carbon are used up when 20.00 kJ of heat is absorbed? 20.0kJ 3molC 12.0g 1.549g 464.8kJ 1mol C
17) Calcium chloride is a compound frequently found in first aid hot packs. It gives off heat when dissolved in water. The following reaction takes place. 2+ - CaCl2(s) Ca (aq) + 2Cl (aq) H = -81.4 kJ a) Calculate H when one mole of calcium chloride precipitates from solution. +81.4 kJ b) What is H when 10.0 g of calcium chloride precipitates? 81.4kJ 1mol CaCl 10.0g 2 7.33kJ mol 111.1g
18) Consider the dissociation of water into ions. + - H2O(l) H (aq) + OH (aq) H = +55.8 kJ a) Calculate H when one mole of calcium chloride precipitates from solution -55.8 kJ b) What is H when 10.0 g of calcium chloride precipitates? 55.8kJ 1mol H O 1.0g 2 3.10kJ mol 18.0g Chemistry: Principles and Reactions H Chemistry Masterton/Hurley NPHS
19) Nitroglycerine, C3H5(NO3)3(l), is an explosive most often used in mine or quarry blasting.
It is a powerful explosive because four gases (N2, O2, CO2, and steam) are formed when nitroglycerine is detonated. In addition, 6.26 kJ of heat is given off per gram of nitroglycerine detonated. a) Write a balanced thermochemical equation for the reaction. 6.28 kJ 227.0 g 4 mol ΔH 5684.1 kJ - 5680 kJ g mol
4C3H5(NO3)3 6N2 + O2 + 12CO2 + 10H2O H = -5680 kJ
b) What is H when 4.65 moles of products are formed? 5684.1kJ x 29mol product 4.65mol product x 911kJ
20) Strontium metal is responsible for the red color in fireworks. Fireworks manufacturers use strontium carbonate, which can be produced by combining strontium metal, graphite 3 (C), and oxygen gas. The formation of one mole of SrCO3 releases 1.220 x 10 kJ of energy. a) Write a balanced thermochemical equation for the reaction. 3 Sr + C + 3/2 O2 SrCO3 H = -1.22 x 10 kJ b) What is H when 10.00 L of oxygen at 25 ºC and 1.00 atm are used by the reaction? PV = nRT (1 atm)(10.0 L) = (n)(0.0821 L·atm/mol·K)(298 K) n = 0.4087 mol
1.22 103kJ x 1.5mol O2 0.4087mol x 332kJ
21) A typical fat in the body is glyceryl trioleate C57H104O6. When it is metabolized in the body, it combines with oxygen to produce carbon dioxide, water, and 3.022 x 104 kJ of heat per mole of fat. a) Write a balanced thermochemical equation for the metabolism of fat. 4 C57H104O6(s) + 80O2 57CO2 + 52H2O H = 3.022 x 10 kJ b) How many kJ of energy must be evolved in the form of heat if you want to get rid of five pounds of this fat by combustion? 3.022 104 kJ 1mol 453.6g 5lb 7.753104 kJ mol C 57H104O6 884.0g 1lb c) How many nutritional calories is this (1 nutritional calorie = 1 x 103 calories)? 3.022 104 kJ 1kcal 18,530kJ 4.184kJ Chemistry: Principles and Reactions H Chemistry Masterton/Hurley NPHS
27) To produce silicon, used in semiconductors, from sand (SiO2), a reaction is used that can be broken down into three steps:
SiO2(s) + 2C(s) Si(s) + 2CO(g) H = +689.9 kJ
Si(s) + 2Cl2(g) SiCl4(g) H = -657.0 kJ
SiCl4(g) + 2Mg(s) 2MgCl2(s) + Si(s) H = -625.6 kJ
a) Write the thermochemical equation for the overall reaction for the formation of
silicon from silicon dioxide; CO and MgCl2 are by-products.
2C + 2Cl2 + 2Mg + SiO2 Si + 2CO + 2MgCl2 H = -592.7 kJ b) What is the H for the formation of one mole of silicon? H = 689.9 kJ -657.0 kJ – 625.6 kJ = -592.7 kJ c) Is the overall reaction exothermic? exothermic