Power and Energy Conversion EXAMPLE 14.1 Power and Energy Conversion How much electrical energy, in joules, is consumed by a 75-W bulb burning for 1.0 hr? Solution From the definition of a watt, 1 W = 1 J/s, we know that 75 W = 75 J/s. The bulb burns for 60 min 1 h 1 h x x 60 s 1 min = 3600 s = 270,000 J 75 J 1 s 3600 s x How much electrical energy, in joules, dos a 650-W microwave oven consume in heating a cup of coffee for 1.5 min? Exercise 14.1A (a) How much electrical energy, in kilojoules, does a 1200-W space heater consume when it runs for 8.0 h? (b) How much energy is that in kilowatt hours? Exercise 14.1B

C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(g) + 2201 kJ EXAMPLE 14.2 Energies of Chemical Reactions Burning 1.00 mol of propane releases 2201 kJ of energy. C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(g) + 2201 kJ How much energy is released when 15.0 mol of propane is burned? Solution We start with 15.0 mol C3H8 and use the balanced equation to form a conversion factor, just as we did with chemical conversions in Chapter 6. 2201 kJ 1 mol C3H8 15.0 mol C3H8 x = 33,000 kJ The reaction of nitrogen and oxygen to form nitrogen monoxide (nitric oxide) requires an input of energy. N2(g) + O2(g) + 18.07 kJ  2 NO(g) How much energy is absorbed when 5.05 mol of N2 reacts with oxygen to NO? Exercise 14.2A Photosynthesis is an endothermic reaction in which carbon dioxide and water are converted to sugars in a reaction that requires 15,000 kJ of energy for each 1.00 kg of glucose produced. How much energy, in kilojoules, is required to make 1.00 mol of glucose? Exercise 14.2B

(3 x C) + (8 x H) = (3 x 12.0 u) + (8 x 1.0 u) = 36.0 + 8.0 u = 44.0 u EXAMPLE 14.3 Energy Changes in Chemical Reactions How much energy is released when 225 g of propane (see Example 14.2) is burned? Solution The formula mass of propane (C3H8) is (3 x C) + (8 x H) = (3 x 12.0 u) + (8 x 1.0 u) = 36.0 + 8.0 u = 44.0 u The molar mass is therefore 44.0 g. Next, we use the molar mass to convert grams of propane to moles of propane. Now, proceeding as in Example 14.2, 225 g C3H8 x 1 mol C3H8 44.0 g C3H8 = 5.11 mol C3H8 5.11 mol C3H8 x 2201 kJ = 11,200 kJ How much energy, in kilocalories, is released when 4.42 g of methane is burned? CH4 + 2 O2  CO2 + 2 H2O + 803 kJ Exercise 14.3A How much energy, in kilojoules, is absorbed when 0.528 g of N2 is converted to NO (see Exercise 14.2A)? Exercise 14.3B