EXAMPLE 14.1
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
1 h x
60 min
1 h x
3600 s x
60 s
1 min
= 3600 s
75 J
1 s
= 270,000 J
Exercise 14.1A
How much electrical energy, in joules, dos a 650-W microwave oven consume in heating a cup of coffee for
1.5 min?
Exercise 14.1B
(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?

EXAMPLE 14.2
Burning 1.00 mol of propane releases 2201 kJ of energy.
C
3
H
8
(g) + 5 O
2
(g)  3 CO
2
(g) + 4 H
2
O(g) + 2201 kJ
How much energy is released when 15.0 mol of propane is burned?
Solution
We start with 15.0 mol C
3
H
8 and use the balanced equation to form a conversion factor, just as we did with chemical conversions in Chapter 6.
15.0 mol C
3
H
8
2201 kJ x
1 mol C
3
H
8
= 33,000 kJ
Exercise 14.2A
The reaction of nitrogen and oxygen to form nitrogen monoxide (nitric oxide) requires an input of energy.
N
2
(g) + O
2
(g) + 18.07 kJ  2 NO(g)
How much energy is absorbed when 5.05 mol of N
2 reacts with oxygen to NO?
Exercise 14.2B
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?

EXAMPLE 14.3
How much energy is released when 225 g of propane (see Example 14.2) is burned?
Solution
The formula mass of propane (C
3
H
8
) 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.
225 g C
3
H
8 x
1 mol C
3
H
8
= 5.11 mol C
3
H
8
44.0 g C
3
H
8
Now, proceeding as in Example 14.2,
2201 kJ
5.11 mol C
3
H
8 x = 11,200 kJ
1 mol C
3
H
8
Exercise 14.3A
How much energy, in kilocalories, is released when 4.42 g of methane is burned?
CH
4
+ 2 O
2
 CO
2
+ 2 H
2
O + 803 kJ
Exercise 14.3B
How much energy, in kilojoules, is absorbed when 0.528 g of N
2 is converted to NO (see Exercise 14.2A)?