Question 7.3: Heat of Reaction: Mass to Mole Conversion How much heat is r......
Heat of Reaction: Mass to Mole Conversion
How much heat is released during the combustion of 7.50 g of methane (molar mass = 16.0 g/mol)?
CH_4(g) + 2 O_2(g) → CO_2(g) + 2 H_2O(l) ΔH = -213 \frac{kcal}{mol\ CH_4} = -891 \frac{kJ}{mol\ CH_4}
ANALYSIS We can find the moles of methane involved in the reaction by using the molecular weight in a mass to mole conversion, and then use ΔH to find the heat released.
BALLPARK ESTIMATE Since 1 mol of methane (molar mass = 16.0 g/mol) has a mass of 16.0 g, 7.50 g of methane is a little less than 0.5 mol. Thus, less than half of 213 kcal, or about 100 kcal (418 kJ), is released from combustion of 7.50 g.
Going from a given mass of methane to the amount of heat released in a reaction requires that we first find the number of moles of methane by including molar mass (in mol/g) in the calculation and then converting moles to kilocalories or kilojoules:
7.50 \cancel{g\ CH_4} × \frac{1\ \cancel{mol\ CH_4}}{16.0\ \cancel{g\ CH_4}} × \frac{-213\ kcal}{1\ \cancel{mol\ CH_4}} = -99.8 kcal
or
7.50 \cancel{g\ CH_4} × \frac{1\ \cancel{mol\ CH_4}}{16.0\ \cancel{g\ CH_4}} × \frac{-891\ kJ}{1\ \cancel{mol\ CH_4}} = -418 kJ
The negative sign indicates that the 99.8 kcal (418 kJ) of heat is released.
BALLPARK CHECK Our estimate was -100 kcal (-418 kJ)!