Question 7.4: Heat of Reaction: Mole Ratio Calculations How much heat is r......
Heat of Reaction: Mole Ratio Calculations
How much heat is released in kcal and kJ when 2.50 mol of O_2 reacts completely with methane?
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 Since the ΔH for the reaction is based on the combustion of 1 mol of methane, we will need to perform a mole ratio calculation.
BALLPARK ESTIMATE The balanced equation shows that 213 kcal (891 kJ) is released for each 2 mol of oxygen that reacts. Thus, 2.50 mol of oxygen should release a bit more than 213 kcal, perhaps about 250 kcal (1050 kJ).
To find the amount of heat released by combustion of 2.50 mol of oxygen, we include in our calculation a mole ratio based on the balanced chemical equation:
2.50 \cancel{mol\ O_2} × \frac{1\ \cancel{mol\ CH_4}}{2\ \cancel{mol\ O_2}} × \frac{-213\ kcal}{1\ \cancel{mol\ CH_4}} = -266 kcal
or
2.50 \cancel{mol\ O_2} × \frac{1\ \cancel{mol\ CH_4}}{2\ \cancel{mol\ O_2}} × \frac{-891\ kJ}{1\ \cancel{mol\ CH_4}} = -1110 kJ
The negative sign indicates that the 266 kcal (1110 kJ) of heat is released.
BALLPARK CHECK The calculated answer is close to our estimate (- 250 kcal or – 1050 kJ).