Textbook Question
For the evaporation of water, H2O(l) → H2O(g), AT 100°C, ∆H = +9.72 kcal/mol (+40.7 kJ/mol).How many kilojoules are released when 10.0 g of H2O(g) is condensed?
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8. Quantities in Chemical Reactions
Thermochemical Equations
Multiple Choice
The creation of liquid methanol is accomplished by the hydrogenation of carbon monoxide:
CO (g) + 2 H2 (g) → CH3OH (l) ∆Hº = – 128.1 kJ
How much heat (in kJ) is released when 125.0 g CO reacts with 2.32 x 102 g H2?
A
275 kJ
B
-572 kJ
C
-128 kJ
D
281 kJ
Verified step by step guidance1
First, determine the molar mass of carbon monoxide (CO) and hydrogen (H2). The molar mass of CO is approximately 28.01 g/mol, and the molar mass of H2 is approximately 2.02 g/mol.
Calculate the number of moles of CO and H2 present in the given masses. Use the formula: \( \text{moles} = \frac{\text{mass}}{\text{molar mass}} \). For CO, use 125.0 g, and for H2, use 232 g.
Identify the limiting reactant by comparing the mole ratio from the balanced chemical equation. The equation shows that 1 mole of CO reacts with 2 moles of H2. Calculate the moles of each reactant and determine which one is the limiting reactant.
Use the enthalpy change (∆Hº = –128.1 kJ) to calculate the heat released. Since the reaction releases 128.1 kJ per mole of CO reacted, multiply the moles of the limiting reactant by the enthalpy change to find the total heat released.
Ensure the sign of the heat change is negative, indicating that the reaction is exothermic and releases heat. The final value should reflect the total heat released based on the limiting reactant.
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