Question 1
Multiple choiceEasy1 markThe figure shows the alcohol burned in a calorimetry experiment. A 0.230 g sample is combusted at constant pressure, warming 100.0 g of water from 20.0 °C to 26.0 °C. For the estimate, use the heat gained by the water as the heat released by combustion. The specific heat capacity of water is 4.18 J g⁻¹ K⁻¹. What molar enthalpy change of combustion is obtained, in kJ mol⁻¹?
Show answer and explanation
Answer: A. The structure gives the formula C₂H₆O, so its molar mass is 46.07 g mol⁻¹. The water gains Q = mcΔT = 100.0 × 4.18 × 6.0 = 2508 J = 2.508 kJ. The amount of ethanol is 0.230/46.07 = 0.00499 mol, giving ΔH = −2.508/0.00499 = −502 kJ mol⁻¹.
B: uses the molar mass of CO₂, 44.0 g mol⁻¹, instead of ethanol’s. C: doubles the ethanol amount by applying the coefficient 2 for CO₂ to the fuel amount. D: reverses the sign, treating heat gained by the water as a positive enthalpy change for combustion.
Remember: heat released by an exothermic reaction gives a negative enthalpy change.