The heat of combustion of ethane (C2H6) is -337.0 kcal at 25°C. The heat of the reaction when 3 g of ethane is burnt will be:
Correct answer: C. -33.7 kcal
- A. -3.37 kcal
- B. +3.37 kcal
- C. -33.7 kcal
- D. 33.7 kcal
Explanation
Option C is correct.The heat of combustion of ethane (C2H6) is -337.0 kcal at 25°C. This means that 337.0 kcal of heat is released when 1 mole of ethane is burned in oxygen. The heat of combustion of ethane is a state function, which means that it depends only on the initial and final states of the system, and not on the path taken. In this case, the initial state is 3 g of ethane and the final state is the products of combustion, which are carbon dioxide and water. The heat of the reaction when 3 g of ethane is burnt is -33.7 kcal. This is less than the heat of combustion of 1 mole of ethane, because only 3 g of ethane is burned. The heat of the reaction can be calculated using the following equation: Heat of reaction = Heat of combustion × Mass of ethane / Molar mass of ethane Substituting the values from the previous equations, we get: Heat of reaction = -337.0 kcal × 3 g / 30.07 g/mol Heat of reaction = -33.7 kcal Therefore, the heat of the reaction when 3 g of ethane is burnt is -33.7 kcal.
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Thermochemistry measures energy changes in chemical reactions and distinguishes exothermic from endothermic processes. Work covers systems, surroundings and state functions, internal energy, the first law of thermodynamics, enthalpy and Hess's law, including the sign conventions used when heat enters or leaves a system.
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