How much heat is evolved by 100 g of water when its temperature decreases from 25°C to 5°C? (heat capacity is 4.2 J/g·K)
Correct answer: D. -8400 J
- A. 84,000 J
- B. -2000/4.2 J
- C. 2000/4.2 J
- D. -8400 J
Explanation
Using the formula q = mcΔT, where ΔT = Tfinal - Tinitial = 5°C - 25°C = -20°C, q = (100 g)(4.2 J/g°C)(-20°C) = -8400 J. The negative sign indicates that heat is evolved (released).
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About Thermochemistry and Energetics of Chemical Reactions
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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