1. Section 7.5 B) Reasonable efficiency for converting chemical energy to electr

Question

1. Section 7.5

B) Reasonable efficiency for converting chemical energy to electrical energy via combustion is 30%. Calculate how many pounds of coal must be burned to run your reading lamp for two hours of studying for a chemistry exam. A typical reading lamp has a 75 W bulb. Assume that coal is pure graphite [C(s)]. The combustion of each mole of graphite requires one mole of oxygen gas and produces one mole of carbon dioxide gas. Hf(CO2 (g)) = -393.5 kJ/mol. (1 W = 1 J/s, 1 lb = 453.592 g) Give your answer to two decimal places

A) The enthalpy (heat) of melting of a substance is 23200 Joules per mole. Its entropy of melting is 60 Joules/degree-mole. What is its equilibrium melting temperature?

Explanation / Answer

starting with the reaction
.. 1 C + 1 O2 ---> 1 CO2.. dHrxn = -32.808 kJ/g C..

using that dimensional analysis you've been practicing so diligently..
(2 hr x 75W) x (3600sec / 1hr) x (1J/s / 1W) x (1kJ / 1000J) x (1g C / 32.808kJ) x (100g burned C / 30g C needed) x (1lb / 454g) = 0.12g C

2 x 75 x 3600 / 1000 / 32.808 x 30 / 100 / 454 = 2412962.784

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