Calculate the standard enthalpy of formation of liquid methanol, CH_3OH(l), usin

Question

Calculate the standard enthalpy of formation of liquid methanol, CH_3OH(l), using the following information: C(graph) + O_2 rightarrow CO_2(g) deltaHdegree = -393.5 kJ/mol H_2(g) + (1/2)O_2 - H_2O(1) deltaHdegree = -285.8 kJ/mol CH_3OH(l) + (3/2)O_2(g) rightarrow CO_2(g) + 2H_2O(l) deltaHdegree = -726.4 kJ/mol Calculate the change in internal energy (deltaE) for a system that is absorbing 35.8 kJ of heat and is expanding from 8.00 to 24.0 L in volume at 1.00 atm. Question: How would it change if the volume is compressed? Calculate the energy of the green light emitted, per photon, by a mercury lamp with a frequency of 5.49 times 10^14 Hz. How many photons are contained in a burst of yellow light (589 nm) from a sodium lamp that contains 609 kJ of energy?

Explanation / Answer

A - dHorxn = dHo(products) - dHo(reactants)

-726.4 = (-393.5 + 2 x -285.8) - (dHof[CH3OH(l)])

Thus, standard enthalpy of formation for liquid CH3OH,

dHof[CH3OH(l)] = -238.7 kJ/mol

B - we have,

change in internal energy dE = q - w

w = -P(V2 - V1) = -1(24 - 8) = -16 L.atm x 101.325 = -1.61 kJ

Thus with q = -35.8 kJ

we get,

dE = -35.8 - (-1.61) = 34.2 kJ

If the volume is compressed, the change in internal energy would be = -34.2 kJ

C - Energy = hv

for the given frequency,

energy = 6.626 x 10^-34 x 5.49 x 10^14 = 3.64 x 10^-19 per photon

D - energy of yellow light,

wavelength = 589 nm = 5.89 x 10^-7 m

E = hc/l = 6.626 x 10^-34 x 3 x 10^8/5.89 x 10^-7 = 3.37 x 10^-22 kJ

number of photon = 609/3.37 x 10^-22 = 1.80 x 10^24 photon

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