109

[Jade Fosburgh Discussion 2C]

Coal fired steam engines get their power by using the heat from burning coal to boil water. Suppose that coal of density 1.5 g/cm^3 is carbon. The combustion of carbon is described by the equation


C(s) + O2(g) --> CO2(g) and delta H = -394 kJ

a) calculate the heat produced when a lump of coal of size 7 cm x 6 cm x 5 cm is burned.


The solutions manual did:

heat = (7 cm x 6 cm x 5 cm)*(1.5g/cm^3)(-394 KJ/mol) / 12.01 g/mol

What is this equation they are using for heat? I thought it was q = mC(deltaT)

[Samantha Kan 2L]

For this problem they are finding the volume of the coal in cm^3 and using the given density to find the grams of carbon burned. Since the delta H given is in kJ/mol, you need to convert the grams of carbon burned to the mols of carbon burned, which is why they have the (12.01 g/mol) in the equation. To find the total amount of heat produced, you then multiply the moles of carbon previously calculated and the delta H given, which should leave you with a value in the units of kJ. Hope this helps!

[Cam Bear 2F]

For this question, why in question a) is q=-1.0x10^4 kJ but in question b) the solutions manual uses positive q (converted to joules 1.0x10^7 J) to solve the problem?