Question

What is the difference between the internal energy of combusiton and the heat of combustion?

For example, tungsten wire has a heat of combustion of 1400 cal/g, what would the internal enery of combustion be?

Answer 1

internal energy --> "U" property, it only accounts for thermal changes

heat of combustion -- >"H" property, it accounts for U + PV changes, therefore ,any pressure or specially volume changes due to combustion are accounted

Typoically dH > dU for combustion

mol of tungsten = mass/MW = 1/183.84 = 0.0054395

note htat

W + 3/2O2 = WO3

1.5 mol of O2 --> 0 mol of gas

0.0054395 mol of W --> requires 1.5*0.0054395 = 0.00815925 mol of gas

PV = nRT

v = nRT/P = (0.00815925 *0.082)(298) / 1 = 0.19937 L = 0.19937*10^-3 m3

W= -P*(V-Vi)

W = -101325*(0- 0.19937*10^-3) = 20.201 J = 20.201*10^-3 J

Hcombustion = U + PV = (1400)(4.184) + 20.201*10^-3 = 5857.62 J