Standard enthalpies of formation The following table provides standard enthalpie

Question

Standard enthalpies of formation

The following table provides standard enthalpies of formation for methane (CH4), carbon dioxide (CO2), and water (H2O).

By definition, the enthalpy of formation of any element in its standard state is 0 kJ/mol.

Part C

Another way to produce energy is by burning natural gas, which is mostly methane (CH4). The reaction

CH4(g)+2O2(g)?CO2(g)+2H2O(g)

shows the combustion of methane. Assuming complete combustion, what mass of methane is needed to produce the same amount of energy as the fusion reaction in Part A?

Express your answer to three significant figures and include the appropriate units.

PART A =

?7.82

Explanation / Answer

First find dHr=?

Therefore,

dHr=dHf(products) - dHf(reactants)

dHr=-393.5 + 2(-285.83) - (-74.8) - 2(0)

dHr = -890.36 kJ/mol

Now,

Combustion of 1 mole of CH4 gives off 890.36kJ of heat.

Assume x gm of CH4 is needed to produce 7.82kJ of heat,

Therefore,

{x.(1mole of CH4)}/16.04 = (x/16.04) moles of CH4

Since you know that 1 mole produces 890.36 kJ of heat upon combustion, you can say that (x/16.04) moles will produce,

(X/16.04)*(890.36) kJ = 7.82kJ

Gives, x=0.1408 gm

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