help

[jennypear]

Propane, C3H8(g), is used in many instances to produce heat by burning:

C3H8(g) + 5O2(g) --> 3CO2(g) + 4H2O(g)

The standard enthalpy of reaction, , is -2,044 kJ. How many grams of oxygen would be needed to react with excess propane to produce 213.0 kJ of heat?


heat of formation of co2 and h2o were given previously
so i started by finding the heat of formation of c3h8
hrx=((sumHproduct) - (sum of Hreact))
-2044kJ=[3mol*-394KJ/mol)+(4mol*-286kJ/mol)]-x
x=-282kJ

not sure how to proceed from here?

[garytse86]

Propane, C3H8(g), is used in many instances to produce heat by burning:

C3H8(g) + 5O2(g) --> 3CO2(g) + 4H2O(g)

The standard enthalpy of reaction, , is -2,044 kJ. How many grams of oxygen would be needed to react with excess propane to produce 213.0 kJ of heat?



I think: the standard enthalpy of reaction is the enthalpy change per mole of c3h8 and per 5 moles of oxygen.

2044/5 = 408.8= per 1 mole of oxygen

so 213/408.8 moles gives 213 kj.

mass = moles * RMM

so mass = 213/408.8 * 32
=16.7 grams.

I hope this is right..... (Which I seriously doubt)

[wolfson_1123]

Just to say: The standard heat of formation (DH°f) for O2(g) is zero.

Yes your calculations look fine :-)