My question is more conceptual. It is expressed in a series of two problems:
What energy change occurs when 1.2 moles of methane combust with an enthalpppy change of -434 KJ?
CH4+2O2 TO CO2+2H20
I understand it is simply the moles of mthane times the enthalpy change yet I am curious if a stoichiometry set-up exists which our teacher neglegected to teacch us. That would manifest if given moles of oxygen. I would infer you either convert to methane, as 434 is per mol of methane. Or set up stoich and end up dividing by 4 somewhere.
Another example would be:
The heat of formation of Fe2O3 is -826 KJ/mol. Calc heat if 30 g of iron reacts.
4Fe+3O2 TO 2Fe2O3
As the heat is for the product the moles of iron must be converted to iron oxide, Hence:
30 g * 1 mole Fe/55.35 g * 2 mole Fe2O3/4 mole Fe * -826 kJ/1 mole Fe2O3
Any help would be greatly appreciated.
a. Enthalpy for the reaction is 434 kJ and that is for 16 g CH4 (1 mol).
So 434 x 1.2/1 = ? or you can do
1.2 mol CH4 = 1.2*16 = 19.2 and
434 kJ x 19.2/16 = ?
b. dHf for 1 mol = 826 kJ; therefore, dH rxn as written is 826/mol x 2 mol = 1652 kJ.
1652 kJ x (30g/4*atomic mass Fe) = ? kJ.
Thanks!
To answer your first question about the energy change in the combustion of methane:
The given balanced equation for the combustion of methane is: CH4 + 2O2 → CO2 + 2H2O
The enthalpy change for this reaction is -434 kJ per mole of methane (CH4). You correctly mentioned that the energy change can be calculated by multiplying the number of moles of methane by the enthalpy change.
So, the energy change for 1.2 moles of methane would be:
Energy change = 1.2 moles CH4 × (-434 kJ/mol CH4) = -520.8 kJ
Now, regarding your question about the stoichiometry setup with given moles of oxygen, let's say you are given moles of oxygen instead of moles of methane. In that case, you need to determine the stoichiometric ratio between methane and oxygen in the balanced equation.
In the balanced equation, you can see that 1 mole of methane reacts with 2 moles of oxygen. So, you would need to convert the moles of oxygen to the corresponding moles of methane. This can be done using the stoichiometric ratio:
Given moles of oxygen → Moles of oxygen × (1 mole CH4 / 2 moles O2)
Now, you have the moles of methane and you can proceed with the calculation using the enthalpy change.
Moving on to your second question about the heat calculation with the given mass of iron:
The given balanced equation for the reaction is: 4Fe + 3O2 → 2Fe2O3
The heat of formation of Fe2O3 is given as -826 kJ/mol Fe2O3. You correctly mentioned that the moles of iron (Fe) need to be converted to moles of iron oxide (Fe2O3) in order to calculate the heat.
Using the molar mass of iron (Fe) and the given mass of iron, you can convert the mass of iron (in grams) to moles of iron:
Moles of iron = Mass of iron (g) × (1 mole Fe / molar mass of Fe)
Then, using the stoichiometric ratio between iron and iron oxide in the balanced equation (4Fe + 3O2 → 2Fe2O3), you can convert the moles of iron to moles of iron oxide:
Moles of iron oxide = Moles of iron × (2 moles Fe2O3 / 4 moles Fe)
Finally, you can use the given heat of formation of Fe2O3 to calculate the heat change:
Heat change = Moles of Fe2O3 × (Heat of formation of Fe2O3)
I hope this explanation helps you understand the process for calculating heat changes in these reactions. Feel free to ask if you have any further questions!