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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: moreche28 on February 15, 2020, 05:00:24 AM

Title: Calculating the amount of released energy.
Post by: moreche28 on February 15, 2020, 05:00:24 AM

There's a chemistry problem I'm trying to solve. Burning 1 kg of syntin or 1 kg of hydrogen will release more E? We predict that syntin decomposes into H2O and CO2. Their standard ΔH of formation: ΔH (CO2, g) =-393,5kJ/mol; ΔH(H2O) =-241,8 kJ/mol. Many thanks! :)

Title: Re: Calculating the amount of released energy.
Post by: Borek on February 15, 2020, 07:32:53 AM

You have to show your attempts at solving the problem to receive help, this is a forum policy.

Title: Re: Calculating the amount of released energy.
Post by: moreche28 on February 16, 2020, 02:39:14 PM

Oh, thank you. :) I will do so.

Title: Re: Calculating the amount of released energy.
Post by: Enthalpy on February 16, 2020, 07:02:25 PM

The usual combustion of Syntin is with pure oxygen, and despite high pressure, the high temperature makes about as much CO as CO₂.

Anyway, you can compute a heat of combustion, or you can answer qualitatively that the same mass of hydrogen releases more combustion heat than a hydrocarbon, because it contains many more moles. I know no exception. The margin is big.

"Energy" isn't accurate enough, and "E" seems to designate the internal energy in some countries. You're probably talking about the enthalpy H.

Title: Re: Calculating the amount of released energy.
Post by: moreche28 on February 18, 2020, 10:51:15 AM

Thank you, sir. :) So, I thought about a possible way how to calculate it... Here it is.

a)   Burning 1 kg of H2.

2 H2 (g) + O2 (g) => 2 H2O (g)

Mr = 2
Q1 = 2 . - 241,8 = - 483,6 kJ.mol-1
n = m/M = 1000 g / 2 g.mol-1 = 500 mol
E = 500 . - 483,6 = - 241 800 kJ = - 241,8 MJ

b)   Burning 1 kg of syntin.

C10H16 + 14 O2 => 10 CO2 + 8 H2O

Mr = 136,238
Q2 = (10 . – 393,5) + (8 . - 241,8) = - 5889,4 kJ.mol-1
n = m/M = 1000/136,238 = 7,34 mol
E = 7,34 . - 5889,4 = - 43228,196 kJ = - 43,228196 MJ.

Is that right? :D I presume not, but otherwise I cannot come to any other solution. Thanks.
   

Title: Re: Calculating the amount of released energy.
Post by: mjc123 on February 18, 2020, 01:18:22 PM

In a, what you have shown is the burning of 2 moles of H₂. Therefore the enthalpy of combustion is 241.8 kJ/mol, and E = 120.9 MJ.
The answer for b looks correct, but it has far too many sig figs.

Title: Re: Calculating the amount of released energy.
Post by: Enthalpy on February 19, 2020, 02:25:46 PM

I don't see the enthalpy of formation of Syntin anywhere in the enthalpy of combustion. It should be there. +133kJ/mol (Syntin is nicely endothermic).

(The enthalpy H is an energy. It's the one used in such computations. "Energy" isn't accurate enough. But for high school, this difference is normally ignored.)

500 moles H₂ produce 500 moles H₂O. -241.8kJ/mol is the enthalpy of formation for gaseous water at 298K. You compute here the lower calorific power of the fuel. Condensed water releases more heat, which defines the higher calorific power. And at a different temperature, the result would differ too.

Syntin is a rocket fuel, hence burned with oxygen. At that price, I don't imagine torching it in a stove. But in a rocket, the hot combustion would produce much CO, so computing with CO₂ is quite artificial.

We can guess your symbols Q, n, m... but they are no universal convention.