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Thermodynamics/Combustion of ethane

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QUESTION: The combustion of ethane (C2H4) is an exothermic reaction.
-The subscripts 'g' and 'l' written after each chemical formula represents 'gas' or 'liquid':

C2H4(g) + 3O2(g) -> 2H2O(l) + 2CO2(g)
And the ΔH = -1401kJ/mol

Calculate the amount of heat liberated when 4.79g of C2H4 react with excess oxygen.

-So first, I adjusted the given standard molar enthalpy change of ethane (by multiplying it by the 4 moles on the product side) in order to include it in the chemical equation:
(-1401kJ/mol)(4mol) = -5604kJ

So the equation would look like the following:
C2H4(g) + 3O2(g) -> 2H2O(l) + 2CO2(g) + 5604kJ

Since there is 1 mole of C2H4 reacting to release 5604kJ of energy, then that means that there are 28.054g of C2H4 reacting (due to its molar mass of 28.054g/mol).

28.054g -> -5406kJ
4.79g -> x
x = -923kJ

So the amount of heat liberated seems to be 923kJ. However in the question, it asks "... with excess oxygen", in which I am unsure of what that means and if it affects the answer that I just obtained. Could you please explain to me why sometimes questions would ask to calculate something when the reaction occurs with a sufficient vs. excess amount of a substance? (Example: "Calculate the amount of heat liberated when 4.79g of C2H4 react with EXCESS oxygen." vs. "Calculate the amount of heat liberated when 4.79g of C2H4 react with SUFFICIENT oxygen." What is the difference?)

ANSWER: Hi K,

Two things.

1. There is no need to multiply by 4. One mole of C2H4 reacts to give -1401 kJ.

2. In bomb calorimiters it is normal practice to have an excess of O2 in order to ensure complete combustion. It is a practical expedient and does not affect the result. Incomplete combustion would lead to false results. In this instance 'sufficient' and 'excess' mean the same thing that is complete combustion is obtained.

I hope this helps.
Best wishes,
Kevin

---------- FOLLOW-UP ----------

QUESTION: Hi, I am still confused about how to solve problems in which the enthalpy change either is or is not included in the chemical equation. May I ask that you solve (showing all your steps) the following equations:

1) The enthalpy change IS NOT included in the chemical equation...

The following synthesis reaction has an enthalpy change of -411kJ/mol:

Na(s) + 1/2Cl2(g) -> NaCl(s)

What mass of sodium (Na) would be needed for the reaction to release 2000kJ?

2) The enthalpy change IS included in the chemical equation...

Below is how iron oxide (Fe2O3) is formed:

4Fe(s) + 3O2(g) -> 2Fe2O3(s) + 1648.4kJ

Using 100.0g of iron and sufficient oxygen (O2), what energy is released by the reaction?

-By the way, for problem 1), I got 114.95g of sodium. For problem 2), I got 1476kJ of energy is released. I'm guessing that one of them is wrong, and now I'm unsure of how to solve exactly depending on whether the enthalpy change of the reaction is included or excluded from the chemical equation.

Answer
Hi K,

Its all in the units used. In the first case it is kJ per mol. In the second case it just the heat evolved for the given chemical equation in this case 4 mol Fe.

Q1. One mol of Na reacts to give -411kJ. So 2000 kJ/411 kJ mol=4.86 mol of Na. Mol wt of Na =23; 23*4.86=111.9g of Na.

Q2. 4 mol of Fe gives 1648.4 kJ. One mol of Fe would give 1648.4/4= 412.1Kj. 100. g of Fe has 100/55.8 (mol wt Fe 55.8) =1.79 mol. Heat evolved 1.79*412.1=738.5.

I hope this helps.
Best wishes
Kevin.

Thermodynamics

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Kevin

Expertise

Thermodynamics, chemistry, chemical reactions, kinetics, chemical reaction safety, dust explosion technology, static electricity. General science.

Experience

Over 40 years experience as a practicing thermochemist in industry. Head of the fire and explosion laboratory of a major European chemical company (Ciba-Geigy). Now retired.

Organizations
Institute of Chemical Engineers. Royal Society of Chemistry.

Education/Credentials
Chartered Chemist, Member of the Royal Society of Chemistry (C.Chem MRSC). Msc Sheffield.

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