How to calculate heat from enthelpy of reaction

In summary: However, the enthalpy of combustion is often given per mole of reactant, rather than per mole of reaction. This can lead to confusion, because the two quantities are not equivalent. The correct way to state the equation is: q = ΔrH° × n(X) / ν (X), where n(X) is the number of moles of reactant and ν (X) is the number of moles of product.
  • #1
CroSinus
32
1
1. Homework Statement

The chemical reaction is given:
2C2H2 + 5O2 -> 4CO2 + 2 H2O. ΔrH* = -1299 kJ / mol.
How much heat is released when 50 grams of C2H2 is burned?​

Homework Equations

The Attempt at a Solution


First, I convert mass into moles.
M(C2H2) = 2*12+2 = 26 g/mol

n(C2H2) = m/M = 50 g / 26 gmol-1
n(C2H2) = 1,923 mol

Now I say: if 1299 kJ of heat is released when 2 moles of C2H2 are burned, then for 1.923 moles of the same thing I get 1249.05 kJ of heat (energy). Am I right?

Please be so king to comment on my answer! Let me know if I am right or wrong? Thank you very much.
CroSinus​
 
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  • #2
Logic looks OK, although you are told 50 g which suggests you should not list you answer with 6 significant digits.
 
  • #3
Thank you for your swift answer. :-)

I do not know about digits. Should I write only 1249.1 kJ / mol?

The confusing thing is the unit: kJ per mol. Per mol of what?
 
  • #4
Actually I have just checked and the enthalpy as listed is per mole of acetylene reacting. IMHO this is ambiguous and should be explained more precisely. Enthalpy can be given "per mole of a reacting substance" or "per mole of a reaction as written" (yes, technically there is no such thing as "mole of reaction", but it is nothing unusual to have it written like that). No wonder we were both wrong.

The answer should contain 2 significant digits, so (already correcting the mistake we did) 2.5×103 kJ, or just 2.5 MJ. Not that it matters much, but it is in general a mistake to present the number as if it was highly accurate when it is not.
 
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  • #5
Thank you once again for dealing with my chemistry problems.

Yes, "per mole of a reaction as written", to my thinking, that is the right interpretation of the unit kJ / mol. If I want to calculate the amount of energy (heat) released in a reaction when ΔrH° is given, should I use the formula: q = ΔrH° × n(X) / stoichiometric coefficient (X)? Where X denotes either a chosen reactant, or a chosen product. I usually come across the formula: q = ΔrH° × n(X), but without the stoichiometric coefficient. So the complete formula should be written as: q = ΔrH° × n(X) / ν (X)?

Thanks,
CroSinus
 
  • #6
CroSinus said:
Yes, "per mole of a reaction as written", to my thinking, that is the right interpretation of the unit kJ / mol.

That's the problem: it is not necessarily right. Giving enthalpy of combustion per mole of acetylene completely burned is perfectly valid and unambiguous, as long as it is clearly stated.
 

FAQ: How to calculate heat from enthelpy of reaction

1. How do you calculate heat from enthalpy of reaction?

The formula for calculating heat from enthalpy of reaction is q = ΔHrxn * n, where q is the heat released or absorbed, ΔHrxn is the enthalpy change of the reaction, and n is the number of moles of the substance undergoing the reaction.

2. What is enthalpy change of a reaction?

Enthalpy change of a reaction, denoted as ΔHrxn, is the difference in enthalpy between the products and reactants in a chemical reaction. It is a measure of the amount of heat released or absorbed during a reaction at constant pressure.

3. How is enthalpy change measured?

Enthalpy change is typically measured experimentally using a calorimeter. The reactants are placed in the calorimeter and the temperature change is recorded to calculate the heat released or absorbed. This value can then be used to calculate the enthalpy change of the reaction.

4. Can enthalpy change be negative?

Yes, enthalpy change can be negative. A negative enthalpy change indicates that the reaction is exothermic, meaning heat is released during the reaction. A positive enthalpy change indicates an endothermic reaction, where heat is absorbed.

5. How does enthalpy change relate to bond energies?

Enthalpy change is related to bond energies in a chemical reaction. Breaking bonds requires energy, which is an endothermic process and increases the enthalpy of the system. Forming bonds releases energy, which is an exothermic process and decreases the enthalpy of the system. The net change in bond energies determines the overall enthalpy change of the reaction.

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