Calculate Delta Hrxn for The Following Reaction C2h6 O2
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The enthalpy change (ΔHrxn) for a chemical reaction is a measure of the heat absorbed or released during the reaction. For the combustion of ethane (C2H6) with oxygen (O2), we can calculate ΔHrxn using standard enthalpies of formation.
What is ΔHrxn?
ΔHrxn (delta H reaction) represents the change in enthalpy (heat content) when one mole of a substance reacts completely. For combustion reactions, a negative ΔHrxn indicates an exothermic process where heat is released to the surroundings.
In the case of ethane (C2H6) combustion, the balanced chemical equation is:
2 C2H6 + 7 O2 → 4 CO2 + 6 H2O
This reaction shows ethane combining with oxygen to produce carbon dioxide and water.
How to Calculate ΔHrxn
To calculate ΔHrxn for the combustion of ethane, we use the standard enthalpies of formation (ΔHf°) of the reactants and products. The formula is:
ΔHrxn = ΣΔHf°(products) - ΣΔHf°(reactants)
Where:
- ΔHf°(products) is the sum of the standard enthalpies of formation of all products
- ΔHf°(reactants) is the sum of the standard enthalpies of formation of all reactants
Standard enthalpies of formation are typically reported in kilojoules per mole (kJ/mol) at standard temperature and pressure (25°C and 1 atm).
Example Calculation
Let's calculate ΔHrxn for the combustion of ethane using standard enthalpies of formation:
| Compound | ΔHf° (kJ/mol) | Coefficient | Total ΔHf° |
|---|---|---|---|
| C2H6 (ethane) | -84.7 | 2 | -169.4 |
| O2 (oxygen) | 0 | 7 | 0 |
| CO2 (carbon dioxide) | -393.5 | 4 | -1574 |
| H2O (water) | -285.8 | 6 | -1714.8 |
Using the formula:
ΔHrxn = [4*(-393.5) + 6*(-285.8)] - [2*(-84.7) + 7*0]
ΔHrxn = [-1574 + (-1714.8)] - [-169.4]
ΔHrxn = -3288.8 - (-169.4)
ΔHrxn = -3119.4 kJ/mol
The negative value indicates this is an exothermic reaction, releasing 3119.4 kJ of energy per mole of ethane burned.
Interpretation of Results
The calculated ΔHrxn of -3119.4 kJ/mol means:
- The reaction releases 3119.4 kJ of heat energy per mole of ethane combusted
- This energy is transferred to the surroundings as heat
- The reaction is highly exothermic, typical for most combustion reactions
Note: Actual heat released may vary slightly due to experimental conditions and heat capacity of the system.
FAQ
What units are used for ΔHrxn?
ΔHrxn is typically measured in kilojoules per mole (kJ/mol) or calories per mole (cal/mol).
Is ΔHrxn always negative for combustion reactions?
Yes, combustion reactions are almost always exothermic, resulting in negative ΔHrxn values.
How does ΔHrxn relate to bond energies?
ΔHrxn is related to the difference in bond energies between reactants and products, but it's more directly calculated using standard enthalpies of formation.