Calculate The Heats of Combustion for The Following Reactions C2h4

Calculating the heat of combustion for C2H4 (ethylene) reactions involves determining the energy released when one mole of ethylene burns completely in oxygen. This calculation is essential in chemistry, engineering, and environmental science for understanding reaction energetics and fuel efficiency.

Introduction

The heat of combustion (ΔH°c) is a measure of the energy released when a compound undergoes complete combustion with oxygen. For C2H4 (ethylene), the combustion reaction is:

C2H4 + 3O2 → 2CO2 + 2H2O

This reaction releases energy that can be calculated using standard enthalpies of formation. The heat of combustion is typically expressed in kilojoules per mole (kJ/mol).

Formula

The heat of combustion can be calculated using the standard enthalpies of formation (ΔH°f) of the products and reactants. The formula is:

ΔH°c = ΣΔH°f(products) - ΣΔH°f(reactants)

For the combustion of C2H4:

ΔH°f(C2H4) = -14.2 kJ/mol
ΔH°f(O2) = 0 kJ/mol (element in standard state)
ΔH°f(CO2) = -393.5 kJ/mol
ΔH°f(H2O) = -285.8 kJ/mol

Calculation

Using the formula and standard enthalpies:

ΔH°c = [2 × ΔH°f(CO2) + 2 × ΔH°f(H2O)] - [ΔH°f(C2H4) + 3 × ΔH°f(O2)]

ΔH°c = [2 × (-393.5) + 2 × (-285.8)] - [-14.2 + 3 × 0]

ΔH°c = [-787 - 571.6] - [-14.2]

ΔH°c = -1358.6 + 14.2

ΔH°c = -1344.4 kJ/mol

The negative sign indicates that the reaction is exothermic, releasing energy.

Example

Suppose you have 5 grams of C2H4. First, convert the mass to moles:

Molar mass of C2H4 = 28.05 g/mol

Moles of C2H4 = 5 g / 28.05 g/mol ≈ 0.178 mol

Then calculate the total heat released:

Total heat = Moles × ΔH°c

Total heat = 0.178 mol × (-1344.4 kJ/mol) ≈ -239.4 kJ

This means burning 5 grams of C2H4 releases approximately 239.4 kJ of energy.

FAQ

What is the standard heat of combustion for C2H4?

The standard heat of combustion for C2H4 is -1344.4 kJ/mol, meaning one mole of C2H4 releases 1344.4 kJ of energy when burned completely.

How do I calculate the heat of combustion for other compounds?

Use the same formula with the standard enthalpies of formation for the specific reaction. Ensure you account for all reactants and products.

What units are used for heat of combustion?

Heat of combustion is typically expressed in kilojoules per mole (kJ/mol) or kilocalories per gram (kcal/g).

Is the heat of combustion always negative?

Yes, for combustion reactions, the heat of combustion is always negative because energy is released (exothermic process).

Where can I find standard enthalpies of formation?

Standard enthalpies of formation can be found in chemistry reference books, online databases like NIST, or educational websites.