How to Calculate Delta H F

How to Calculate Delta H F
••• ABGlavin/iStock/GettyImages

In a chemical reaction, both reactants and the products they form have what are called "heats of formation." Expressed by the symbol "ΔHf" (delta HF), heats of formation are an important part of understanding energy transfer during chemical reactions. In order to calculate ΔHf for any product or reactant, you must have on hand the total amount of heat the reaction produces (ΔH), as well as the ΔHf value for all the other reactants and/or products, all of which your chemistry problem will provide you.

Step 1: Set Up the Equation

Arrange your given ΔHf and ΔH values according to the following equation: ΔH = ΔHf (products) - ΔHf (reactants).

For example, imagine that you want to know ΔHf for acetylene, C2H2, for the reaction C2H2 (g) + (5/2)O2 (g) --> 2CO2 (g) + H2O (g), the combustion of acetylene, the ΔH of which is -1,256 kJ/mol.

You know that the ΔHf of CO2 is -394 kJ/mol and the ΔHf of H2O is -242 kJ/mol. Elemental reactants and products such as oxygen gas have no "heat of formation" by definition; they exist is their form naturally.

Knowing all of this, you can write the following: ΔH = ΔHf (products) - ΔHf (reactants), or

-1,256 = (2 × (-394) + (-242)) - ΔHf(C2H2),

which you can rearrange as follows:

ΔHf(C2H2) = [2 ×(-394) + (-242)] +1,256.

Note that you must multiply the ΔHf of CO2 by two because of the "2" coefficient in front of it in the reaction equation.

Step 2: Solve the Equation

Solve your equation for ΔHf. In the case of the example ΔHf(C2H2),

ΔHf(C2H2) = [2 ×(-394) + (-242)] - (-1,256).

= (-1,030) + 1,256 = 226 kJ/mol.

Step 3: Validate the Sign

Adjust your ΔHf value's sign depending on whether it is for a product or a reactant. Product ΔHf values will always be negative, while those for reactants are always positive. As C2H2 is a reactant, its ΔHf is positive. Therefore, ΔHf(C2H2) = 226 kJ/mol.

Tips

  • ΔHf and ΔH values are always given in kilojoules per moles, where a "kilojoule" is the international unit of heat or energy and a "mole" is a unit which describes a very large number of molecules of a compound.

Related Articles

How to Determine Delta H
How to Find Heat Reaction When Zn Reacts With HCl
How to Write & Balance a Decomposition Reaction
How to Calculate an Isolated Yield
How to Calculate Equilibrium Pressures
How to Calculate Entropy Change
What Is the Number Written to the Left of the Chemical...
How to Calculate the Stoichiometric Ratio
How to Solve a Parabola
How to Calculate HCO3 From CO2
How to Tell If a Reaction Will Occur
How to Determine If There Was a Reaction in a Chemical...
How to Calculate Heat Loss During Pipeline Depressurization
How to Solve a Neutralization Equation
How to Explain What Happens When We Burn Magnesium...
Steps in Finding Percent Yield
Two Examples of Endothermic Processes That Are Spontaneous
How to Find the Number of Moles Needed to React
How to Write the Net Ionic Equation for the Reaction...

Dont Go!

We Have More Great Sciencing Articles!