Standard Enthalpy of Formation: Explanation & Calculations

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  • 0:02 Energy of Chemical Reactions
  • 1:28 Standard Enthalpy of Formation
  • 3:18 Calculating Enthalpy Change
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Lesson Transcript
Instructor: Nicola McDougal

Nicky has taught a variety of chemistry courses at college level. Nicky has a PhD in Physical Chemistry.

In this video lesson, we learn about the standard enthalpies of formation of substances. We will also learn how we can use these values as one way to calculate the standard enthalpy change of a chemical reaction.

Energy of Chemical Reactions

Some chemical reactions are very energetic. A great example is the thermite reaction. This is a classic chemical reaction that TV and filmmakers love to use because it is so dramatic. In fact, the makers of Breaking Bad used it to destroy a lock on a warehouse door.

This chemical reaction is a fairly simple one. Aluminum powder and iron oxide react together to form aluminum oxide and molten iron. It is a spectacular, highly exothermic reaction where lots of heat is given out.

Now, technically speaking, this reaction is not explosive because no gas is formed. But it is so energetic it produces very high, localized temperatures. In the correct amounts, it is certainly possible to create enough heat to melt a lock, though it could easily set fire to the door and the surroundings at the same time!

A great question you might ask would be how much energy is needed to melt and destroy a lock. Not that I am suggesting you do this for real, but it is fun to know in theory. To answer this great question, we need to figure out the energy, or enthalpy change, of the reaction.

In this lesson, we are going to learn one way of calculating the enthalpy change of chemical reactions using standard enthalpies of formation.

Standard Enthalpy of Formation

The standard enthalpy of formation is defined as the enthalpy change when 1 mole of compound is formed from its elements under standard conditions. Standard conditions are 1 atmosphere pressure and 25 degrees Celsius. The symbol you will see is delta H0f, where the subscript f stands for formation and little 0 tells you it is standard conditions.

It is worth knowing that some textbooks use a different term for standard enthalpy of formation and that is 'standard heat of formation.' These are exactly the same thing so please do not be confused. These terms are simply interchangeable.

You will find standard enthalpies of formation for compounds in data tables in any textbook. You will never need to remember them. I have a table with just a few of them here. The units are given in kJ/mol.

Compound delta Hf (kJ/mol) Compound delta Hf (kJ/mol)
AgBr(s) -100.4 Al2O3(s) -1676.7
CH4(g) -74.8 CO2(g) -393.5
Fe2O3(s) -824.2 H2O(l) -285.8
NH3(g) -46.1 NaOH(s) -425.6

You will find most enthalpies of formation are negative. This means that the formation of most compounds from their elements is exothermic.

You will notice from my table, and from most data tables, that elements are not included. This is a really important point. All elements are assigned 0.00 kJ/mol. This is for two reasons. Firstly, we know there is no such thing as zero energy because energy cannot be created nor destroyed. So we need to have an arbitrary zero that we can compare everything against. Elements were chosen as this arbitrary zero because we don't make elements -- we make compounds. Elements are found naturally on Earth.

Calculating Enthalpy Change of Formation

We have just learned that there's an enthalpy value when compounds are formed. It turns out we can use this to calculate the change in enthalpy, delta H, for a chemical reaction.

The reason this works is because enthalpy is a state property. This means that it doesn't matter how we get there; all that matters is where we start and where we finish. So, for any reaction, delta H can be figured out by imagining that the reaction takes place in two steps.

  1. Reactants are converted to elements.
  2. Elements are converted to products.

This means we can figure out the enthalpy change of any reaction using this very useful equation:

Enthalpy Change

Do not be scared by the weird symbols. The funny E sign is just the symbol for 'the sum of.' So all we are doing is adding up all of the enthalpies of formation for the products and then subtracting all of the enthalpies of formation for the reactants.

Okay, so let's return to our thermite reaction and calculate the enthalpy change of reaction using enthalpies of formation.

Here we have the balanced chemical equation for the thermite reaction. 2 moles of aluminum react with 1 mole of iron oxide, forming 2 moles of iron and 1 mole of aluminum oxide.

Thermite Reaction

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