2.2 Heat of Formation

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Turn now to a special type of chemical reaction, one in which we form one mole of a compound from its elements. We are interested in how much energy is either absorbed or released during this synthesis reaction.

For example, the formation reaction for liquid water is described by the following equation:

H_{2 (g)} + ½O_{2 (g)} → H₂O_(l) + 285.8 kJ

The amount of energy released during this reaction, 285.8 kJ, is referred to as thestandard heat of formation. Since the reaction is exothermic we would indicate the heat of formation, ΔH^o_f,as -285.8 kJ.

Standard
Heat of Formation

ΔH^o_f

the amount of energy
gained or lost when
1 mole of the substance is
formed from its elements
under standard conditions
(25°C, 1 atm = 101.3 kPa)

Key items to note regarding this definition:

one mole of the compound is formed
from its elements

Important
Data Table

Heats of Formation

Pick a format:

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Keep in mind that a heat of formation is just a special case of a heat of reaction - it's just that in this particular type of reaction one mole of the compound forms from its elements.

You will find the standard heats of formation listed for a variety of compounds in the Table of Thermochemical Data. Print a copy of this table and keep it handy.

The heat of formation for pure elements, such as H₂(g), O₂(g), Al(s), etc. is 0 kJ·mole^-1. You'll find it useful to remember this.

FullSizeRender (23).jpg

Figure 1. Set up of titration apparatus for MgO Calorimetry

Writing Heat of Formation Reactions

Writing heat of formation reactions is an important skill. Keep the following points in mind:

Balance the equation so that one mole of the compound is produced.
Remember the diatomic molecules and write them correctly (H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂).
The reactants must be elements, not polyatomic ions.

Examples of polyatomic ions are hydroxide, OH^-, carbonate, CO₃^2-, and ammonium, NH₄⁺.

Reference Chart

Table of Common Ions

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Return for a moment to our earlier reaction:

H₂(g) + ½O₂(g) → H₂O(l) + 285.

If 285.8 kJ of energy are released during the formation of one mole of H₂O(l) (as is shown in the balanced equation), how much energy do you imagine would be released if two moles of water were produced?

If you predicted 571.6 kJ of energy you're right! It shouldn't be hard for you to determine why. It is useful, for you however, to see how you can use unit analysis (also called factor analysis or dimension analysis) to solve this question. You likely learned about unit analysis in your Chemistry 20 course.

Our formation reaction tells us that 285.8 kJ of energy are released for every one mole of H₂O. This can be written mathematically as:

285.8 kJ

mol H₂O

If we want to know how much energy is released for 2 moles of water, you should be able to see how the unit "mol H₂O" cancels out when we multiply:

285.8 kJ

~~mol H₂O~~

2 ~~mol H₂O~~

571.6 kJ

Our new equation looks like this:

2 H₂(g) + O₂(g) → 2 H₂O(l) + 571.6 kJ

Since the equation does not form one mole of the compound, it is not a strict heat of formation reaction. Be careful! Watch this.