Chemistry 30

2.2 Heat of Formation

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: H2 (g) + ½O2 (g) → H2O(l) + 285.8 kJ The amount of energy released during this reaction, 285.8 kJ, is referred to as the standard heat of formation. Since the reaction is exothermic we would indicate the heat of formation, ΔHof, as -285.8 kJ.

Standard Heat of Formation ΔHof 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: Word | RTF | PDF

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.

Writing Heat of Formation Reactions

It will be important for you to write heats of formation reactions. Try question 1 of the practice questions at this time.

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, CO32-, and ammonium, NH4+. Reference Chart Table of Common Ions Word | RTF | PDF

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 1

=

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.

Compete the practice questions.