Chemistry 30

If ΔH_f^o is large and negative, energy is released when the compound is formed.

For example, ΔH_f^o for MgO(s) is -601.7 kJ. Thus:

Mg(s) + ½O₂ → MgO(s) + 601.7 kJ

Conversely, energy is required to decompose the compound. The decomposition reaction is the reverse of the formation reaction, and thus will be:

MgO(s) + 601.7 kJ → Mg(s) + ½O₂

Notice that when we reverse an equation, the energy term moves to the other side of the equation. This should make sense to you, and will be important to understand.

Since a large amount of energy must be supplied for MgO to break down into its elements (601.7 kJ), we conclude that MgO is a stable compound. If a sample of MgO were sitting on your desk, it would remain as MgO and not break down into solid magnesium and oxygen gas.

If ΔH_f^o is small and negative (only slightly exothermic), little energy is required to decompose the compound. These compounds are often unstable & decompose easily.

Let's look at HBr(g) as an example. ΔH_f^o for this compound is -36.4 kJ. Our equation looks like this:

½ H₂(g) + ½ Br₂(g) → HBr(g) + 36.4 kJ

Again, stability of a compound refers to the reverse of the formation reaction, or

HBr(g) + 36.4 kJ → ½ H₂(g) + ½ Br₂(g)

Since only a small amount of energy (36.4 kJ) is required for this reaction to occur, it is possible that this compound will break down spontaneously on its own. We therefore conclude that HBr is likely an unstable compound.

Finally, if ΔH_f^o is positive (endothermic), the compound is likely to be unstable.

ΔH_f^o for CS₂ is +117.4 kJ. The heat of formation reaction:

C(s) + 2 S(s) + 117.4 kJ → CS₂(g)

Reversing this to see the decomposition reaction:

CS₂(g) → C(s) + 2 S(s) + 117.4 kJ

we see that energy is released when this compound breaks down. Since we now know that exothermic reactions tend to occur spontaneously (Section 2-3), we conclude that CS₂ is an unstable compound.

Complete these practice questions before continuing to the next section.