- •5.1.1 Thermochemistry
- •Example 5.1. Calculating heat of a reaction
- •Example 5.2. Applying Hess’s law by combining thermochemical equations
- •Example 5.3. Calculating h °r from standard enthalpies of formation
- •Example 5.4. Using the heats of combustion to calculate h°f
- •5.1.2 Bond Energy and Heat Effect of Reaction
- •Example 5.5. Calculating bond energies from thermodynamic data
- •5.1.3 Spontaneous and Nonspontaneous Reactions. Entropy and Gibbs Energy
- •Example 5.6. Predicting the sign of entropy change
- •Example 5.8. Caculating the temperature the reaction startes
5.1.3 Spontaneous and Nonspontaneous Reactions. Entropy and Gibbs Energy
A spontaneous reaction is a reaction that, once begun, takes place without outside assistance. An example is a reaction between hydrogen and oxygen. Once the mixture is ignited, the reaction to form water continues until all of one reactant is consumed. The reverse reaction, decomposition of water to hydrogen and oxygen, is nonspontaneous. This reaction will occur when an electric current is passed through aqueous solution (containing an electrolyte such as NaOH). Thus decomposition continues as long as the electric current is supplied.
Predicting, whether a given reaction will occur spontaneously or not under a given set of conditions is important for chemists. The reaction between hydrogen and oxygen and many other spontaneous reactions are exothermic. So we can assume that one of the driving forces that determine whether a reaction is spontaneous is a tendency to give off energy. However, there are also rare endothermic spontaneous reactions. Dinitrogen tetroxide decomposes spontaneously at room temperature (the reaction is not complete):
N2O4(g) = 2 NO2(g) |
ΔH° = 57.2 kJ/mol |
Dissolution is another spontaneous process, which can be endothermic. For example, ammonium nitrate dissolves spontaneously in water, even though energy is absorbed when this reaction takes place.
|
H2O |
|
|
NH4NO3(s) |
|
NH4+(aq) + NO3(aq) |
ΔH° = 28.05 kJ/mol |
Therefore, there is another factor that influences whether reaction is spontaneous or not. This factor is entropy.
Entropy. Thermodynamic characteristic, called entropy (denoted as S) is a measure of the disorder of the system5. Entropy of a substance depends on its aggregate state:
Ssolid Sliqud Sgas
For example, for water Sice S(H2O)liqud (70.1) S(H2O)gas(188.7)
The entropy of a substance (mixture) increases with increasing temperature. For a gas the entropy increases with volume increase (greater space is favorable for disorder).
The standard entropy change for a reaction can be calculated, using sums of the standard entropies of all products and reactants. Each S° should be multiplied by the corresponding coefficient in the equation:
S°r = ΣS°(products) – ΣS°(reactants).
Predicting the sign of entropy change for chemical reaction. For some chemical reactions, it is possible to predict the sign of S. For example, let us consider the reaction
2 Cu(NO3)2(s) 2 CuO(s) + 4 NO2(g) + O2(g)
which occur when copper nitrate is heated. The reactant is a solid, and, as we know, solids have low entropies. Among the products there is one solid, but there are also two gases. The gases have much larger entropies that any of the solids. Therefore, for this reaction S 0.
When gases are present among both the reactants and products, we can compare amounts of matter of gases before and after the reaction. If the total amount of matter of gases increased after the reaction, S 0. For example, S is positive for decomposition of N2O4(g) (NO2(g) is formed). Generally, decrease of the complexity of the particles in the system (for example, dissociation of diatomic molecule like H2, Cl2 or HI) lead to increase of entropy. Dissolution of solids also results in great increase of entropy.