Box 48 Chemical energy

The study of energy change is called thermodynamics. For example, the combustion of graphite carbon yields energy:

The total energy released, or the energy change in going from reactants to products, is termed the change in Gibbs free energy (AG), which is measured in kilojoules per mole (kJ mol-1). If energy is released, i.e. the products have lower free energy than the reactants, AG is considered negative. AG° for the burning of graphite under standard temperature (25°C) and pressure (1 atm), indicated by the superscript is -394.4kJmol-1. Tables of AG° for various reactions are widely available and values of AG° for different reactions can be calculated by simple arithmetic combination of tabulated values. Any reaction with a negative AG value will in theory proceed spontaneously—the chemical equivalent of water flowing downhill — releasing energy. The reverse reaction requires an input of energy, i.e.:

Since an energetically favoured reaction proceeds from reactants to products, there is a relationship between AG and the equilibrium constant (K) for a reaction.

where T is the absolute temperature (measured in Kelvin (K)) and R is the universal gas constant (8.314Jmol-1K-1), relating pressure, volume and temperature for an ideal gas (see Box 3.1).

Converting equation 3 to decimal logarithms gives:


The total energy released in a chemical reaction has two components, enthalpy and entropy. Change in enthalpy (AH, measured in J mol-1) is a direct measure of the energy emitted or absorbed by a reaction. Change in entropy (AS, measured in Jmol-1K-1) is a measure of the degree of disorder. Most reactions proceed to increase disorder, for example by splitting a compound into constituent ions or atoms. Enthalpy and entropy are related:

In most reactions the enthalpy term dominates, but in some reactions the entropy term is important. For example, the dissolution of the soluble fertilizer potassium nitrate (KNO3) occurs spontaneously. However, AH for the reaction

is +35 kJ and the solution absorbs heat (gets colder) as KNO3 dissolves. Despite the positive enthalpy, the large increase in disorder (entropy) in moving from a crystalline solid to ions in a solution, gives an overall favourable energy balance or negative AG for the reaction.

Electrode potentials (E°, Box 4.3) are a measure of energy transfer and so can be related to G:

where n is the number of electrons transferred and F is the universal Faraday constant (the quantity of electricity equivalent to one mole of electrons = 6.02 x 1023e-).

of material is greater on steeper slopes. Conversely, the potential for dissolution and transport of dissolved material is lower on steep slopes because the contact time between soil water and mineral solids is lower. The form of a slope—whether it is linear, concave or convex—also influences water movement, and potentially

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