5. Chemical Thermodynamics and Chemical Kinetics

5.1. Chemical Thermodynamics

5.1.1 Thermochemistry

5.1.2 Bond Energy and Heat Effect of Reaction.

5.1.3 Spontaneous and Nonspontaneous Reactions. Entropy and Gibbs Energy

Thermodynamics (thermos = heat, dynamics = flow) is the science dealing with energy and its transformation. It studies the energy changes that accompany chemical and physical processes. Thermodynamics is an exact science; it can predict whether a given process will occur spontaneously or not under a given set of conditions. However, it gives no information with regard to the rate at which a given change will proceed. Thermodynamics deals only with the states of the system; it explains why a change occurs but does not consider the mechanism of the process. Thermodynamics deals with matter on a macroscopic level.

Some common terms. Energy is a fundamental property of matter, usually defined as capacity to do work. An object (for example, a particle) can possess kinetic energy and potential energy. The total amount of energy an object has is therefore the sum of its kinetic and potential energy. Kinetic energy is energy an object has when it is moving, it is equal

E_kinetic = 1/2 m²

where m is the object’s mass and  is its speed. Potential energy is energy an object has when it is either attracted to or repelled from other object. The SI unit for energy is joule, J; one joule is 1 kgm²/s² (it can be derived from the definition of kinetic energy).

Energy of atoms and molecules. For any object, average kinetic energy of its atomic-sized particles is directly proportional to the absolute temperature of the object (measured in Kelvins). So, heat is not another kind of energy, it is kinetic energy of atoms and molecules.

Atoms consist of electrically charged particles (nuclei and electrons) that attract and repel each other. Therefore, electrons and nuclei possess potential energy. There are potential energy changes when electrons are transferred between atoms (during the formation of ions) and when atoms share electrons (during the formation of molecular substances).

System and surrounding; state of a system. A system is a part of universe we are studying. (It might be two reagents in a beaker, for example). Outside the system are the surroundings. Energy of a system depends on its state (temperature, pressure, amount of matter and aggregate states of all substances). When a change takes place in a system, we say that the system goes from one state to another. Energy can be transferred between objects as heat, light, electricity, etc., although object can have only kinetic and potential energy.

5.1.1 Thermochemistry

Thermochemistry is a part of thermodynamics; it is a quantitative study of heat effects of chemical reactions. Thermochemistry focuses on energy changes, accompanying a reaction, particularly on the system’s energy exchange with its surrounding.

Heat effect of a reaction (heat of a reaction). Exothermic and endothermic reactions. All substances possess some internal energy. The internal energy includes all forms of energy except the kinetic and potential energy of a macroscopic object. The important part of the internal energy is the energy of chemical bonds. Destruction of some bonds and formation of new ones in a chemical reaction is usually accompanied by energy absorption or release. When total internal energy of products is less, than that of reagents, the energy is realized and the reaction is called an exothermic reaction. If total internal energy of products is greater, than that of reagents, the energy is absorbed and the reaction is called an endothermic reaction. Heat effect of a reaction is denoted as ΔH_r (r – reaction). The algebraic signs that ΔH_r has for exothermic and endothermic reactions are the following:

Exothermic reaction	ΔHr  0
Endothermic reaction	ΔHr  0

Enthalpy and enthalpy change. A thermodynamic characteristic, called enthalpy (denoted as H) is a measure of the total energy of a system¹.

Enthalpy change (ΔH) is an energy transferred from the system to the surrounding at constant pressure through heating and expansion work². The enthalpy change can be measured, in contrast to enthalpy itself. The magnitude of the enthalpy change depends only on the initial and final states of the system. Therefore, no matter how we go from one state to another, the net energy change and therefore the net ΔH, is the same³:

ΔH = H_final  H_initial

The enthalpy change for the reaction, ΔH_r. Chemical reactions can occur under different conditions, for example, at constant pressure or at constant volume. Reactions at constant pressure (in open container) are the most common ones. The heat of a reaction at constant pressure is equal to the enthalpy change for the reaction, ΔH_r. (When a reaction occurs at constant volume the heat of reaction is equal to the change of the internal energy of substances).

Thermochemical equation is a chemical equation written together with the ΔH_r. Heat effect of a reaction, given in a thermochemical equation always refers to the amounts of matter of substances defined by coefficients in the equation. Fractional coefficients can be used in the thermochemical equation. In addition to formulas of the reagents and products their aggregate states (solid, liquid, or gaseous) and conditions (temperature, pressure) are given in the thermochemical equation.

The thermochemical equation for the synthesis of 1 mole of P₄O_10(s) can be written as

P4(s) + 5 O2(g) → P4O10(s)

ΔHr = – 2984 kJ