Chemical reaction: Encyclopedia II - Chemical reaction - Thermochemistry

Chemical reaction - Thermochemistry

See main article: Thermochemistry.

Thermochemistry deciphers whether a specific chemical reaction can or cannot occur. Thermodynamics (or what is now known as equilibrium thermodynamics) understands the reaction in terms of the initial and final states of the reaction mixture.

Reactions very seldom occur directly. Usually, reactants must collide to form an activated complex. This complex has a higher internal energy than the original reactants combined, having gained some from the kinetic energy of the reactant substances' collision. This energy allows for the rearrangement of bonds which constitutes the reaction. In some reactions, the reactants may pass through several reactive intermediates before becoming products.

Thermodynamics does not attempt to figure out the process by which a reaction occurs. This field of study is taken up by the field of chemical kinetics. Another question "How fast is the reaction?" is also left completely unanswered by it. Chemical kinetics attempts to put all these phenomena into perspective.

Chemical reaction - Chemical equilibrium

Every chemical reaction is, in theory, reversible. In a forward reaction the substances defined as reactants are converted to products. In a reverse reaction products are converted into reactants.

Chemical equilibrium is the state in which the forward and reverse reaction rates are equal, thus preserving the amount of reactants and products. However, a reaction in equilibrium can be driven in the forward or reverse direction. This is done by changing the reaction conditions such as temperature or pressure. Le Chatelier's principle can be used to predict whether products or reactants will be formed.

Although all reactions are reversible to some extent, some reactions can be classified as irreversible. An irreversible reaction is one that "goes to completion." This phrase means that nearly all of the reactants are used to form products. These reactions are very difficult to reverse even under extreme conditions.

Chemical reaction - Exothermic reactions

According to energy balance criteria, that is, chemical reaction equilibria criteria, any closed system will tend to minimize its free energy. Without any outside influence, any reaction mixture, too, will try to do the same. For many cases, an analysis of the enthalpy of the system will give a decent account of the energetics of the reaction mixture. The enthalpy of a reaction is calculated using standard reaction enthalpies and the Hess' law of constant heat summation. Many of these enthalpies may be found in beginners' books on thermodynamics. For example, consider the combustion of methane in oxygen:

By calculating the amounts of energy required to break all the bonds on the left ("before") and right ("after") sides of the equation using collected data, it is possible to calculate the energy difference between the reactants and the products. This is referred to as ΔH, where Δ (Delta) means difference, and H stands for enthalpy, a measure of energy which is equal to the heat transferred at constant pressure. ΔH is usually given in units of kilojoules (kJ) or in kilocalories (kcal).

If ΔH is negative for the reaction, then energy has been released often in the form of heat. This type of reaction is referred to as an exothermic reaction (literally, outside heat, or throwing off heat). An exothermic reaction is more favourable and thus more likely to occur. An example reaction is combustion, known from everyday experience, since burning gas in air produces heat.

Chemical reaction - Endothermic reactions

A reaction may have a positive ΔH. If a reaction has a positive ΔH, it consumes energy as the reaction moves towards completion. This type of reaction is called an endothermic reaction (literally, inside heat, or absorbing heat).

The above rule, "Exothermic reactions are favourable", is usually true. However, there may be situations where exothermic reactions may not be favourable. This happens when the stability obtained due to loss of enthalpy is off set by a corresponding decrease in entropy (a measure of disorder). The exact rule is that a reaction is favourable when the Gibbs free energy of that reaction is negative where ΔG = ΔH − TΔS; ΔG being the change in Gibbs free energy, ΔH being the change in enthalpy, and ΔS is the change in entropy

A reaction is called spontaneous if its thermodynamically favoured, by that meaning that it causes a net increase on entropy. Spontaneous reactions (in opposition to non-spontaneous reactions) do not need external perturbations (such as energy supplement) to happen. In a system at chemical equilibrium, it is expected to have larger concentrations of the substances formed by the spontaneous direction of the process.

Thus, in a global isolated system (which it strictly isn't, see entropy), spontaneous reactions may be understood to occur without human interference. Most spontaneus reactions in this system are exothermic (such as rusting) or metamorphosis, thus increasing the global entropy, though photosynthesis is an important exeption (in a global system).

Adapted from the Wikipedia article "Thermochemistry", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki