Applications of thermodynamics form the heart of physical chemistry. With the First Law of Thermodynamics we can find some elementary applications of thermodynamics to processes relevant to the atmosphere. As an example, consider the elementary reaction

where the g in parentheses indicates that the chemical species is in the gaseous state (the solid phase is indicated by s, liquid by l and aqueous solution by aq).

At the molecular level a molecule of CO2 strikes an H2 molecule and the rearrangement collision occurs with a certain probability depending on velocities, spatial orientation of the colliders, etc. The rate at which a reaction proceeds in a system is the product of the likelihood of a collision between the important parties and the probability of rearrangement, given the collision. Sometimes a CO molecule bumps into an H2O molecule and the reverse reaction occurs. That the reaction might go both ways is indicated by the equation

Once equilibrium is established (rate of reactions proceeding to the right equals the rate of those going to the left) in a suitable enclosure, we can consider the matter involved to be a thermodynamic system which can be treated by the methods of equilibrium thermodynamics. The number of moles of the species vCO2, vH2, vCO and vH2O become thermodynamic coordinates or functions along with those we are already acquainted with, M,p, V, U, H, S, G, and T. In fact, we want to know how these coordinates (equilibrium concentrations) vary as a function of the temperature if the pressure is held constant. Fixed pressure is the usual condition for gas phase reactions in the atmosphere since they can be taken as occurring in a small parcel or volume element, whose pressure inside quickly adjusts to that outside (which in our case depends on altitude). In the real atmosphere there are many chemical species in various states of equilibrium. The task of the atmospheric chemist is often to sort out which reactions are important, what the sources of the various species are, what is the feasibility and energetics of chemical reactions and how fast the reactions proceed.

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