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Chemical Education: Software Abstract for Volume 8 B, Number 1 REACT: Exploring Practical Thermodynamic and Equilibrium Calculations Richard W. Ramette Carleton College Northfield, MN 55057 REACT facilitates t h r e e chemistry manipulations t h a t often a r e exasperating, a n d sometimes seem impossible to introductory chemistry students. They are: 1. Balancing equations having unusual or complicated stoichiometry; 2. Looking up standard thermodynamic data for all reactants and products and calculating the changes in free energy, enthalpy and entropy for the reaction; 3. Performing calculations, given an initial set of concentrations and pressures, to find the equilibrium conditions. REACT uses a d a t a b a s e of s t a n d a r d thermodynamic properties for 570 substances. Most of t h e enthalpies a n d free energies of formation, a n d t h e absolute entropies, a r e t a k e n from t h e NBS tables of chemical thermodynamic properties. REACT'S interface m a k e s i t easy t o search t h e database either bv n a m e o r bv formula. Thermodvnamie d a t a a n d molar m a s s a r e displayed for each selected species. T h e user chooses reactants a n d products t o build u p a skeleton equation. REACT balances t h e equation a n d shows i t s thermodynamic properties a n d equilibrium constant, o r standard potential for a half-reaction. The user can e n t e r different temperatures a n d s e e t h e change - i n properties. . . If desired, one c a n e n t e r initial concentrations or pressures for species other t h a n pure solids or liquids. REACT calculates equilibrium values taking account of activity coefficients for aqueous ions. T h e calculation m a y b e repeated for various temperatures. For half-reactions, t h e calculation option i s t o e n t e r concentrations a n d pressures for t h e species i n a half-cell. T h e Nernstian ootential i s calculated a t t h e chosen temperature.
Figure 1. Screen from REACT showing entry of product O,(aq) for the equation needed to answer question 1.
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lure 2 Screen from REACT showng thermodynamc values equatlon needed ta solve problem 1
P e r h a n s t h e best wav to u s e REACT i s to answer oractical chemical Problems t h a t a r e quite i n t i k i d a t incr t o solve bv h a n d a r e m a d e easv " bv " REACT. Below a r e fair examples of t h e type of problems your students can solve with REACT. 1. You are a research environmentalist, and you want to predict the concentration of oxygen in a cold mountain stream a t 5 'C. 2. You have a bottle of 0.1 M acetic acid on your lab desk. It
seems stahle, but does thermodynamics predict an eventual decomposition into graphite and water? 3. What is the balanced equation for this reaction (117
4. Here's a scary thought for you to explore. Consider any body of fresh water, such as your local river or the Great
Lakes, constantly exposed to the nitrogen and oxygen of Earthb atmosphere. You can write a balanced equation for the hypothetical reaction:
Does thermodynamics predict that this reaction is spontaneous, whereby our lakes will be partially converted to nitric acid? It doesn't seem to happen: a bottle of pure water exposed to air doesn't gradually become more acidic. But this inhibition is really a matter of kinetics, not thermodynamics. Apparently there is a t least one large activation energy barrier to be overcome, so the reaction rate is infinitesimal. Now for the scary part: conceivably an industrial pollutant may one day enter Earth's water and act as a catalyst for this reaction, allowing it to reach equilibrium quickly. You are called in as a consultant to make a realistic prediction of what concentration the nitric acid would reach a t equilibrium assuminganly one small northern lake (in winter, at 0 "C) is infected with the catalyst. Assume that the air composition (21% oxygen, 78% nitrogen) remains constant. Because REACT deals with s u c h questions so quickly, and because users can s e t u p equations so easily, t h e prog r a m c a n serve students a s a n enjoyable learning tool for thermodynamic a n d equilibrium questions.
