chemistry

Thermochemistry 0) - heat is absorbed by the reacting system "> 0) - heat is absorbed by the reacting system"/> 0) - heat is absorbed by the reacting system Thermochemistry deals with determining quantities of heat produced or absorbed by a reaction both by measurement and by calculation. It rests on Lavoisier and Laplace's law and on Hess's law: Lavoisier and Laplace's law: The energy change accompanying any reaction is equal and opposite to energy change accompanying the reverse process Hess' law: The energy change accompanying any reaction is the same whether the process occurs in one or in several steps These statements preceded the first law of thermodynamics (1845) and helped in its formulation. References P. Atkins, J de Paula, “Physical Chemistry: Thermodynamics, Structure and Change”, 10th Edition, W. H. Freeman, 2014 D. A. McQuarrie, J. D. Simon,“Physical Chemistry: A Molecular Approach”, 1st Edition, University Science Book...

Solved Examples - Entropy changes and thermodynamic equilibrium Entropy Changes ΔS and Thermodynamic Equilibrium – Solved Examples   In a previous post entitled “ Entropy, Free Energy and Thermodynamic Equilibrium ” the Boltzmann definition of entropy was given and how entropy changes ΔS are associated with chemical processes was discussed. Below, some examples are given regarding entropy changes ΔS and chemical reactions. Example #1 Choose the reaction expected to have the greatest increase in entropy: a)  N 2 (g) + O 2 (g) ———› 2 NO(g) b)  H 2 O (l)    ———›   H 2 O (g) c)  2 XeO 3 (s)  ———›   2 Xe(g) + 3 O 2 (g) d) C(s) + O 2 (g)   ———›  CO 2 (g)   Answer: The reaction with the greatest increase in the moles of gas will have the greatest increase in entropy . Answer (c) is correct. In general, when a reaction involves gaseous ...

Entropy, free energy and thermodynamic equilibrium Entropy, Free Energy and Thermodynamic Equilibrium Chemical reactions are performed by mixing the reactants and regulating external conditions such as temperature and pressure . Two basic questions though arise: Is it possible for the reaction to occur at the selected conditions? If the reaction proceeds, what determines the ratio of products and reactants at equilibrium?   Both questions are answered by chemical thermodynamics : Thermodynamics can tell us whether a proposed reaction is spontaneous (possible) under particular conditions even before the actual experiment. Thermodynamics can also predict the ratio of products and reactants at equilibrium provided that the reaction is spontaneous . Note: Thermodynamics cannot answer though how fast a reaction will proceed. The field of Chemical Kinetics studies reaction rates. After many years of observation scientists conclude...

Entropy, free energy and thermodynamic equilibrium Entropy, Free Energy and Thermodynamic Equilibrium Chemical reactions are performed by mixing the reactants and regulating external conditions such as temperature and pressure . Two basic questions though arise: Is it possible for the reaction to occur at the selected conditions? If the reaction proceeds, what determines the ratio of products and reactants at equilibrium?   Both questions are answered by chemical thermodynamics : Thermodynamics can tell us whether a proposed reaction is spontaneous (possible) under particular conditions even before the actual experiment. Thermodynamics can also predict the ratio of products and reactants at equilibrium provided that the reaction is spontaneous . Note: Thermodynamics cannot answer though how fast a reaction will proceed.   After many years of observation scientists concluded that the characteristic common to all spontaneous...

Phase changes - energy changes - heating curves Phase Changes - Energy Changes - Heating Curves   Many important properties of liquids and solids relate to the ease with which they change from one state to another. Water for example, when heated it evaporates that is changes from liquid to the gas state. In general, each state of matter (solid, liquid, gas)  can change into either of the the other two states. Figure I.1 shows these transformations which are called phase changes or changes of state . What happens when a solid is heated? Typically, it melts to form a liquid. If the heating continues, the liquid at some point boils and forms the vapor phase (gas). This process can be represented by a heating curve : a plot of temperature versus time for a process where energy is added at a constant rate. The heating curve of water is shown in Fig. I.2. There are five separate zones on the graph (heating curve) of Fig. I.2:   Zone 1 (Ice) : As energy ...