Chap15Thermo - Chapter 15 Thermochemistry Key Notes...

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Chapter 15: Thermochemistry Key Notes: Fundamentals Aspects Thermochemistry is an area of chemistry that deals with the relationship between energy and chemical reactions. Chemists would like to know if a proposed chemical reaction is going to be endo- or exothermic, if it will proceed to give products, or if it will produce a mixture of reactants and products. Thermochemistry seeks to answer these questions by looking at thermodynamic properties of the substances involved in a reaction. Computational chemistry allows one to calculate properties for these substances in order to provide the answers chemists require, without resorting to experimental means of determining this information. Internal Energy The internal energy ( U ) is the sum of the kinetic and potential energies of the particles that make up a system. Kinetic energy involves the motion of the electrons, nuclei, and the molecules themselves while potential energy is present in chemical bonds between atoms and in intermolecular forces. Internal energy is a state function . A state function depends only on the present state of the system and is completely determined by variables such as temperature and pressure. As a system changes from one state to another, the internal energy changes from one definite value to a new definite value. The change in internal energy ( U ) equals the difference in internal energy between the final and initial states. In terms of a chemical reaction, the change in internal energy is the difference between the internal energy of the products and that of the reactants. Enthalpy The enthalpy ( H ) is a property of a substance that can be used to calculate the heat produced or absorbed in a chemical reaction. Enthalpy is also a state function, and we calculate the enthalpy change for a chemical reaction by finding the difference in enthalpy between the products and reactants. Enthalpy is related to internal energy via its precise definition: H = U + PV where P is the pressure and V is the volume. Chemical reactions that release energy (exothermic) are favored. Entropy The entropy ( S ) is a measure of the amount of disorder, or randomness, in a system. Entropy is another state function. For reactions involving different phases, we can often predict the sign of the entropy change. Solids have a more ordered structure since the constituent units (atoms, molecules, or ions) have definite locations. Liquids have less order (more entropy) as the units now move
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freely within the liquid volume. Gases have the least order (most entropy), as the constituent units are free to move throughout the volume of the container that holds them. Chemical reactions that involve an increase in entropy are favored. Gibbs Free Energy The Gibbs free energy ( G ), or Gibbs energy, is the thermodynamic quantity defined by the equation: G = H TS where T is the temperature. As a chemical reaction proceeds, both H and S change. These changes, denoted using the symbol, allow the change in the Gibbs energy to be calculated: G = H T S If G for a reaction is positive, the reaction is nonspontaneous.
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