1. Qualitatively determine whether simple chemical or physical changes are spontaneous.
2. Qualitatively predict whether the sign of S is positive or negative for a chemical or physical change.
3. On the basis of probability, determine which of two states has the higher entropy.
4. Calculate the standard entropy of reaction from the standard molar entropies of products and reactants.
5. Determine whether a reaction is spontaneous by determining the sign of S_total.
6. Use the equation G = H - TS to calculate the free energy of reaction and to determine the temperature at which a nonspontaneous reaction becomes spontaneous.
7. Calculate the standard free energy of reaction from standard free energies of formation.
8. Calculate the free energy of reaction for a system having nonstandard pressures and concentrations.
9. From the standard free energy of reaction, calculate the value of the equilibrium constant.

What factors determine the direction and extent of a chemical reaction? Some reactions, such as the combustion of hydrocarbon fuels, go almost to completion. Others, such as the combination of gold and oxygen, hardly occur at all. Still others—for example, the industrial synthesis of ammonia from N₂ and H₂ at 400-500°C—result in an equilibrium mixture that contains appreciable amounts of both reactants and products.

As we saw in Section 13.5, the extent of any particular reaction is described by the value of its equilibrium constant K: A value of K much larger than 1 indicates that the reaction goes far toward completion, and a value of K much smaller than 1 means that the reaction does not proceed very far before reaching an equilibrium state. But what determines the value of the equilibrium constant, and can we predict its value without measuring it? Put another way, what fundamental properties of nature determine the direction and extent of a particular chemical reaction? For answers to these questions, we turn to thermodynamics, the area of science that deals with the interconversion of heat and other forms of energy.

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