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General Chemistry Principles, Patterns, and Applications, 2011

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Device Energy Conversion Approximate Efficiency (%)<br />

large electrical generator mechanical → electrical 99<br />

chemical battery chemical → electrical 90<br />

home furnace chemical → heat 65<br />

small electric tool electrical → mechanical 60<br />

space shuttle engine chemical → mechanical 50<br />

mammalian liver cell chemical → chemical 30–50<br />

spinach cell light → chemical 30<br />

internal combustion engine chemical → mechanical 25–30<br />

fluorescent light electrical → light 20<br />

solar cell light → electricity 10<br />

inc<strong>and</strong>escent light bulb electricity → light 5<br />

yeast cell chemical → chemical 2–4<br />

St<strong>and</strong>ard Free-Energy Change<br />

We have seen that there is no way to measure absolute enthalpies, although we can measure changes in<br />

enthalpy (ΔH) during a chemical reaction. Because enthalpy is one of the components of Gibbs free<br />

energy, we are consequently unable to measure absolute free energies; we can measure only changes in<br />

free energy. Thest<strong>and</strong>ard free-energy change (ΔG°) is the change in free energy when one substance or a<br />

set of substances in their st<strong>and</strong>ard states is converted to one or more other substances, also in their<br />

st<strong>and</strong>ard states. The st<strong>and</strong>ard free-energy change can be calculated from the definition of free energy, if<br />

the st<strong>and</strong>ard enthalpy <strong>and</strong> entropy changes are known, using Equation 18.26:<br />

Equation 18.26<br />

ΔG° = ΔH° − TΔS°<br />

If ΔS° <strong>and</strong> ΔH° for a reaction have the same sign, then the sign of ΔG° depends on the relative magnitudes<br />

of the ΔH° <strong>and</strong> TΔS° terms. It is important to recognize that a positive value of ΔG° for a reaction<br />

does not mean that no products will form if the reactants in their st<strong>and</strong>ard states are mixed; it means only<br />

that at equilibrium the concentrations of the products will be less than the concentrations of the reactants.<br />

Note the Pattern<br />

A positive ΔG° means that the equilibrium constant is less than 1.<br />

E X A M P L E 8<br />

Saylor URL: http://www.saylor.org/books<br />

Saylor.org<br />

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