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19.7 Entropy Changes in Chemical Reactions:
We can calculate the standard change for a chemical reaction by calculating the difference between the products and the reactants using the standard entropies for many common substances.
In the third method, we determine the free energy change for a stepwise reaction from the free energy changes of each step.
We discuss what is free at the end of this section.
We can use Equation 19.11 to estimate the changes in free energy.
The value of -70.9 kJ is calculated for the reaction in For Practice 19.6.
The free energy of formation of pure elements is zero.
All changes in free energy can be measured.
The complete list can be found in Appendix IIB.
The compounds form spontaneously from their elements.
Positive free energies of formation do not spontaneously form from their elements and are less common.
A process that is nonspontaneous can be made spontaneously with another process that is highly spontaneous.
The way to generate hydrogen gas is a nonspontaneous reaction between water and carbon monoxide.
The metabolism of food is the main cause of the nonspontaneous ones.
Nonspontaneous reactions are necessary to sustain life.
The coefficients for C are 1, and 3 in the reaction of interest.
The energy released by a chemical reaction can be used to do work.
In an automobile engine, we use the energy released by the gasoline to move the car forward.
The change in free energy is less than the change in enthalpy for a reaction.
74.6 kJ of heat energy is given off by the reaction.
The amount of energy available for useful work is only 50.5 kJ.
Only a maximum of 50.5 kJ is available to do work because the reaction produces 74.6 kJ of heat in the surroundings.
The amount of energy available to do work is what is lost to the surroundings.
The weight of sand matches the pressure at each increment.
In a reversible process, the free energy is drawn out in infinitesimally small increments that match the amount of energy that the process is producing in that increment.
The weight of sand almost matches the pressure of the expanding gas when grains of sand are removed one at a time.
The process is close to being fully reversible because each sand grain needs an infinitesimally small mass.
The definition of surroundings is heat.
There are only a few reactions that reach the theoretical limit.
The free energy is drawn out in infinitesimally small changes in a mally small amount that match the amount of energy that the reaction is variable.
Real reactions do not achieve the theoretical limit of free energy.
Consider the discharge of a battery.
They make an electrical current.
The free energy can be harnessed to do work.
An electric motor can be wired.
The electrical current makes the motor turn.
The heat is lost to the surroundings because of the resistance in the wire.
Slowing down the rate of current flow can decrease the amount of free energy lost.
A loss of energy in an energy exchange is when current is tax.
The battery needs more energy than it can get from the work because some of the energy is lost as heat.
The battery resistance in the wire is what happens when the battery is discharged.
It went through a cycle in which it returned to its charged state.
The amount of energy required to replenish the battery will be more than the amount of work done by the people around it.
The system may return to its original state, but the surroundings do not, resulting in the permanent dispersal of energy to the surroundings.
It takes more energy than the theoretical limit to make a real nonspontaneous reaction.
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