Concept 8.2: The free-energy change of a reaction tells us whether or not the reaction occurs spontaneously Flashcards
In 1878, J. Willard Gibbs, a professor at Yale, defined a very useful function called
the Gibbs free energy of a system (without considering its surroundings), symbolized by the letter G.
is the portion of a system’s energy that can perform work when temperature and pressure are uniform throughout the system, as in a living cell.
Free energy
The change in free energy, ▲G , can be calculated for a chemical reaction by applying the following equation:
▲G=▲H-T▲S
▲H symbolizes the change in the system’s
enthalpy (in biological systems, equivalent to total energy)
▲S is the change in the system’s entropy; and T is the absolute temperature in
Kelvin (K) units
Using chemical methods ▲G, we can measure for any
reaction.
More than a century of experiments has shown that only processes with a negative ▲G are
spontaneous
For ▲G to be negative,
▲H must be negative (the system gives up enthalpy and H decreases) or T▲S must be positive (the system gives up order and S increases), or both
When ▲H and T▲S are tallied, ▲G has a negative value for all
spontaneous processes
In other words, every spontaneous process decreases the system’s free energy, and processes that have a positive or zero ▲G are
never spontaneous.
This information is immensely interesting to biologists, for it allows us to predict which kinds of change can happen without an
input of energy
This principle is very important in the study of metabolism, where a major goal is to determine which reactions can supply energy for
cellular work.
Another way to think of ▲G is to realize that it represents the difference between the free energy of the final state and the free energy of
the initial state: ▲G=G final state -G initial state
▲G can be negative only when the process involves a loss of free energy during the change from initial state to
final state
Because it has less free energy, the system in its final state is less likely to change and is therefore
more stable than it was previously.
Another term that describes a state of maximum stability is
equilibrium
Recall that most chemical reactions are reversible and proceed to a point at which the forward and backward reactions occur at the
same rate.
The reaction is then said to be at chemical equilibrium, and there is no further net change in the .
relative concentration of products and reactants
As a reaction proceeds toward equilibrium, the free energy of the mixture of reactants and products
decreases
For a system at equilibrium, G is at its lowest
possible value in that system.
Any change from the equilibrium position will have a positive ▲G and will not be .
spontaneous
systems never spontaneously move away from
equilibrium.
A process is spontaneous and can perform work only when it is moving
toward equilibrium.
Based on their free-energy changes, chemical reactions can be classified as either
exergonic (“energy outward”) or endergonic (“energy inward”).
Proceeds with a net release of free energy. Because the chemical mixture loses free energy (G decreases), ▲G is negative for an exergonic reaction.
exergonic reaction
▲G as a standard for spontaneity, exergonic reactions are those that occur
spontaneously
The magnitude of ▲G for an exergonic reaction represents the maximum amount of work the
reaction can perform.
The greater the decrease in free energy, the greater the amount of work that can be
done.
cells contain many different molecules that can engage in a variety of
chemical reactions
when molecules react, for example when they collide and exchange parts their atoms and bonds are
rearranged
reactants are rearranged to form
products
reactant and product molecules store
potential energy in the arrangement of their atoms and bonds
chemical reactions involve changes in bonding and changes in
energy
we can plot potential energy on a graph and compare the potential energy of
reactants and products
using molecules as an example, we can image two chemical reactions. molecules AB and CD react to form molecules
AC and BD or vice versa
study graph video in animation Exergonic and Endergonic reactions
chemical reactions that release energy are called
exergonic reactions
you may call exergonic reactions
downhill reactions
other reactions, they absorb energy from their surroundings
endergonic reactions are uphill changes
exergonic reactions occur
spontaneously or, that is without a net addition of energy
the potential energy of the molecule
decrease
it is easier for a cell to carry out a reaction that does not need
additional energy input
a downhill change is easier than an
uphill change
endergonic reactions do not occur
spontaneously
it is harder for a cell to carry out a reaction that needs additional
energy input
learn C6 H12 O6 + 6 O2 → 6 CO2 + 6 H2 O ▲G= -682kcal/mol (-2,870kj/mol)
For each mole (180 g) of glucose broken down by respiration under what are called
“standard conditions”
learn
1 M of each reactant and product, 25°C, pH 7), 686 kcal (2,870 kJ) of energy is made available for work
Because energy must be conserved, the chemical products of respiration store 686 kcal less free energy per mole than the
reactants
The phrase “energy stored in bonds” is shorthand for the
potential energy that can be released when new bonds are formed after the original bonds break, as long as the products are of lower free energy than the reactants.
is one that absorbs free energy from its surroundings
endergonic reaction
Because this kind of reaction essentially stores free energy in molecules (G increases), ▲G is
positive.
If a chemical process is exergonic (downhill), releasing energy in one direction, then the reverse process must be
endergonic (uphill), using energy
If for respiration, which converts glucose and oxygen to carbon dioxide and water, then the reverse process—the conversion of carbon dioxide and water to glucose and oxygen—must be strongly
endergonic, with ▲G=+680kcal/mol .
Plants get the required energy—686 kcal to make a mole of glucose—from the environment by capturing
light and converting its energy to chemical energy
Because systems at equilibrium are at a minimum of G and can do no work, a cell that has reached metabolic equilibrium is
dead!
Like most systems, a living cell is not in
equilibrium.
The constant flow of materials in and out of the cell keeps the metabolic pathways from ever reaching
equilibrium, and the cell continues to do work throughout its life.
The key to maintaining this lack of equilibrium is that the product of a reaction does not accumulate but instead becomes a reactant in the next step; finally,
waste products are expelled from the cell.
