Free Energy and Chemical Reactions

1. Free Energy and Chemical Reactions Unit 3: Bioenergetics Honors Biology Monkemeier

2. Chemical Reactions • During chemical reactions chemical bonds are broken and new chemical bonds are formed. • The breaking of chemical bonds in the reactants results in the formation of new bonds in the products

3. Heat and Chemical Bonds • It takes energy to break the chemical bonds that hold the atoms in a molecule together. • Heat energy, because it increases atomic motion, makes it easier for the atoms to pull apart. • Both chemical bonding and heat have a significant influence on a molecule: forming a chemical bond decreases disorder while heat increases the motion of the particles and increases disorder.

4. Free Energy • The net effect of the chemical bond energy and the heat energy on a molecule = the amount of energy actually available to break and subsequently form other chemical bonds = Free Energy of the molecule. • Free energy is defined as the energy available to do work.

5. Free Energy and Cells • In a molecule within a cell, where pressure and volume usually do not change, the free energy is denoted by the symbol G (for “Gibbs’ Free Energy”, which limits the system being considered to the cell) • G = energy contained within a molecule’s chemical bonds - the energy unavailable because of disorder (entropy)

6. Gibb’s Free Energy • H = Enthalpy = chemical bond energy of a molecule • S = entropy or measure of disorder • T = absolute temperature which is oC + 273.. Increasing temperature increases motion and increases entropy.

7. Gibbs’ Free Energy • Chemical reactions break some bonds in the reactants and form new ones in the products. • Consequently, reactions can produce changes in free energy. • The change in free energy or ∆ G is a fundamental property of chemical reactions.

8. Gibb’s Free Energy, Endergonic Reactions! • In some reactions, the ∆ G is positive, which means that the products of the reaction contain more free energy than the reactants; the bond energy (∆H) is higher, or the disorder (S) in the system is lower. • Such reactions DO NOT PROCEED spontaneously because they require an input of energy. • + ∆G reactions require an input of energy and are said to be ENDERGONIC.

9. Gibbs’ Free Energy , Exergonic Reactions! • In other reactions ∆G is negative. In this case, the products of the reaction contain less free energy than the reactants; either the bond energy is lower, or the disorder is higher or both. • Such reactions tend to proceed spontaneously. • Any chemical reaction tends to proceed spontaneously if the difference in disorder (T ∆S) is GREATER than the difference in bond energies between the reactants and products (∆H) • These reactions release free energy as HEAT and are called EXERGONIC.

10. What does spontaneously mean? • Note that spontaneous does NOT MEAN the same as instantaneous! • A spontaneous reaction may proceed very slowly! • Spontaneous reactions are exergonic reactions!

11. Equilibrium Constant • Because chemical reactions are reversible, a reaction that is exergonic in the forward direction will be endergonic in the reverse direction. • For each reaction, an equilibrium exists at some point between the relative amounts of reactants and products. • This equilibrium has a numeric value and is called the equilibrium constant. • Another way to think about free energy changes is: an exergonic reaction has an equilbrium favoring the products, and an endergonic reaction has an equilibrium favoring the reactants.

12. Interpreting Graphs

13. Activation Energy • Most spontaneous reactions require an input of energy to get started. • The energy needed to destabilize existing chemical bonds and initiate a chemical reaction is called ACTIVATION ENERGY. • The rate of an exergonic reaction depends on the activation energy required for the reaction to begin. • Reactions with large activation energies tend to proceed more slowly because fewer molecules succeed in overcoming the initial energy hurdle.

14. Activation Energy – Can YOU find it in the graphs below?

15. Activation Energy • The rate of chemical reactions can be increased in two ways: • Increasing the energy of the reacting molecules (by heating reactants) • Lowering the Activation Energy required (by adding a catalyst)

16. Catalysts • Catalysts work by lowering the activation energy of the reaction. • Catalysts speed up the rate of chemical reactions. • In living systems, enzymes act as catalysts!