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1. Big Idea 5: The laws of thermodynamics describe the essential role of energy and explain and predict the direction of changes in matter. 5.C. The attraction between the electrons of one atom and the protons of another explains the force of attraction in a chemical bond. As a bond forms, the atoms or ions become more stable and lower in potential energy. Thus, bond formation is an exothermic process and bond breaking (bond dissociation energy or bond energy) is an endothermic process. The repulsion between two nuclei (and core electrons) explains why atoms repel one another at close distance. The distance at which atoms are most stable and lowest in potential energy is known as the bond length. A graph of energy vs. distance can be used to determine bond length and bond length. Larger ionic charges result in stronger bonds. Shorter bonds tend to be stronger bonds. Double covalent bonds are stronger than single covalent bonds and triple bonds are the strongest covalent bonds. http://chemistry.tutorvista.com/organic-chemistry/chemical-bonds.html

2. Most chemical changes involve an absorption of energy in order to break existing bonds followed by a release of energy as new bonds form. The net energy change, ∆H, is negative in an exothermic reaction in which the products are lower in potential energy (more stable). The potential energy released by the reactants is transformed into the kinetic energy of the products. With the exception of an isolated system, some of the kinetic energy of the products is usually transferred to the surroundings as heat in an exothermic reaction. • For an endothermic reaction; ∆H is positive, the products have higher potential energy and lower kinetic energy, thus cooler. Exothermic reactions appear to be “thermodynamically favored” as the products are energetically more stable (higher average bond energy) than the reactants. Activation Energy, Ea, is the additional potential energy the reactants must gain to break bonds and form the activated complex.

3. 5.D. The same essential electrostatic attractions (+/-) that result in bond formation are responsible for weaker attractions between non-bonded atoms. In molecular systems, such intermolecular forces of attraction are responsible for the formation of solids and liquids. The role that energy plays in phase changes, breaking and forming IFA’s, is similar to the role energy plays, breaking and forming bonds, in chemical changes. • At the particulate scale, physical and chemical changes are typically distinguished by the type of interactions (bond vs. only IFA) that change during a process. However, a process such as dissolving salt in water involves breaking ionic bonds and forming molecule-ion attractions. A case could be made for either classification in some changes. Large biomolecules often demonstrate non-covalent attractions between molecules and within the same molecule resulting in highly specific molecular shapes, attractive sites and reactive sites. Enzymes are just one example of the vital role that structure-function relationships play in biochemistry. • The molecular structure and interactions between and within synthetic polymers can be modified to change macroscopic properties. Kevlar is shown here.

4. 5.E. Chemical or physical processes are driven by a decrease in enthalpy or an increase in entropy, or both. The standard change in Gibbs free energy, ∆Go, can tell us whether a given process is thermodynamically favored in the forward or reverse direction and to what extent at equilibrium. ∆Go = ∆Ho – T ∆So. A strongly favored process would have a large, negative ∆Go. Entropy can be thought of as the disorder or randomness of a given state. Energy is more dispersed when there are more possible micro-states (energetic positions) for matter. A pure, perfect crystal at absolute zero has zero entropy. Entropy increases: when a solid becomes a liquid, when a liquid becomes a gas, when a solute and solvent are mixed, when the number of product molecules is greater than the number of reactant molecules, when the temperature increases, when a sample of gas is allowed to increase its volume. A complex molecule has greater entropy than a simpler molecule. We will use the phrase thermodynamically favored in place of the term “spontaneous” to avoid misconceptions. Thermodynamically favored means the reaction “tends to occur” or the formation of products are favored at equilibrium. Note: a favored reaction can occur unperceptively slowly or it may not occur because its activation energy is too high. One can use the equation, ∆Go= ∆Ho– T ∆So, to qualitatively predict the direction and magnitude of physical and chemical changes based on the sign and magnitude of ∆Go. Note: Temperature is a multiplier (amplifier) of the value of ∆So. Note: Values for ∆Go and ∆Hoare typically reported in kJ/mol, but values for ∆So are typically reported in J/mol! Tis always the Kelvin temperature!

5. What conditions could lead to a thermodynamically favored process?