Thermodynamics
Thermodynamics
Bond Dissociation Energy
In every compound, there is a certain amount of energy in all of the bonds. This heat energy, or enthalpy (H), can be measured. In a reaction, bonds are broken and bonds are formed. There is an exchange of energy each time this happens. Energy must be put into the system in order to break a bond. Imagine trying to break a chain and the energy you exert trying to do that with the sweat on your brow and strained muscles. Remember that it always takes energy to break a bond. Since we are using energy to break a bond, the bond dissociation energy (BDE) is positive. If a weak bond is broken, the BDE is a small positive number, because it is easy and takes little energy to break the bond. If a strong bond is broken, the BDE is a large positive number, because it is difficult and takes a lot of energy to break the bond.
Energy is released when bonds are formed. The individual ions do not like to be by themselves with a charge on them. They would much prefer to be in a neutral compound. So, when they form a bond and become a compound, they give a sigh of relief. They can relax and have less energy. A negative BDE represents the energy that is lost when this bond is formed. If a weak bond is formed, the BDE is a small negative number. If a strong bond is formed, the BDE is a large negative number (the ion feels more secure and relaxes more). The following is an example reaction to form methanol.
Enthalpy
When a reaction occurs, we look at all of the bonds that are broken (absorbing energy) and all of the bonds that are formed (releasing energy) and add up all of these BDE’s. This overall measurement on how much energy (heat) is released or absorbed is called the heat of reaction or the change in enthalpy (ΔH). The symbol, Δ, means “change in”. Overall, if more energy is absorbed than released, the reaction is called an endothermic reaction (+ΔH). If more energy is released than absorbed, the reaction is called an exothermic reaction (-ΔH). Reactions that are exothermic and give off energy are typically favored.
Breaking bonds Endothermic = + ΔH
Forming bonds Exothermic = - ΔH
11. Predict if ΔH is positive or negative for the following situations.
a) In a reaction, one strong bond is broken and one weak bond is formed. ___ ΔH
b) In a reaction, one weak bond is broken and one strong bond is formed. ___ ΔH
There is a method chemists use to quickly communicate the changes in energies of the reactants and products throughout a reaction. A Reaction-Energy Diagram can be drawn. The reaction coordinate is like the timeline of the reaction, from the reactants to the products. The energies of all species along this reaction coordinate are mapped out. Below is the reaction-energy diagram for our one-step reaction to make methanol.
Reaction Coordinate Diagram
So, from a reaction-energy diagram, we can see the relative energies of the reactants and products. We can also determine the activation energy of the reaction, the change in enthalpy (ΔH), whether ΔH is positive (endothermic) or negative (exothermic). We know that ΔH is the difference in energy between the products and the reactants. If the products have less energy than the reactants, heat is given off and the reaction is exothermic (-ΔH). This is the case in this reaction-energy diagram.
Various parts of a reaction-energy diagram
The activation energy (Ea) of a reaction is the amount of energy it takes to get from the energy of the products to the energy of the transition state. It is the amount of energy needed to get up the first hill to make the reaction go. For instance, if there is a high activation energy, perhaps the reaction may need to be heated in order to proceed.
12. Match the following statements with its reaction-energy diagram.
A. Endothermic, high activation energy
B. Endothermic, low activation energy
C. Exothermic, high activation energy
D. Exothermic, low activation energy
Catalysts lower the activation energy of a reaction and speed up the reaction. Catalysts are not consumed in the process. They must be regenerated so only a small amount of catalyst is needed. In biology, certain proteins called enzymes act as catalysts. Notice, catalysts only change the energy of the transition state by lowering the Ea. Catalysts do not influence the energies of the reactants or products at all.
Even though exothermic reactions (-ΔH) are more likely to proceed, more needs to be considered.
Entropy
Entropy (S) is the amount of disorder or randomness. The change in the entropy (ΔS) of a reaction is a consideration. If the products of a reaction are more disordered than the reactants, ΔS is positive. If the products are more ordered, ΔS is negative. How can we predict ΔS? Reactions that go to more disorder (+ΔS) are favored.
If two reactants react to form one product, there is less disorder in the system. ΔS is negative.
If one reactant forms two products, there is more disorder in the system. ΔS is positive.
So, ΔH and ΔS both influence whether a reaction will proceed towards the products. If a reaction proceeds or not is measured by the Gibbs free energy change (ΔG). If ΔG is positive, energy must be put into the system for the reaction to proceed. If ΔG is negative, the reaction will proceed spontaneously. The relationship between ΔG, ΔH, and ΔS can be expressed by the following equation.
ΔG = ΔH - TΔS
ΔG = Change in Gibbs free energy (Negative proceeds)
ΔH = Change in enthalpy (Negative helps)
T = Temperature (High temp. Either helps or hurts, depending on ΔS)
ΔS = Change in entropy (Positive helps)
Usually, ΔH is a greater term than TΔS, so the change in energy of the system is a good predictor of the spontaneity of the reaction. So, if ΔH is negative then ΔG is usually negative and the reaction is spontaneous (or proceeds).