Nuggets of Knowledge for Chapter 7 – Introduction to ReactionsChem 2310I. Introduction to Chemical ReactionsWhat a chemical reaction is• A chemical reaction involves the making and breaking of chemical bonds to form new compounds. ◦ Since chemical bonds are made of shared or transferred electrons, when a chemical reaction occurs, it is the electrons that change. The nucleus is not changed, so the same atoms are present at the beginning and the end. Only the connections between the atoms change.How to represent chemical reactions• In order to talk about chemical reactions without using specific examples, we need a way to represent reactions in a general way.◦ For example, we can use this format: A + B → C + D. A and B are reactants, while C and D are products. B◦ In organic chemistry we often use this format: A → C + D. ▪ Sometimes in organic chemistry, we refer to organic reactants as starting materials, and inorganic reactants are reagents. We think of a reagent causing a change to a starting material. In the equation above, A is a starting material, and B is a reagent.Reversibility of reactions (moved)• Most reactions are reversible. The reaction can run in the “forward” direction (A + B --> C + D) or in the “backward” direction (C + D --> A + B). This is usually written by drawing equilibrium arrows between the reactants and products. Which direction is forward and which is backward depends on how we have chosen to write it.Energy diagrams for chemical reactions• The energy of a reaction can be represented by an energy diagram, in which energy is on the y-axis and time on the x-axis.◦ The energy pathway of a reaction always starts low, goes up to a high point, and goes down again. The high energy point in the middle is called the transition state. ▪ Since the bonds that are breaking and being formed are half-way through the process, the transition state is higher in energy than either the products or the reactants. ◦ The energy needed to get from the starting materials to the transition state is called the activation energy. Molecules must have this much energy in order to react.◦ Enthalpy ( H) is the difference between the energy of the reactants and products, and given by H of the products – H of the reactants.▪ If the energy of the reactants is lower than the products, then the reaction is endothermic. If the energy of the reactants is higher than the products, then the reaction is exothermic.• Endothermic reactions absorb energy, while exothermic reactions give off energy. Endothermic reactions have a positive  H, while exothermic reactions have a negative  H. • The reason for the difference in energy is the energy of the chemical bonds. ◦ In an exothermic reaction, stronger bonds which are lower in energy are being formed, so the excess energy is given off. ◦ In an endothermic reaction, weaker bonds which are higher in energy are being formed, so additional energy is needed.How free energy relates to enthalpy (moved)•  H isn't the whole story – entropy is involved too•  G comes from a combination of the change in enthalpy and entropy:  G =  H – T S.o  H is the change in enthalpy, which comes from the energy of the bonds being broken and formed. o  S is the change in entropy, and can be significant if the number of molecules changes. If the number of molecules increases, the entropy increases, and  S is positive. o  S usually much smaller than  H. If the  H is near zero (bonds formed and broken are about the same energy), then  S can be important.II. Kinetics of Reactions• The kinetics of a reaction tells you how fast a reaction occurs; in other words, how many molecules of product are formed per second.◦ This matters because it tells us how long we will have to wait for a reaction to be done, or how fast an undesirable reaction will occur.• To understand kinetics, we must look at what is happening on the molecular level. In order for a reaction to occur, two molecules must collide so that their orbitals will come close enough to react.◦ They must also have the correct orientation; in other words, the right atoms must bump into each other at the right angles.◦ They must also have sufficient energy to react; they must have equal to or greater than the activation energy.◦ Therefore, anything which affects the frequency of collisions or the energy of the molecules will affect the rate of the reaction.• The activation energy and orientation are always the same for a given reaction; however the concentration of the reactants and the temperature of the reaction may be changed.◦ The lower the activation energy for a reaction is, the faster the reaction will be. ◦ The higher the probability that the molecules will collide with the correct orientation, the faster the reaction will be. The probability factor is represented by A, and has to do with how narrow the acceptable approach angle is.◦ A higher concentration of reactants will cause the reaction to speed up, since it causes more collisions between molecules. Concentration is represented by putting the molecule in brackets [ ].◦ A higher temperature will cause the reaction to speed up for two reasons: 1) the molecules are moving faster, so there are more collisions, and 2) more molecules have enough energy to overcome the activation energy. Temperature is represented by T.• The rate law of a reaction is a mathematical expression: rate = kr[A]x[B]y.◦ Rate is the number of molecules formed per second.◦ kr is the rate constant; it is given by the equation kr = AeEa/RT. A gives the probability that the molecules will collide at an appropriate angle, while the rest of the equation gives the proportion of molecules with enough energy to react.◦ [A] and [B] are the concentrations of the reactants.◦ X and y give the order of the reaction; by adding the exponents for all reactants, you can get the overall order of the reaction.• The rate of the reaction can be affected differently by the concentration of different reagents, depending on the role they play in the reaction. These must be determined experimentally, but once they are known, they can be used to help determine how the reaction proceeds.◦ If the mechanism is known it can also be used to determine the order of the reactants. The mechanism