Factors Affecting the Rate of Reaction Kinetics Lesson # 2 Factors Affecting the Rate of Reaction

Collision Theory Collision theory states that chemical reactions can occur only if reactants collide Futhermore, they must collide at a specific orientation and with enough kinetic energy that any bonds in the reactants will break and new bonds will form, making the products. The rate of a reaction depends on the frequency and the proportion of collisions that convert reactants into products. An increase in the frequency of effective collisions leads to a higher reaction rate. Orientation means that the specific atoms in a molecule need to make contact in order to react. If the atoms are large or the shape hinders this interaction, then no effective collision can occur.

Activation Energy Activation energy (Ea) is the minimum energy that reactant molecules must possess for a reaction to be successful. Think of our potential energy diagrams from thermochemistry. We know that the pathway doesn’t really matter, and that the sum of the steps is the same as the net change in enthalpy. BUT, the molecules must have enough energy to start to allow that reaction to occur in the first place – this is activation energy.

Ea Analogy Think of activation energy like a potential energy barrier or hill. Think of a ball rolling up a hill. As it rolls up the hill, the ball slows down as some of the kinetic energy changes to potential. If there is enough kinetic energy to reach the top of the hill, the ball will roll down the other side (a reaction will occur). If not, it will roll back the way it came (no reaction). In terms of chemical reactions, the kinetic energy has to be high enough to break all the bonds in the compounds.

Ea (continued) The unstable arrangement of atoms (bonds not completely broken or reformed) is called the activated complex or transition state, and is always located at the highest potential energy value.

Factor 1: Chemical Nature of Reactants Whether an element or compound is deemed “reactive” is a chemical property. This could do with arrangement of electrons, activity, bond type, bond strength, amount of bonds, etc. Sodium is considered reactive and gold is not. This is just based on chemical properties of each substance. Single bonds will break apart faster than double bonds (based on bond energies). Ions will react faster than molecules as they free versus bonded to other entities.

Factor # 2 – Concentration Concentration is the amount of solute dissolved in a solution. The more solute in solution, the closer the molecules will be to one another (think filling a room with 5 people versus 50). The more particles, the greater the probability that collisions will occur, meaning the rate of reaction will increase.

Factor # 3 – Surface Area Surface area is the amount of surface that is exposed on a particle. For example, a solid piece of magnesium will react with acid much slower than magnesium ground up into powder. As a solid, only the surfaces that touch the acid may have the possibility of colliding and reacting, but when ground up, each tiny particle is now exposed and has the opportunity to collide and react. Breaking apart solids into smaller pieces is effectively increasing the concentration.

Factor # 4 - Temperature In general, most reaction rates increase with increasing temperature. Many reactions are greatly affected by temperature – even noticing that an increase in 10 degrees will often double the rate of reaction. This is why sugar dissolves better in hot water versus cold water. This is also why we keep food in the fridge – it slows down the reaction of oxygen with the food, causing spoiling. Cold-blooded animals take energy from the sun, so on warm, sunny days these animals are most active.

Temperature (continued) Temperature can be related to collision theory because temperature is a measure of the average kinetic energy of a system. An increase in temperature means an increase in kinetic energy. Molecules with more kinetic energy are able to overcome the activation energy and break bonds to form new compounds. Also, increasing temperature means that particles are vibrating faster, causing stronger collisions, and increasing the probability of an effective collision.

Maxwell-Boltzmann Distributions A Maxwell-Boltzmann distribution shows the relationship between the number of entities (atoms, molecules, etc.) of a reactant versus its kinetic energy. Only if the molecules can achieve energy greater than or equal to the activation energy will react.

MBD and Temperature With increasing temperature, a greater number of entities can achieve this energy. Most reaction rates increase exponentially with increasing temperature.

Factor # 5 - Catalysts A catalyst is a substance that changes the rate of a chemical reaction but remains unchanged during the reaction. Catalysts can participate in reactions over and over without being used up. This means that they are environmentally friendly (as long as they are disposed of properly once done with). They also allow reactions to be done at lower temperatures which conserve energy. Biological catalysts are called enzymes (large protein molecules). When the catalyst is the same state as the reactants, it is called a homogeneous catalyst. When it is not, it is called a heterogeneous catalyst.

Catalysts (continued) Unlike temperature, catalysts do not increase the number of collisions between reactants, nor do they increase the kinetic energy of reactants. Instead, catalysts provide an alternative pathways for the reaction at a lower activation energy. Since the energy required to reach the activated complex is lower, a greater fraction of reactants will be able to form products. It is important to note that although the pathway is different using a catalyst, the initial energy of reactants and final energy of product is unchanged, according to Hess’s Law.

Reaction Pathway with Catalysts RATE VIDEO