Factors Affecting Reaction Rates

6.4 and 6.5 FACTORS AFFECTING REACTION RATES Homogeneous reactions are chemical reactions in which all chemical species are in the same phase. (all reactants and products are gases, all reactants and products are aqueous solutions, all reactants and products are solids, etc.) example: 2H2 (g) + O2 (g) à 2H2O (g) Heterogeneous reactions have reactants in two or more phases. example: Mg (s) + 2HCl (aq) à MgCl2 (aq) + H2 (g) Factors Affecting the Rate of a Homogenous or Heterogeneous Reaction: 1. Temperature Maxwell-Boltzmann Distribution Curve: The two areas are the same because the number of particles doesn't change. 6.4 and 6.5 FACTORS AFFECTING REACTION RATES Homogeneous reactions are chemical reactions in which all chemical species are in the same phase. (all reactants and products are gases, all reactants and products are aqueous solutions, all reactants and products are solids, etc.) example: 2H2 (g) + O2 (g) à 2H2O (g) Heterogeneous reactions have reactants in two or more phases. example: Mg (s) + 2HCl (aq) à MgCl2 (aq) + H2 (g) Factors Affecting the Rate of a Homogenous or Heterogeneous Reaction: 1. Temperature Maxwell-Boltzmann Distribution Curve: 6.4 and 6.5 FACTORS AFFECTING REACTION RATES 1. Temperature Maxwell - Boltzmann Distribution Curve: Ea = activation energy barrier • The minimum amount of KE which a molecule must possess in order to react. • i.e. the minimum increase in PE (conversion of KE) during the collision of reactant particles necessary to initiate a reaction • or create an "activated complex"; • hence it acts as a barrier to the reaction eg. shaded area of curve: at T1 only the moles in the blue shaded area have enough energy to successfully react -However at T2 both the blue shaded area and the red shaded area have particles with sufficient KE to successfully react • Therefore at the higher temperature, there are more particles with the energy necessary to overcome the activation energy barrier and react • These are called "effective collisions" i.e. when two particles collide and go on to form a product or products • higher temp == faster particles = more KE = harder collisions = faster rate! **If two particles collide but just bounce off one another, this is an ineffective collision and does not lead to a reaction, (therefore they did not have sufficient KE). FACTORS AFFECTING REACTION RATES And 6.6 COLLISION THEORY Collision Theory: Chemical reactions occur due to collisions of the reacting particles The rate of a reaction is proportional to the number of collisions. Very few collisions result in reactions because of these two necessary conditions: 1) the orientation of particles 2) must have a certain minimum threshold KE called Ea Q10 rule The Q10 temperature coefficient is a measure of the rate of change of a biological or chemical system as a consequence of increasing the temperature by 10 °C. As a very general rule: The rate of a reaction doubles for each 10oC rise in temperature. Transition State Theory When molecules collide they slow down, stop, & fly apart again. Therefore, because of Cons. of Energy, KE is converted to PE at the time of collision. Transition state theory describes a hypothetical transition state that exists between reactants and products during a chemical reaction. The species formed in this hypothetical transition state is called the activated complex. This theory is used to explain how chemical reactions take place. If the rate constant has been experimentally determined, the theory can also be used to determine the standard Gibbs free energy, enthalpy and entropy of the reaction. It is closely related to Collision Theory. According to TST, between the state where molecules are reactants and the state where molecules are products, there is a state known as the transition state. In the transition state, the reactants are combined in a species called the activated complex. The theory suggests that there are three major factors that determine whether a reaction will occur: 1. The concentration of the activated complex 2. The rate at which the activated complex breaks apart 3. The way in which the activated complex breaks apart: whether it breaks apart to reform the reactants or whether it breaks apart to form a new complex, the products. Collision theory proposes that not all reactants that combine undergo a reaction. However, assuming the stipulations of the collision theory are met and a successful collision occurs between the molecules, transition state theory allows one of two outcomes: a return to the reactants, or a rearranging of bonds to form the products. ACTIVATION ENERGY: (PE DIAGRAM) - an artistic representation of the change in stored energy (converted from KE) as the reaction proceeds. ACTIVATED COMPLEX: is a transitory, unstable, high PE intermediate in the rxn process. -it is the turning point at which A + B is forming C + D Therefore Ea is the minimum amount of energy that the colliding species must have to form the activated complex. ACTIVATION ENERGY: (PE DIAGRAM) - an artistic representation of the change in stored energy (converted from KE) as the reaction proceeds. ACTIVATED COMPLEX: is a transitory, unstable, high PE intermediate in the rxn process. -it is the turning point at which A + B is forming C + D Therefore Ea is the minimum amount of energy that the colliding species must have to form the activated complex. Can you draw a graph representing each of the following? NOTE: Ea Ea ____________________ Temp ___________________ Rate CONSIDER THE FOLLOWING: CONSIDER: A [reactant] B [reactant] ________________ time C [product] _________________ time D [product] ________________ time __________________ time WHICH GRAPH (above) BEST REPRESENTS: i) product concentration vs. time ii) reactant concentration vs. time A B rate rate ________________ time C __________________ time D rate rate ________________ time __________________ time WHICH GRAPH (above) BEST REPRESENTS: i) rate of reactants used vs. time ii) rate of products produced vs. time FACTORS AFFECTING REACTION RATES continued 2. Concentration of Reactants The rate of a reaction is directly proportional to the concentration of the reactants. As the concentration of the reactants increases there will be more "effective collisions" between reactant molecules per unit of time. Consider the Chemical reaction: A + B ® C v α [A] x [A] = concentration of A in moles per litre where v = velocity or rate of a reaction measured in moles / L / s i.e. v is concentration = moles / L time s also v α [B] y x and y are exponents which must be found experimentally therefore v α [A] x[B] y v = k [A] x[B] y this is the rate equation for the reaction A + B ® C The concentration of the reactants can be increased by either adding more of the reactants per unit volume (i.e. you can’t just put more reactants in; you must put proportionally more moles of the reactant in the total volume of the reaction). or decreasing the size of the container - which applies to reactions in the gaseous phase. For gases, pressure is the same as concentration: PV = nRT n = moles = concentration V L n=P at constant temperature, R and T are constant V RT therefore n ∝ P (at constant T) V This is a GOOD TIME TO DO the IODINE CLOCK REACTION 3. Nature of Reactants i.e. the chemical properties of the substance (not the physical properties such as solid, liquid or gas) for example, the bonding (double vs. single vs. triple - or in other words saturated vs. unsaturated), the molecule geometry, the available electrons, etc. In general the greater the number of bonds which must be broken in a collision between two molecules, the higher is the activation energy barrier. Recall the breaking of bonds requires energy. The larger and more complex the reactant molecules are, the slower the reaction because few molecules will possess sufficient KE to overcome Ea. Monatomic species usually react much more rapidly than polyatomic species. (polyatomic reacts slower because many more bonds to break) Eg. FACTORS AFFECTING REACTION RATES And 6.7 CATALYSIS 4. Catalyst A substance which: speeds up a chemical reaction but remains unchanged at the end of the reaction. A catalyst is thought to: lower the activation energy barrier by providing an alternative reaction pathway or a more favourable collision geometry. (more favourable collisions = faster rate) In Biology, it is called an enzyme. In Chemistry, it is called a catalyst. If Ea is lowered then: the number of molecules present of A and B with sufficient KE to collide effectively and overcome the activation energy barrier is increased. Therefore the rate of the reaction increases. INHIBITORS: Chemicals which slow or retard the reaction rate without being consumed themselves eg. food preservatives medicine fire retardants on clothing Many different types of catalysts: a) IONS eg. Mn +2 in the reaction of MnO4 - with Fe +2 The Mn +2 ion makes this reaction go faster b) MOLECULES eg. MnO2 in making O2(g) out of KClO3(s) MnO2(s) 2KClO3 (s) -------------> 2KCl (s) + 3O2(g) heat c) ACIDS (H+ or H3O+) Acid decomposition of formic acid HCOOH (l) ⇄ CO (g) + H2O (l) this is a slow reaction Adding an acid (i.e. a hydrogen ion) speeds up the reaction Proposal mechanism: (3 step mechanism) HCOOH + H+ ⇄ HCOOH2+ HCOOH2+ ⇄ HCO+ + H2O (l) HCO+ ⇄ H+ + CO (g) d) ENZYMES (proteins) large protein molecules found in all living organisms -proteins are long chains of amino acids NH2 amino group COOH R changes from one amino acid to another carboxyl group Your body needs 20 different amino acids to grow and function properly. Though all 20 of these are important for your health, only nine amino acids are classified as essential. These are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. Enzymes have an active site: An enzyme speeds up a reaction by lowering the Ea for both the forward and reverse reactions. Each enzyme has a specific active site, therefore only one substrate molecule can fit into the active site, therefore each different reaction will have a different specific enzyme. FACTORS AFFECTING REACTION RATES continued for Heterogeneous reactions, add : 5. Surface Area Therefore more collisions per unit of time between the reacting molecules. FASTEST ------------------------------------------------------->SLOWEST Aqueous > gas > solids Ions in or solution liquids 6. Agitation (stirring) increased KE = (eg. ultrasound stirring device) more particles brought into contact = increased possibility of reaction = increased rate