v AB + C à AC + B Ø Bonds breaking

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1 Chemistry Study Guide 3 v Kinetics reaction rates Ø Catalyst Ø Temperature Ø Concentration Ø Bonds Ø Surface area v Kinetics Ø Kinetic energy is directly proportional to the temperature Ø Gasses will react when Enough energy Collisions Right orientation v Reactants Ø Catalysts lower activation energy Ø Temperature proportional to KE Ø Concentration Ø Nature of reactants Surface area (solid) Reactivity (bond strength) S < l < g Aqueous Ø AB + C à AC+ B Ø No effect on rate Inert (non- reactive) gas v Maxwell Boltzmann distribution v AB + C à AC + B Ø Bonds breaking

2 Ø New bonds forming Ø Activated complex/transition state Same Ø Highest point new bonds formed old bonds break Very unstable v Collision theory of kinetics Ø For most reactants for a reaction to take place the reacting molecules must collide with each other On average about 10 9 collisions per second Ø Once molecules collide they may react together or they may not depending on two factors Whether the collision has enough energy to break the bonds holding reactant molecules together Whether the reacting molecules collide in the proper orientation for new bonds to form v Effective collisions kinetic energy factor Ø For a collision to lead to overcoming the energy barrier the reacting molecules must have sufficient kinetic energy so that when they collide the activated complex can form v Effective collisions Ø Collisions in which these two conditions are met (and therefore lead to a reaction) are called effective collision Ø The higher the frequency of the effective collisions the faster the reaction rate Ø When two molecules have an effective collision a temporary high energy (unstable) chemical species is formed the activated complex v Catalysts Ø Homogeneous catalysts are in the same phase as the reactant particles Ø Heterogeneous catalysts are in a different phase than the reactant particles Solid catalytic converter in the cars exhaust system v Enzymes Ø Because many of the molecules are large and complex most biological reactions require a catalyst to proceed at a reasonable rate Ø Protein molecules that catalyze biological reactions are called enzymes Ø Enzymes work by absorbing the substrate reactant onto an active site that orients the substrate for reaction Enzyme + substrate ß à enzyme substrate fast Enzyme substrate à enzyme + product v Collision theory and the frequency factor of the Arrhenius equation Ø The Arrhenius equation includes a term A called the frequency factor Ø The frequency factor can be broken into two terms that relate to the two factors that determine whether a collision will be effective v Collision Frequency Ø The collision frequency is the number of collisions that happen per second Ø The more collisions per second there are the more collisions can be effective and lead to product formation

3 v Orientation factor Ø The orientation factor p is a statistical term relating the frequency factor to the collision frequency Ø For most reactions p < 1 Ø Generally the more complex the reactant molecules the smaller the value of p Ø For reactions involving atoms colliding p ~ 1 because of the spherical nature of the atoms Ø Some reactions actually can have a p > 1 Generally involve electron transfer Ø The proper orientation results when the atoms are align in such a way that the old bonds can break and the new bonds can form Ø The more complex the reactant molecules the less frequently they will collide with the proper orientation Reactions when symmetry results in multiple orientations leading to reaction have a p slightly less than 1 Ø For most reactions the orientation factor is less than 1 v Molecular interpretation of factors affecting rate reactant nature Ø Reactions generally occur faster in solution than in pure substances Mixing gives more particle contact Particles separated allowing more effective collisions per second Forming some solutions breaks bonds that need to be broken Ø Some materials undergo similar reactions at different rates either because they have

4 A higher initial potential energy and are therefore closer in energy to the activated complex Because their reaction has a lower activation energy v Molecular interpretation of factors affecting rate temperature Ø Increasing the temperature raises the average kinetic energy of the reactant molecules Ø There is a minimum amount of kinetic energy needed for the collision to be converted into enough potential energy to from the activated complex Ø Increasing the temperature increases the number of molecules with sufficient kinetic energy to overcome the activation energy v Molecular interpretation of factors affecting rate concentration Ø Reaction rate generally increases as the concentration or partial pressure of reactant molecules increases Except for zero order reactions Ø More molecules leads to more molecules with sufficient kinetic energy for effective collision Distribution the same just bigger curve v Factors affecting reaction rate: nature of the reactants Ø Nature of the reactants means what kind of reactant molecules and what physical condition they are in Small molecules tend to react faster than large molecules Gasses tend to react faster than liquids which react faster than solids Powdered solids are more reactive than blocks More surface area for contact with other reactants Certain types of chemicals are more reactive than others Ex: potassium metal is more reactive than sodium Ions react faster than molecules No bonds need to be broken v Factors affecting reaction rates: temperature Ø Increasing temperature increases reaction rate Chemists rule of thumb for each 10 degree C rise in temperature the speed of the reaction doubles For many reactions Ø There is a mathematical relationship between the absolute temperature and the speed of a reaction discovered by Svante Arrhenius v Factors affecting reaction rate: catalysts Ø Catalysts are substances that affect the speed of a reaction without being consumed Ø Most catalysts are used to speed up a reaction these are called positive catalysts Catalysts used to slow a reaction are called negative catalysts Ø Homogeneous = present in same phase Ø Heterogeneous = present in different phase v Factors affecting reaction rate: reactant concentration

5 Ø Generally the larger the concentration of reactant molecules the faster the reaction Increase the frequency of reactant molecule contact Concentration of gases depends on the partial pressure of the gas Higher pressure = higher concentration Ø Concentrations of solutions depend on the solute to solution ratio molarity v Chemical kinetics Ø The study of the actors that effect the rates of chemical reactions Such as temperature Ø Lizards and other cold blooded creatures are ectotherms animals whose body temperature matches their environments temperature Ø When a lizards body temperature drops the chemical reactions that occur in the body slow down as do all chemical reactions when cooled) Ø This causes the lizard to become lethargic and to slow down Ø The speed of the chemical reaction is called its reaction rate Ø The rate of a reaction is a measure of how fast the reaction makes products Or uses reactants Ø The ability to control the speed of a chemical reaction is important v Defining rate Ø Rate is how much a quantity changes in a given period of time Ø The speed you drive your car is a rate the distance your car travels (miles) in a given period of time (1 hour) So the rate of your car has units in mi/hr Ø Speed = change in distance/change in time Ø The rate of a chemical reaction is generally measured in terms of how much the concentration of a reactant decreases in a given period of time Or product increases Ø For reactants a negative sign is placed in front of the definition Ø Rate = change in concentration/change in time Ø Fate = change in product/change in time = - change in reactant/change in time v Reaction rate changes over time Ø As time goes on the rate of a reaction generally slows down Because the concentration of the reactants dereases Ø At some time the reaction stops either because the reactants run out or because the system has reached equilibrium v Reaction rate and stoichiometry Ø In most reactions the coefficients of the balanced equation are not all the same Ø For these reactions the change in the number of molecules of one substance is a multiple of the change in the number of molecules of another Ø To be consistent the change in concentration of each substance is multiplies by 1/coefficient v Average rate Ø The average rate is the change in measure concentrations in any particular time period

6 Linear approximation of a curve Ø The larger the time interval the more the average rate deviates form the instantaneous rate v Instantaneous rate Ø The instantaneous rate is the change in concentration at any one particular time Slope at one point in the curve Ø Determined by taking the slope of a line tangent to the curve at the particular point First derivative of the function v Measuring reaction rate Ø To measure the reaction rate you need to be able to measure the concentration of at least one component in the mixture at many points in time Ø There are two ways of approaching the problem For reactions that are complete in less than 1 hour it is best to use continuous monitoring of the concentration For reactions that happen over a very long period of time sampling of the mixture at various points in time can be used When sampling is used often the reaction in the sample is stopped by a quenching technique v Continuous monitoring Ø Polarimetry measuring the change in the degree of rotation of plane- polarized light caused by one of the components over time Ø Spectrophotometry measuring the amount of light of a particular wavelength absorbed by one component over time The component absorbs the complementary color Ø Total pressure the total pressure of a gas mixture is stochiometrically related to partial pressures of the gasses in the reaction v Sampling the reaction mixture at specific times Ø At specific times during the reaction drawing off aliquots (samples from the reaction mixture) and doing quantitative analysis Titration of one of the components Gravimetric analysis Ø Gas chromatography can measure the concentrations of carious components in a mixture For samples that have volatile components Separates mixture by adherence to surface v Factors affecting reaction rate: nature of reactants Ø Nature of the reactants means what kind of reactant molecules and what physical condition they are in Small molecules tend to react faster than large molecules Gases tend to react faster than liquids which react faster than solids Powdered solids are more reactive than blocks More surface area for contact with other reactants

7 Certain types of chemicals are more reactive than others K more reactive than Na Ions react faster than molecules No bonds need to be broken v The rate law Ø The rate law of a reaction is the mathematical relationship between the rate of the reaction and the concentrations of the reactant And homogeneous catalysts as well Ø The rate law must be determined exponentially Ø The rate of a reaction is directly proportional to the concentration of each reactant raised to a power Ø For the reaction aa + bb à products the rate law would have the form given below n and m are called orders for each reactant k is called the rate constant Ø Rate = k[a] n [B] m v Reaction order Ø The exponent on each reactant in the rate law is called the order with respect to the reactant Ø The sum of the exponents on the reactants is called the order of the reaction v Finding the rate law the initial rate method Ø The rate law must be determined experimentally Ø The rate law shows how the rate of a reaction depends on the concentration of the reactants Ø Changing the initial concentration of the reactant will therefore affect the initial rate of the reaction v Rate = k[a] n Ø If a reaction is zero order the rate of the reaction is always the same Doubling [A] will have no effect on the reaction rate Ø If a reaction is a first order the rate is directly proportional to the reactant concentration Doubling [A] will double the rate of reaction Ø If a reaction is second order the rate is directly proportional to the square of the reactant concentration Doubling [A] will quadruple the rate of reaction v Determining the rate law when there are multiple reactants Ø Changing each reactant will effect the overall rate of the reaction Ø By changing the initial concentration of one reactant at a time the effect of each reactant s concentration on the rate can be determined Ø In examining results we compare differences in rate for reactions that only differ in the concentration of one reactant v Finding the rate law graphical methods Ø The rate law must be determined experimentally Ø A graph of concentration of reactant vs. time can be used to determine the effect of concentration on the rate of a reaction

8 Ø This involves using calculus to determine the area under the curve v Integrated rate laws Ø For each reaction A à products the rate law depends o the concentration of A Ø Applying calculus to integrate rate law gives another equation showing the relationship between the concentration of A and the time of the reaction this is called the integrated rate law v Half life Ø The half life t1/2 of a reaction if the length of time it takes for the concentration of the reactant to fall to ½ of its initial value Ø The half life of the reaction depends on the order of the reaction v Zero order reactions Ø Rate = k[a] 0 = k Constant rate reactions Ø [A] = - kt + [A]initial Ø Graph of [A] vs. time is straight line with slope = - k ad y- intercept = [A] initial Ø t1/2 = [Ainitial] /2k Ø When rate = M/sec, k = M/sec v First order reactions Ø Rate = k[a] 1 = k[a] Ø In[A] = - kt + in[a]initial Ø Graph In[A] vs. time gives straight line with slope = - k and y- intercept = in[a]initial Used to determine the rate constant Ø t1/2 = 0.693/k Ø The half life of a first order reaction is constant Ø When rate = M/sec, k = s - 1

9 v Second order reactions Ø Rate = k[a] 2 Ø 1/[A] = kt + 1/[A]initial Ø Graph 1/[A] vs. time gives straight line with slope = k and y- intercept = 1/[A]initial Used to determine rate constant Ø t1/2 = 1/(k[A]0]) Ø when rate = M/sec, k = M - 1 *s - 1

10 v Rate law Ø k = rate constant Ø A à product Ø A + B à product Ø n, m = order of reaction Ø Rate = k[a] m [B] n Ø Overall order of reaction = add exponents Ø Order of reaction with respect to concentration of [A] means only A v Zero order reaction Ø Rate = k[a] 0 = k Ø M/s Molarity per second Ø Concentration doesn t affect rate v First order reaction Ø Rate = k[a] Ø Rate is directly proportional to the concentration v 2 nd order reaction Ø Rate = k[a] 2 Ø Rate is proportional to [ ] 2 Concentration to 2 nd power v Rate constant Ø K is temperature dependent Ø K directly related to temperature Ø t1/2 = 0.693/k