Chapter 6 Kinetics IB Chemistry Fall 2010 Tam

Standards Define the term rate of reaction Describe suitable experimental procedures for measuring rates of reactions Analyse data from rate experiments Describe the kinetic theory in terms of the movement of particles whose average energy is proportional to temperature in kelvins Define the term activation energy, Ea Describe the collision theory Predict and explain, using the collision theory, the qualitative effects of particle size, temperature, concentration and pressure on the rate of a reaction Sketch and explain qualitatively the Maxwell–Boltzmann energy distribution curve for a fixed amount of gas at different temperatures and its consequences for changes in reaction rate Describe the effect of a catalyst on a chemical reaction Sketch and explain Maxwell–Boltzmann curves for reactions with and without catalysts.

Rates of Reaction The rate of a chemical reaction is a measure of the amount of reactants being converted into products per unit amount of time. Rate of rxn = increase in product conc./ time Or = decrease in reactant conc./time Expressed as change in conc. per unit time or mol dm -3 s -1. On a graph, the steepness or gradient, of the curve is a measure of the change in conc. Per unit time (rate of rxn). Rate at chosen value of time can be obtained from tangent to curve and measuring its gradient.

Rate of rxn is not constant due to curve, but usually fastest at start and slows down as rxn proceeds. Common to compare initial rate of rxn by taking tangent to curve at t=0, when comparing rates of rxn under different conditions.

Measuring rates of rxn Every reaction differs- measure conc of reactants or products and technique used vary Usually conc not measured directly- signal related to conc. Ex. Colored precipitate formed- measure color change; gas formed- measure volume change.

Common Techniques Change in vol. of gas produced- Collect gas and measure change in vol. at regular time intervals for graph of vol. vs. time Glass Gas syringe apparatus to collect gas or use an inverted burette or measuring cylinder filled with water, but gas must have low solubility in water.

Common Techniques 2 Change in mass- for rxn giving off gas, corresponding decrease in mass measured by standing rxn mixture directly on balance. Unlikely to work well with H 2 produced- too light to give significant changes in mass. This method allows for continuous readings- graph of mass vs. time.

Common Techniques 3 Change in transmission of light: colorimetry/ spectrophotometry- a reactant/product is colored giving characteristic absorption in visible region (wavelengths ~ nm) Indicator may be used to generate colored compound Colorimeter/ spectrophotometer passes light of specific wavelength through soln and measures light absorbed by rxn of components. Conc. Of colored compound increases, it absorbs proportionally more light, so less is transmitted. A photocell generates electric current from light transmitted and then recorded on meter or PC. Method allows for continuous readings, graph of absorbance vs. time. Use standard curve to convert abs. values into conc. based on readings of known conc.

Common Techniques 4 Change in conc. Measured using titration- measure conc. Of one of reactant/product by titrating it against a known “standard”. Can’t be done continuously b/c technique involves chemically changing the reaction mixture. Samples must be withdrawn from mixture at regular intervals and analyzed by titration. Problem from time needed to complete titration while mixture continues to react. Use “quenching” to overcome problem, a substance is introduced effectively stopping rxn at time of withdrawal of sample.

Common Techniques 5 Change in conc measured using conductivity- Total electrical conductivity of a soln depends on total conc of its ions and on their charges. Conductivity meter- immersing inert electrodes in the soln. Apparatus can be calibrated using soln of known conc so readings can be converted to conc of ions present. Sharp decrease in conc of ions will give corresponding decrease in electrical conductivity of soln as rxn proceeds.

Common Techniques 6 Non-continuous methods of detecting change during a rxn: “clock rxns” Difficult to record continuous change in rate or rxn.- measure time for rxn to reach a certain fixed pt. Time taken to reach pt for same rxn under diff. conditions can then be compared and used as means of judging different rates of rxn.

Collision Theory Kinetic-molecular theory- particles in substance move randomly due to kinetic energy they possess. Temp (K) is proportional to average kinetic energy of the particles in a substance. Increasing temp -> increase avg kinetic energy of particles Solid  Liquid  Gas

Maxwell- Boltzmann Distribution Curve # of particles in a gas at a particular temp. show a range of values of kinetic energy Shows the # of particles that have a particular value of kinetic energy (or probability of that value occurring) vs. values for kinetic energy Area under curve represents total # of particles in sample.

How reactions happen Kinetic energy of particles cause collisions and energy from collisions may result in some bonds between reactants being broken and some new bonds forming resulting in products Rate of rxn will depend on # of collisions b/t particles successful in forming products Not all collisions will be successful due to energy of collision & geometry of collision.

Energy of Collision Particles must have a certain minimum value for their kinetic energy for a collision to lead to rxn. Energy is needed to overcome repulsion b/t molecules and often to break some bonds in the reactants before they can react When energy is supplied, reactants achieve transition state from which products can form. Activation energy (E A )- minimum value of kinetic energy which particles must have before they are able to react. Rate of rxn depends on proportion of particles that have values of kinetic energy greater than E A

Geometry of collision Collisions b/t particles are random and unlikely to occur with particles in many different orientations Can be crucial in determining whether or not collisions will be successful and proportion of collisions leading to a rxn.

Collision Theory Rate of rxn depends on frequency of collisions which occur b/t particles possessing both: Values of kinetic energy greater than activation energy Appropriate collision geometry.

Factors affecting rate of rxn Temperature Concentration Particle size Pressure Catalyst

Temperature Increase temp increase rate of all rxns. B/c temp is a measure of avg kinetic energy of particles so higher temp means increase in avg kinetic energy and more particles with energy exceeding E A. With increased temp. area under curve is the same b/c area proportional to total # of particles & is a constant in sample of study, but peak of curve shifts to right resulting in: Increase in collision frequency and more collisions involving particles with higher values of kinetic energy, specifically higher than activation energy. Consequently increase in # of successful collisions and hence increase rate of reaction. Many rxns double their rxn rate for every 10 o C increase in temp.

Concentration Increasing conc of reactant increases rate of rxn, b/c as conc increases, frequency of collisions b/t reactant particles increases so frequency of successful collisions increases As reactants are used up, their conc falls and rate of rxn decreases, giving typical rate curve

Particle Size Decreasing particle size increases the rate of rxn, b/c subdividing a large particle into smaller parts increases total surface area and allows more contact and higher probability of collisions b/t reactants. Ex. Stirring solns may help decrease particle size and increase rate.

Pressure For rxns with gases, increasing pressure increases rate of rxn because higher pressure compresses the gas, effectively increasing its concentration and therefore increase frequency of collisions.

Catalyst Substance increasing rate of rxn w/o itself undergoing a permanent change. Most work by providing alternate route for rxn with lower activation energy. Temp not increased, more particles have enough energy greater than E A to have successful collisions. Catalysts have equal effect on both forward and reverse rxn so they don’t change position of equilibrium or the yield. Every biological rxn is controlled by an enzyme Specific enzymes for a particular rxn so many enzymes exist.