1 1 IB Topic 6: Kinetics 6.1: Rates of Reaction 6.1.1Define the term rate of reaction 6.1.2Describe suitable experimental procedures for measuring rates of reactions 6.1.3Analyse data from rate experiments

Define the term rate of reaction Rate of Reaction Some reactions occur rapidly  Inflation of an airbag (pg. 111 IBCC)  Explosion of nitrogen triiodide Some reactions occur slowly  Reaction of pigments in paintings with light & pollutants (pg IBCC)  Tarnishing of silver/Iron rusting

Define the term rate of reaction Rate of Reaction An increase in concentration of one of the products per unit time or as the decrease in concentration of one of the reactants per unit time. Units of rate of reaction: moles per dm 3 per second (mol dm -3 s -1 )

Describe suitable experimental procedures for measuring rates of reactions Measuring the Rate of Reaction  Mass or volume changes for gaseous reactions  Change in pH for reactions involving acids or bases  Change in conductivity measurements for reactions involving electrolytes  Use of a spectrophotometer or colorimeter for reactions involving color changes

Describe suitable experimental procedures for measuring rates of reactions Measuring the Rate of Reaction  The graph shows the volume of CO 2 produced against time when excess CaCO 3 is added to HCl Measuring the Rate of Reaction  The graph shows the concentration of a reactant against time 5

Analyse data from rate experiments Consider the hypothetical aqueous reaction A(aq)  B(aq) A mL flask initially contains mol of A and 0 mol B. The following data are collected: a) Calculate the number of moles of B at each time in the table. b) Graph the change in amount of A & B over time. c) Calculate the average rate of disappearance of A for each 10-minute interval in terms of mol s -1 d) Between t=10 min & t=30 min, what is the average rate of appearance of B in units of mol dm -3 s- 1 Time (min) Moles of A

Analyse data from rate experiments a) Calculate the number of moles of B at each time in the table. Time (min) Moles of A Moles of B

Analyse data from rate experiments b) Graph the change in amount of A & B over time.

Analyse data from rate experiments c) Calculate the average rate of disappearance of A for each 10-minute interval in terms of mol s -1 0 min  10 min: ( )/600 = 2.3 x mol s min  20 min: ( )/600 = 1.5 x mol s min  30 min: ( )/600 = 1.0 x mol s min  40 min: ( )/600 = 0.8 x mol s -1 Time (min) Moles of A

Analyse data from rate experiments d) Between t=10 min & t=30 min, what is the average rate of appearance of B in units of M s- 1 [B] at 10 min = mol/.100 dm 3 =.14 M [B] at 30 min = mol/.100 dm 3 =.29 M Rate = ( )/1200 = 1.3 x mol dm -3 s -1 Time (min) Moles of B

11 IB Topic 6: Kinetics 6.2: Collision Theory 6.2.1Describe 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-Boltzman 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.

Describe the kinetic theory in terms of the movement of particles whose average energy is proportional to temperature in Kelvins. Kinetic Theory  Tiny particles in all forms of matter are in constant motion.  The energy an object has because of motion is called kinetic energy.  The particles in any collection of atoms or molecules at a given temperature have a wide range of kinetic energies, from very low to very high. Overall, there is an average kinetic energy.

Describe the kinetic theory in terms of the movement of particles whose average energy is proportional to temperature in Kelvins. What happens when a substance is heated?  Particles absorb energy, some of which is stored within the particles (potential energy). This does not affect temperature.  The remaining energy speeds up particles (increases the average kinetic energy) which increases temperature.

Describe the kinetic theory in terms of the movement of particles whose average energy is proportional to temperature in Kelvins. Kelvin Temperature  As a substance cools, the average kinetic energy declines and the particles move more slowly.  At some point, the temperature will be low enough so the particles will stop moving and have no kinetic energy. That point is called absolute zero (0 K, o C)  Kelvin temperature of a substance is directly proportional to the average kinetic energy of the particles. The particles in helium gas at 200 K have twice the average as the particles in helium gas at 100K

Define the term activation energy, Ea. Activation energy is the minimum amount of kinetic energy that must be given to the reactants before they will react. (a) Activation energy (Ea) of forward reaction (b) Activation energy (Ea) of reverse reaction (c) Heat of reaction (ΔH) 1) What is the heat of reaction for each of the graphs? Which is exothermic and which is endothermic?

Describe the collision theory.  The two particles (atoms, ions, radicals or molecules) must collide with each other.  They must collide in the correct orientation, so that the reactive parts of each of the two particles come into contact with each other.  They must collide with sufficient kinetic energy to bring about the reaction.

Predict and explain, using the collision theory, the qualitative effects of particle size, temperature, concentration and pressure on the rate of a reaction.  Decreasing the particle size of solid reactants increases the reaction rate.  Increasing the temperature increases the reaction rate.  Only the surface particles of a solid react. Breaking the solid into smaller pieces increases the surface area so more particles available to react.  At higher temperatures the particles move faster and have more energy. Faster movement means more collisions, more energy means more collisions have the activation energy required for reacting.

Predict and explain, using the collision theory, the qualitative effects of particle size, temperature, concentration and pressure on the rate of a reaction.  Increasing the concentration of the reactants increase the reaction rate  Increasing the pressure of a gaseous reaction increases the reaction rate.  More particles present means more collisions.  Increasing pressure translates to increased concentration by either reducing the volume or increasing the number of particles. Higher concentration means more collisions.

Sketch and explain qualitatively the Maxwell- Boltzman energy distribution curve for a fixed amount of gas at different temperatures and its consequences for changes in reaction rate.  Area under the graph directly related to the number of particles present. The average kinetic energy close to the peak of the graph. Area to the right of the Ea depicts the number of particles having sufficient energy to react.  At higher temps, the distribution flattens out as more molecules gain kinetic energy. Area under the curves remain the same since the number of particles remain the same. However more particles have kinetic energies greater than the Ea as temperature increases.

Describe the effect of a catalyst on a chemical reaction.  A catalyst is a substance that increases the rate of a chemical reaction without itself being chemically changed at the end of the reaction.  Catalysts lower the activation energy barrier by positioning reactant particles favorably. More reactant particles possess this lower activation energy so the rate increases.

Sketch and explain Maxwell-Boltzmann curves for reactions with and without catalysts.  A catalyst is a substance that increases the rate of a chemical reaction without itself being chemically changed at the end of the reaction.  Catalysts lower the activation energy barrier by positioning reactant particles favorably. More reactant particles possess this lower activation energy so the rate increases.