Chemical Kinetics Chapter 13

Chemical Kinetics Thermodynamics – does a reaction take place? Kinetics – how fast does a reaction proceed? Reaction rate is the change in the concentration of a reactant or a product with time (mol/(Ls)). A B rate = -  [A] tt rate =  [B] tt  [A] = change in concentration of A over time period  t  [B] = change in concentration of B over time period  t Because [A] decreases with time,  [A] is negative. 13.1

A B 13.1 rate = -  [A] tt rate = [B][B] tt time

Br 2 (aq) + HCOOH (aq) 2Br - (aq) + 2H + (aq) + CO 2 (g) time 393 nm light Detector  [Br 2 ]   Absorption 13.1

Br 2 (aq) + HCOOH (aq) 2Br - (aq) + 2H + (aq) + CO 2 (g) average rate = -  [Br 2 ] tt = - [Br 2 ] final – [Br 2 ] initial t final - t initial slope of tangent slope of tangent slope of tangent instantaneous rate = rate for specific instance in time 13.1

Reaction Rates and Stoichiometry A B Two moles of A disappear for each mole of B that is formed. rate =  [B] tt rate = -  [A] tt 1 2 aA + bB cC + dD rate = -  [A] tt 1 a = -  [B] tt 1 b =  [C] tt 1 c =  [D] tt 1 d

Write the rate for the following reaction: CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O (g) rate = -  [CH 4 ] tt = -  [O 2 ] tt 1 2 =  [H 2 O] tt 1 2 =  [CO 2 ] tt 13.1

Exothermic ReactionEndothermic Reaction The activation energy (E a ) is the minimum amount of energy required to initiate a chemical reaction A + B AB C + D + +

Kinetic Molecular Theory Model of what happens to gas particles during experimentation –Large numbers of molecules in continuous motion –Attractive and repulsive forces are negligible –Energy is transferred between molecules during collisions, but average kinetic energy is unchanged (as long as temp is constant) –Kinetic energy of molecules is proportional to the absolute temperature. (in K)

COLLISION THEORY Collision theory states that... particles must COLLIDE before a reaction can take place not all collisions lead to a reaction reactants must possess at least a minimum amount of energy - ACTIVATION ENERGY plus particles must approach each other in a certain relative way - the STERIC EFFECT

COLLISION THEORY Collision theory states that... particles must COLLIDE before a reaction can take place not all collisions lead to a reaction reactants must possess at least a minimum amount of energy - ACTIVATION ENERGY plus particles must approach each other in a certain relative way - the STERIC EFFECT According to collision theory, to increase the rate of reaction you therefore need... more frequent collisionsincrease particle speedor have more particles present more successful collisionsgive particles more energyor lower the activation energy

CO(g) + NO 2 (g)  CO 2 (g) + NO(g) A Reaction Profile

Particles must collide with the proper geometry or orientation for atoms to come in direct contact and form the chemical bonds of the products. (steric factor)

If both of these conditions are not met, particles will merely collide and bounce off one another without forming products.

istry/essentialchemistry/flash/collis1 1.swf Although, the percentage of successful collisions is extremely small, chemical reactions still take place at a reasonable rate because there are so many collisions per second between reactant particles.

INCREASING THE RATE INCREASE THE SURFACE AREA OF SOLIDS INCREASE TEMPERATURE ADD A CATALYST INCREASE THE PRESSURE OF ANY GASES INCREASE THE CONCENTRATION OF REACTANTS INCREASE THE SURFACE AREA OF SOLIDS INCREASE TEMPERATURE ADD A CATALYST INCREASE THE PRESSURE OF ANY GASES INCREASE THE CONCENTRATION OF REACTANTS The following methods may be used to increase the rate of a chemical reaction

INCREASING SURFACE AREA Increasing surface area increases chances of a collision - more particles are exposed Powdered solids react quicker than larger lumps Catalysts (e.g. in catalytic converters) are in a finely divided form for this reason + In many organic reactions there are two liquid layers, one aqueous, the other non- aqueous. Shaking the mixture improves the reaction rate as an emulsion is often formed and the area of the boundary layers is increased giving more collisions. CUT THE SHAPE INTO SMALLER PIECES SURFACE AREA = 30 sq units SURFACE AREA 9 x ( ) = 54 sq units 1 1 1

Effectincreasing the temperature increases the rate of a reaction particles get more energy so they can overcome the energy barrier particle speeds also increase so collisions are more frequent INCREASING TEMPERATURE ENERGY CHANGES DURING A REACTION As a reaction takes place the enthalpy of the system rises to a maximum, then falls A minimum amount of energy is required to overcome the ACTIVATION ENERGY (E a ). Only those reactants with energy equal to, or greater than, this value will react. If more energy is given to the reactants then they are more likely to react. Typical energy profile diagram for an exothermic reaction

Because of the many collisions taking place between molecules, there is a spread of molecular energies and velocities. This has been demonstrated by experiment. It indicated that...no particles have zero energy/velocity some have very low and some have very high energies/velocities most have intermediate velocities. INCREASING TEMPERATURE MOLECULAR ENERGY NUMBER OF MOLECUES WITH A PARTICULAR ENERGY MAXWELL-BOLTZMANN DISTRIBUTION OF MOLECULAR ENERGY

Increasing the temperature alters the distribution get a shift to higher energies/velocities curve gets broader and flatter due to the greater spread of values area under the curve stays constant - it corresponds to the total number of particles T1T1 T2T2 TEMPERATURE T 2 > T 1 MAXWELL-BOLTZMANN DISTRIBUTION OF MOLECULAR ENERGY INCREASING TEMPERATURE MOLECULAR ENERGY NUMBER OF MOLECUES WITH A PARTICULAR ENERGY

Decreasing the temperature alters the distribution get a shift to lower energies/velocities curve gets narrower and more pointed due to the smaller spread of values area under the curve stays constant - it corresponds to the total number of particles T1T1 T3T3 TEMPERATURE T 1 > T 3 MAXWELL-BOLTZMANN DISTRIBUTION OF MOLECULAR ENERGY INCREASING TEMPERATURE MOLECULAR ENERGY NUMBER OF MOLECUES WITH A PARTICULAR ENERGY

REVIEW no particles have zero energy/velocity some particles have very low and some have very high energies/velocities most have intermediate velocities as the temperature increases the curves flatten, broaden and shift to higher energies T1T1 T2T2 T3T3 TEMPERATURE T 2 > T 1 > T 3 MAXWELL-BOLTZMANN DISTRIBUTION OF MOLECULAR ENERGY INCREASING TEMPERATURE MOLECULAR ENERGY NUMBER OF MOLECUES WITH A PARTICULAR ENERGY

EaEa ACTIVATION ENERGY - E a The Activation Energy is the minimum energy required for a reaction to take place The area under the curve beyond E a corresponds to the number of molecules with sufficient energy to overcome the energy barrier and react. MAXWELL-BOLTZMANN DISTRIBUTION OF MOLECULAR ENERGY NUMBER OF MOLECULES WITH SUFFICIENT ENERGY TO OVERCOME THE ENERGY BARRIER INCREASING TEMPERATURE MOLECULAR ENERGY NUMBER OF MOLECUES WITH A PARTICULAR ENERGY

Explanation increasing the temperature gives more particles an energy greater than E a more reactants are able to overcome the energy barrier and form products a small rise in temperature can lead to a large increase in rate T1T1 T2T2 TEMPERATURE T 2 > T 1 EaEa MAXWELL-BOLTZMANN DISTRIBUTION OF MOLECULAR ENERGY INCREASING TEMPERATURE MOLECULAR ENERGY NUMBER OF MOLECUES WITH A PARTICULAR ENERGY EXTRA MOLECULES WITH SUFFICIENT ENERGY TO OVERCOME THE ENERGY BARRIER

Catalysts provide an alternative reaction pathway with a lower Activation Energy (E a ) Decreasing the Activation Energy means that more particles will have sufficient energy to overcome the energy barrier and react Catalysts remain chemically unchanged at the end of the reaction. ADDING A CATALYST WITHOUT A CATALYSTWITH A CATALYST

The area under the curve beyond E a corresponds to the number of molecules with sufficient energy to overcome the energy barrier and react. If a catalyst is added, the Activation Energy is lowered - E a will move to the left. MOLECULAR ENERGY EaEa MAXWELL-BOLTZMANN DISTRIBUTION OF MOLECULAR ENERGY NUMBER OF MOLECULES WITH SUFFICIENT ENERGY TO OVERCOME THE ENERGY BARRIER ADDING A CATALYST NUMBER OF MOLECUES WITH A PARTICULAR ENERGY

The area under the curve beyond E a corresponds to the number of molecules with sufficient energy to overcome the energy barrier and react. Lowering the Activation Energy, E a, results in a greater area under the curve after E a showing that more molecules have energies in excess of the Activation Energy EaEa MAXWELL-BOLTZMANN DISTRIBUTION OF MOLECULAR ENERGY ADDING A CATALYST EXTRA MOLECULES WITH SUFFICIENT ENERGY TO OVERCOME THE ENERGY BARRIER MOLECULAR ENERGY NUMBER OF MOLECUES WITH A PARTICULAR ENERGY

work by providing an alternative reaction pathway with a lower Activation Energy using catalysts avoids the need to supply extra heat - safer and cheaper catalysts remain chemically unchanged at the end of the reaction. Types Homogeneous Catalysts Heterogeneous Catalysts same phase as reactantsdifferent phase to reactants e.g. CFC’s and ozone e.g. Fe in Haber process Usesused in industry especially where an increase in temperature results in a lower yield due to a shift in equilibrium (Haber and Contact Processes) CATALYSTS - A REVIEW

increasing the pressure forces gas particles closer together this increases the frequency of collisions so the reaction rate increases many industrial processes occur at high pressure to increase the rate... but it can adversely affect the position of equilibrium and yield The more particles there are in a given volume, the greater the pressure The greater the pressure, the more frequent the collisions The more frequent the collisions, the greater the chance of a reaction INCREASING THE PRESSURE

Increasing concentration = more frequent collisions = increased rate of reaction INCREASING CONCENTRATION However, increasing the concentration of some reactants can have a greater effect than increasing others Low concentration = fewer collisionsHigher concentration = more collisions

Concentration A higher concentration of reactants leads to more effective collisions per unit time, which leads to an increasing reaction rate We are not increasing the amount being made for a given balanced equation with limiting reactants, we are only speeding up how quickly those products are made.

CuCO 3(s) + 2HCl (aq)  CuCl 2(aq) + CO 2(g) + H 2 O (l) blue Describe seven different ways to monitor the rate of the above reaction. State how each property would change as the reaction proceeds. 1.Mass of CuCO 3(s) over timedecreases 2.[HCl] over timedecreases 3.[CuCl 2 ] over timeincreases 4.Volume of CO 2 over timeincreases 5.Mass of a open beaker over timedecreases 6.Pressure of a closed beaker over timeincreases 7.Colour of the blue Cu 2+ over timeincreases

CuCO 3(s) + 2HCl (aq)  CuCl 2(aq) + CO 2(g) + H 2 O (l) blue Describe five different ways to increase the rate of the above reaction. 1.Increase the temperature 2.Increase [HCl] 3.Add a catalyst 4.Increase the surface area of CuCO 3(s) 5.Agitate We can't change the nature of the reactant because then we wouldn't have the same reaction. Replacing HCl with H 2 SO 4 would be faster but a different reaction.