Chapter 6: Reaction Rates

Collision theory / requirements for effective collisions

chemical reactions can occur only if reactants collid with….

• Favourable orientation/angle

• Enough energy ( activation energy)

Activation Energy (E_a)

• Minimum amount of collision energy required for a reaction to occur

• Energy used to break the bonds in the reactants

• Distance between the reactant and highest point on energy-time graph

What happens as the E_a increases? decreases?

reaction rate slows, reaction rate speeds up

Activated complex

• Short lived molecular configuration that exists at the top of the energy barrier that the reactants must overcome to become products

• Reactants are partially broken, products are partially formed

• High in energy and unstable due to bonds rearranging

  • Can go forward (products) or backwards (re form reactants)

Enthalpy change (ΔH)

• The difference between the potential energy of the reactants and that of the products

• Determined after product formed

• Has no effect on the rate of reaction

Forward and reverse reactions

• Possible if chemical equation has this symbol: “⇌”

• Mirror images of one another

• Exothermic and Endothermic reverse

Reaction Rate

• Measure of how quickly or slowly reactants are consumed / products are formed

Reaction rate is NOT constant. Explain.

• Beginning of reaction is fastest as concentration of reactants is high, reactants consumed to form products quickly

• Slows down as reactant concentration decreases

• Eventually stops as no more reactant is left

Factors that affect reaction rates

RECALL: Cool People Stay Till Chemistry Night

• Concentration

• Pressure (Increased Pressure)

• Surface Area

• Temperature

• Catalyst

• Nature of Reactant

How does concentration affect reaction rates

• Increases frequency of collisions

• High concentrations increase rate

How does pressure increase reaction rate

• Only for gaseous systems

• Increases frequency of collisions

• Increased pressure increases rate

How does surface area increase reaction rate

• Increases frequency of collisions

• Only in heterogenous systems (reactants in different shapes)

• Increase in surface area increases rate

How does temperature increase reaction rate

• Increases frequency of collisions and energy of collisions

• Increase of temperatures increases rate

  • Molecules collide more often with more energy due to increased KE

  • DOESNT CHANGE E_a, more particles are just able to overcome it.

Maxwell Boltzmann distribution of molecules

Gaussian/Bell curve

Temperature

• Area under curve represents all particles in reaction

• Shaded region represents all particles that pass that have high enough energies to react

• As temp increases, shaded region becomes larger

• Y axis: # of particles, X axis: energy

How does catalyst increase rate of reaction

• Provides an alternative pathway with a lower activation barrier (E_a) which increases reaction rate

• Not consumed in a reaction - Regenerated at the end of the reaction and can be used again

Maxwell Boltzmann distribution of molecules

Gaussian/Bell curve

Catalyst

• Catalyst doesn’t change the curve

• E_a is lowered and moved to the left

Potential energy diagrams with catalyst

• Catalyst will provide alternative pathway that has a smaller E_a

• X: progress of reaction, Y: energy

• Enthalpy is unchanged

How does the nature of reactant affect the rate of reaction?

• similar elements react similarly but at different rates (depending on reactivity series of metal)

  • COLLISION ENERGY

• Reactions in gas or aqueous system react more quickly than solids

  • Due to higher KE

  • COLLISION ENERGY AND FREQUENCY

• Small simple ions and molecules react more quickly than larger molecules

  • Small molecules have less friction due to less surface area —> move more quickly —> increases rate

  • COLLISION ENERGY AND FREQUENCY

• charged particles in solution react more quickly than neutral molecules

  • Neutral molecules collide by chance, whereas charged particles gravitate towards eachother due to electrostatic attraction —> higher change of collision —> increases rate’

  • COLLISION FREQUENCY

• Weak bonds are easier to break (small E_a), strong bonds are harder (higher E_a)

  • Bonds in reactants must be broken first before product formation

  • Bond dissociation energies

  • COLLISION ENERGY

Reaction mechanisms

• The series of steps that take place as a reaction proceeds from reactants to products

• Refers to all the steps together

• Mechanisms are proposed as there is no way to observe reactions at a molecular level

Elementary steps

A single step in a reaction mechanism

Reaction intermediate

• Compound produced in one elementary step and used up in subsequent elementary step

• Does not show up in overall equation

Rate Determining Step (RDS)

• Elementary step with slowest rate

  • Highest E_a

• Determines rate of overall chemical reaction

Overall activation energy

• Distance between the start to the highest point on the graph on reaction mechanism

Rate law

• calculation of reaction rate as a function of reactant concentration

• depends on the RDS of a reaction mechanism

• Must be determined experimentially

• r = k[A]^m[B]ⁿ

  • k is rate constant (depends on temp)

  • m/n number of molecules of A/B collide

Methods of measuring reaction rates

• Volume or pressure over time

  • if reaction involves gases

  • gas syringe(V), gas sensor (P)

• Change in temperature over time

  • endo/exothermic

  • calorimeter

• Change in absorbance over time (colour)

  • spectrophotometer

• Reactions involving ions?:

  • Conductivity over time (ions involved and electricity produced)

  • pH over time (if H or OH ions involved)