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Collision theory / requirements for effective collisions
chemical reactions can occur only if reactants collid with….
Favourable orientation/angle
Enough energy ( activation energy)
Activation Energy (Ea)
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 Ea 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 Ea, 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 (Ea) 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
Ea is lowered and moved to the left
Potential energy diagrams with catalyst
Catalyst will provide alternative pathway that has a smaller Ea
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 Ea), strong bonds are harder (higher Ea)
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 Ea
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)