Reactivity 2.2
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Molecularity
The number of particles taking part in any specified step
Unimolecular
An elementary step that involves a single reactant particle
Bimolecular
An elementary step with two reactant particles
What is the activation energy in a single step reaction?
The minimum energy required to reach the transition state before products are formed.
Rate determining step
The slowest step of a reaction. It has the highest activation energy
Rate of reaction
Increase in conc of one of the products per unit time or decrease in conc of one of the products per unit time
Rate of rxn common units
mol dm^-3 s^-1
What can be used to determine the rate of a chemical reaction?
Any technique that enables the change in conc of one of the products or reactants to be measured over time
Gradient of a conc/time graph=?
Rate of rxn
Collision theory
Particles must collide.. - at the correct orientation - with sufficient energy to break existing bonds in the reactant species (min for this is known as activation energy)
Maxwell-Boltzman energy distribution curve
• Expresses that different particles in a gas at (a specific temp) have a range of kinetic energies • area under curve represents the total number of particles in the sample • can be used to explain the effect of changing temps and the use of a catalyst on the probability of successful collisions occurring
Effect of increasing reactant concentration (or pressure in rxns with gaseous reactants) on rate of rxn
(Initial) Rate of rxn will inc as conc is inc. This is because there are more reactant particles, therefore more collisions, therefore more successful collisions per sec.
Effect of increasing temp on rate of rxn
Rate of rxn will inc as the temp inc. This is because the particles are moving more quickly and are therefore colliding more often with greater energy.
Effect of increasing surface area on rate of rxn
Rate of rxn will inc as the surface area inc. This is because only the surface particles will come into contact with the other reactant/s. Therefore the greater the surface area the greater the amount of available particles.
Effect of catalysts on rate of rxn
The rate of rxn will be increased in the presence of a suitable catalyst. They do this by bringing the reactive parts of the reactants close together, providing an alternate pathway with a lower activation energy. Meaning more of the particles can reach the Ea threshold.
Catalyst
A substance that increases the rate of the reaction without being permanently changed chemically.
If the eq for the rate determining step is: A –> products , what is the molecularity and the rate eq?
Molecularity: Unimolecular Rate eq: k[A]
If the eq for the rate determining step is: 2A –> products , what is the molecularity and the rate eq?
Molecularity: Bimolecular Rate eq: k[A]^2
If the eq for the rate determining step is: A + B –> products , what is the molecularity and the rate eq?
Molecularity: Bimolecular Rate eq: k[A] [B]
What is the activation energy in a single step rxn?
The min energy required to reach the transition state before products are formed
What is formed in a two step rxn?
An intermediate
What is the activation energy in a two step rxn?
Overall Ea is the difference in energy between the reactant state and the highest energy transition state. Each step in a two step rxn has its own Ea, the slow step will have the highest Ea.
Order of rxn with respect to a particular reactant
The power to which it’s conc term in the rate eq is raised
When rate=k[A]^x+[B]^y, what is the overall rxn order?
x+y