Collision Theory – Explains Reaction Rates In order for a reaction to occur, reactant molecules must collide in order to form products Rate depends on number of collisions - FREQUENCY and… The collisions have to be EFFECTIVE, that is, they must collide with the correct orientation and with sufficient energy.

Effective Collisions

Energy Needed Activation energy: The minimum amount of energy that the reactants must have in order to react and form products The activation energy is a barrier that reactants must get over to be converted to products The higher the barrier the larger the investment of energy in order to get the rxn to proceed

ACTIVATED COMPLEX Activated complex: arrangement of atoms at the peak of the activation-energy barrier. An activated complex is a kind of transition molecule which has similarities to reactants & products

How do we make the reaction go faster? There are four things that we can change to make the reaction go faster. They are Temperature Surface area Concentration Using a catalyst

Temperature When temperature increases, the average kinetic energy increases. Therefore: more collisions will take place – increase in frequency collisions will be more effective (more forceful collisions and more have the energy needed to reach the activation barrier) A 10 K increase doubles the reaction

Maxwell-Boltzmann Distribution A Maxwell-Boltzmann curve is a probability distribution representing the kinetic energy (speed) of particles in a gas. The activation energy (Ea) is the energy threshold required for a successful collision. Particles here cannot react (< Ea) Particles here can react (> Ea)

Midterm Mark Distribution

If the mark cut-off for a program is 86%, only students to the right of the mark barrier will qualify.

Maxwell-Boltzmann Distribution As temperature increases, the kinetic energy distributions flatten & shifts to the right.

Temperature and Rate

Surface area If we make the pieces of the reactants smaller we increase the number of particles (frequency) on the surface which can react. This makes the reaction faster. The particles on the surface can react When cut into smaller pieces the particles on the inside can react

http://www.youtube.com/watch?v=yRw4ZRqmxOc

Concentration If we make one reactant more concentrated There are more particles in the same volume to react. (increase in frequency) So the reaction goes faster. There are less red particles in the same volume so there is less chance of a collision There are more red particles in the same volume so there is more chance of a collision so the reaction goes faster

Using a catalyst A catalyst is a chemical which is added to a reaction. It makes the reaction go faster. The catalyst does not get used up in the reaction. It gives the reaction the energy to get started by lowering the activation energy. Greater fraction of effective collisions.

Types of Catalysts Heterogeneous Catalysts The catalyst is in a different phase (state) than the reactants (e.g. solid / liquid). Homogeneous Catalysts The catalyst is in the same phase (state) than the reactants (e.g. gas / gas).

Important Heterogeneous Catalysts FeO catalyst for ammonia production: N2 (g) + 3H2 (g) 2NH3 (g) Ni catalyst for hydrogenation of food oils plant oil + H2 (g) margarine Pt catalyst for pollution control: 2CO(g) + O2 (g) 2CO2 (g) 2NO2 (g) N2 (g) + 2O2 (g) How a car exhaust works FeO (s) Ni (s) Pt(s) Pt(s)

How it works

Notes: 1) The catalyst acts by altering the reaction mechanism but not the net reaction,   2) The activation energy (Ea) for the catalyzed reaction is less than the activation energy of the uncatalyzed. 3) The catalyst is regenerated, so it has not been consumed or permanently changed. 4) The reactants and products are the same for catalyzed and uncatalyzed reactions. 5) The enthalpy of reaction (ΔH) is unchanged. 6) The amount of products obtained from the uncatalyzed reaction are the same as with the catalyzed reaction, but they are obtained more rapidly.

H2O2  H2O + O2 Elephants Toothpaste – Catalyst (KI)

When you eat a meal containing protein, enzymes in your digestive tract break down the protein molecules in a few hrs.. Without enzymes, the digestion of proteins at 37C takes yrs An inhibitor is a substance that interferes with the action of a catalyst An inhibitor could work by reacting with or “poisoning” the catalyst itself

Measuring Reaction Rate Changes Visible colour changes: Can use spectrophotometer to measure colour intensity over time Ion formation: Can use pH meter, or conductivity meter to measure changes over time Gas production: Can collect and measure rate of gas production over time