Chapter 17 Reaction Rates 17.1 & 17.2 – Collision Theory

Collision Theory Reaction rates are actually calculated from experimental data. However, looking at chemical reactions on a molecular level provides us with a clear picture of what reaction rate measures. Have you ever seen a demolition derby in which the competing vehicles are constantly colliding? Each collision results in the demolition of one or more vehicles. In a chemical reaction, reactants must also come together in order to form products.

The Collision Theory The Collision Theory states that atoms, ions, and molecules must collide in order to react. The more collisions that occur between atoms, ions, and molecules; the faster the reaction. However, in reality, detailed calculations of the number of molecular collisions that actually occur in a reaction yield a puzzling result – only a small fraction of collisions produce reactions. For a collision of atoms, ions, and molecules to produce a reaction a collision must have: 1. Correct Orientation 2. Sufficient Energy

Orientation and Activated Complex Correct orientation requires that molecules, ions, and atoms strike each other with the correct atoms in the molecules coming into contact. Animation – Collision Theory If this occurs, an activated complex is formed which is a temporary unstable arrangement of atoms that may break apart to form products. The activated complex is also called the transition state.

Activated Complex Another critical fact that will determine if an activated complex is formed is the presence of enough activation energy (E a ). Animation Even though molecules, atoms and ions collide in the correct orientation, if there is not enough activation energy (E a ) the atoms, molecules and ions will rebound and not form an activated complex and new product.

Factors Affecting Reaction Rates Besides correct orientation and sufficient energy, there are other factors that affect the speed of a chemical reaction. These include: 1.The nature of reactants 2. Concentration 3. Surface Area 4. Temperature 5. Catalyst

Nature of Reactants The nature of reactants strongly affects how quickly a reaction will occur, because some substances react more readily than others. Some substances, depending on their position in the Periodic Table, release electrons easier than others. Elements in families 1A and 2A are much more reactive than elements in families 3A and 4A. Potassium in water

Concentration Reaction speed up when concentrations of reacting particles are increased because there are more atoms, molecules, and ions present to react. This produces more activated complexes and more product.

Surface Area The greater the surface area, the more atoms, ions, and molecules available to react and the faster the reaction.

Temperature In most cases, increasing the temperature will increase the rate of reaction because atoms, ions, and molecules move faster and collide more often. By heating a gas from 20 °C to 30°C, the frequency of collisions increases by 10%, but the rate of reaction increases 100%.

Presence of a Catalyst Catalysts are substances that change the rate of a chemical reaction without being consumed by the reaction. A catalytic converter changes harmful CO and NO to safer CO 2 and N 2 using platinum as the catalyst. 2CO + 2NO -> 2CO 2 + N 2

Inhibitors Inhibitors are catalysts that slow down a chemical reaction. To keep apples from rotting before they are sold, growers add inhibitors to slow the effect of acetylene gas which causes apples to ripen.