It should be clear that if you can increase reaction rates by increasing temperature you can decrease reaction rates by lowering the temperature. You do this every time you put something in the refrigerator. If you want to see the effect of elevated temperatures increased reaction rates you can leave some dairy product out of the refrigerator for a few days and compare its condition with the same age dairy product that was kept cold.

Temperature effects on rates and activation energy diagram

This illustration shows what happens to an exothermic reaction when the temperature is changed. The dotted blue curve shows the energy for a reaction mixture that is heated. The reactants are "part way" up the energy barrier because they are "hot". The dotted magenta curve shows what cooling does to the reactant energy. The energy goes down and the reaction happens with more difficulty. NOTE: The energies of reactants and products have changed. They both have different energies because they were either heated or cooled. The heat of reaction is the slightly different. The relative amounts of reactants and products are slightly different because of the temperature changes.

Concentration

Reaction rates can be increased if the concentration of reactants is raised. An increase in concentration produces more collisions. The chances of an effective collision goes up with the increase in concentration. The exact relationship between reaction rate and concentration depends on the reaction "mechanism". This is the process involving elementary reaction steps. The slowest step controls the rate. The nature of the slow step is not obvious from the balanced equation. Only experimental observation reveals the link between concentration and reaction rates.

 

Catalysts and inhibitors

The reaction energy path controls the speed of the reaction. The molecules follow the path of least resistance, but this path may still require a lot of energy. The activation energy for the path may be high and then the reaction will be slow.

A reaction pathway can be altered by adding nonreacting compounds to the reaction mixture. These molecules can sometimes alter the pathway so the energy needed for reaction is lowered. When this happens the reaction rates are faster. A material that lowers the activation energy is called a catalyst. The four pictures show the effect of a catalyst on hydrogenation of ethylene. The CH2CH2 and H2 molecules are attracted to the metal catalyst. (Graphic by permission of Prentice Hall) . The covalent bonds are weakened because the metal atoms attract electrons away form the "bonded" atoms. The bonds break. (Graphic by permission of Prentice Hall) The free radicals from the original molecules move along the surface of the metal until they collide and form products. (Graphic by permission of Prentice Hall) The ethane, CH3CH3 is not as "electron rich" as the ethylene, so it is not attracted strongly to the metal. The ethane breaks away from the catalyst. The metal catalyst is not consumed in the reaction, but the mechanical action of flowing gases wears or erodes the metal away and the catalyst must be replenished. (Graphic by permission of Prentice Hall) The catalyst is not consumed in the reaction. It merely speeds up the reaction by providing a lower energy path between reactants and products. Catalysts associate with reactant molecules (in biochemistry the reactant molecules are called substrates) and cause a redistribution of electron densities in the reacting molecules (substrates). The bonds that a need to be broken in the reaction are weakened by the association with the catalyst. This makes the reaction occur faster because the weakened bonds are easier to break. When the substrate reacts the catalyst molecule is released and able to repeat the process with another reactant substrate molecule. Enzymes are biological catalysts. Enzymes have a big molecular size. The enzyme is usually very large compared to the reacting substrate. Enzymes have folds and creases and the reactant molecules fit into a definite location in these folds and creases. The substrate molecule ties onto the enzyme molecule at a these "active sites" . The term "lock and key" is applied to this interaction. The enzyme is the "lock" and the substrate is the "key". Problems occur in biological systems when the catalyst is poisoned. This can happen when the active site is "picked" or tied up by a molecule. This kind of thing sometimes happens when heavy metals react with enzymes. One reason why heavy metals are toxic is that they bind to important metabolic enzymes. This keeps the enzyme from carrying out its normal function and the organism suffers.

Inhibitors are the exact opposite of catalysts. They increase the energy of the reaction pathway. In fact they often block the normal pathway. This is desirable when you want to preserve molecules and slow decomposition reactions.