Catalysts.

Catalysts affect the rate of chemical reactions without being chemically changed themselves. They speed up the reactions, as it is cheaper then increasing temperature or pressure, as it is inexpensive as it is not used up.

They provide a different pathway for the reaction, which has lower activation energy. Activation energy is the miniumum amount of energy needed to start the reaction. When the activatation energy is lowered, more particles have the same or greater energy then that level, so more particles react, and the rate is faster.

Cataylsts are in two categories. Heterogenous, and Homogenous. Heterogenous is where the catalyst and reactant are in different phases/states. So, like, a solid catalyst, and liquid reactants. Or a solid catalyst and gas reactants. Homogenous are when they are in the same phase/state. Easy way to remember this, if you can't is to associate them with sexuality (a bit unconventional, but I guess it does the trick, right?) Heterosexual (diff gender - diff phase/state), and Homosexual (same gender - same phase/state) ?

Petrol engine cars have catalytic converters, which reduce the level of polluting gases made. Platinum and Rhodium are the catalysts. They are in a honeycomb shape, which gives a large surface area, on which the reactions take place. It turns nitrogen oxides and carbon monoixde into nigtrogen and carbon dioxide, and hydrocarbons and nitrogen oxides into nitrogen, cardon dioxide and water. The gases make weak bond with the metals. This holds the gases in the right position to react, and then the products are 'released'. The metals, which haven't reacted can then be re-used when these products shift out the way. :D ^_________________^

Zeolites - open structure, allow molecule to fit in it. It means, because they're in small spaces, and, as they're in small spaces, it changes their structure and reactivity. They are used in the petrochemical industry as catalysts.

In hardening fats, a Nickel catalyst is used, which allows the manufacturer to tailor the spreadability of the margerine.