catalysis

hunnymonster

looking for some help to settle an arguement. Def'n a few of us learned in school was that
"a catalysis is a substance that causes a change in the rate of a chemical reaction but is not ultimately changed by the reaction".
Someone suggested that "change" should read "speed up" as we couldn't come up with a single example where a catalyst was used to slow down the reaction.

nitroboy

Catalysts reduce the activation energy required for a reaction to proceed. this means that less energy such as heat or less pressure is required. so the reaction is not necesseraly speeded up.
As for slowing down a reaction, this is usually done by quenching a reaction which can be done by reducing the temperature in an exothermic reaction for example.
There most likely are materials that would quench or slow down a reaction (increase the activation energy required for the reaction to proceed) without undergoing a change themselves, I just can't think of any off hand.
hope that helps.

nitroboy.

Delphi91

As a science teacher, a lot of books annoy me by defining catalyst as something that speeds up a reaction. This is probably because at least at Junior cert level, the only catalysts that students come across do actually speed up the reaction.

I always define a catalyst as something that speeds up or slows down a reaction but is not used up in the reaction.

Capt'n Midnight

http://www.chem4kids.com/files/react_catalyst.html

CATALYSTS SPEED IT UP by Lowering Activation Energy

INHIBITORS SLOW IT DOWN
There is also something called an inhibitor that works exactly the opposite of catalysts. Inhibitors slow the rate of reaction. Sometimes they even stop the reaction completely. You might be asking, "Why would anyone need those?" You could use an inhibitor to make the reaction slower and more controllable. Without them, some reactions could keep going and going and going. If they did, all of the molecules would be used up. That would be bad, especially in your body.

Angel

Something that slows a reaction isn't necessarily catalysing the reaction. For example, some fireworks contain certain clays that delays the combustion of the metal powders that cause the colourful explosions. If the firework was just packed full of these powders, the reaction would be so fast they would be ineffective.

nitroboy

Don't forget that part of the definition of a catalyst is that the catalyst itself is not changed by the reaction, I'd say those clays in the fireworks undergo a chemical reaction due to heat and whatnot.

Capt'n Midnight

The clays are probably just inert bulk. IIRC glycerine is used to make nitro / dynamite more stable