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The RATE of a reaction is the SPEED at which a reaction moves. If a reaction has a low rate that means the molecules combine at a slower rate than a reaction with a high rate. Some reactions take hundreds, maybe even thousands of years while other can happen in less than one second. The rate of reaction depends on the type of molecules which are combining.

There is another big idea with rates of reaction, COLLISION THEORY. The collision theory says that the more collisions in a system, the more likely combinations of molecules will happen. So if there are a higher number of collisions in a system, more combinations of molecules will occur, the reaction will go faster, and the rate of that reaction will be higher.

Reactions happen, no matter what. Chemicals are always combining or breaking down. Over and over again, but not always at the same speed. There are a few things which really affect the speed of the reaction and the number of collisions occurring.

(1) CONCENTRATION: If there is more of a substance in a system, the chances are higher that they will collide and speed up the rate of the reaction. If there is less of something, there will be fewer collisions and the reaction will probably happen at a slower speed.

(2) TEMPERATURE: When you raise the temperature of a system, the molecules bounce around a lot more (because they have more energy). When they bounce around more, they are more likely to collide. That means they are more likely to combine too. When you lower the temperature the molecules are slower and collide less. That drops the rate of the reaction.

(3) PRESSURE: Pressure effects the rate of reaction a lot when you look at gases. When you increase the pressure, the molecules have less space to move around, that makes them collide with each other more often. When you decrease the pressure molecules don't hit each other as much and there are fewer collisions. That lower pressure lowers the rate of reaction.

So you figured out that scientists like to know the rate of the reaction. They like to measure different kinds of rates too. Each rate that can be measured tells scientists something different about the reaction. Here are a few types...

(1) Forward Rate: The rate of the forward reaction when reactants become products.
(2) Reverse Rate: The rate of the reverse reaction when products recombine to become reactants.
(3) Net Rate: The Forward Rate - Reverse Rate.
(4) Average Rate: The Speed of the entire reaction from start to finish.
(5) Instantaneous Rate: The speed of the reaction at one moment in time.

Scientists measure all of these rates by finding out the concentrations of the molecules in the mixture. If you find out the concentration at two different times you can find out which direction the reaction is moving and how fast.

And then there's more. Since many reactions happen with several steps, the rate for each step needs to be measured. There will always be one step which happens the slowest. That slowest step is the RATE LIMITING STEP (the one reaction out of all of them that really determines how fast the reaction can happen).

A catalyst is like adding a bit of magic to a reaction. A reaction has to have a certain amount of energy to happen. If it doesn't, oh well, the reaction probably can't happen. A catalyst lowers the amount of energy needed, so a reaction can happen easier. That energy is called the ACTIVATION ENERGY.

Activation EnergyCatalysts are also used a lot in the human body. Not to cause explosions, but to make very difficult reactions happen. They help very large molecules combine. Something else interesting about catalysts, if you look at the graph, you will also notice that when the activation energy is lower, the products can combine easier also. So the forward and reverse reactions are both accelerated.

There is something called an inhibitor which works exactly the opposite to catalysts. Inhibitors slow the rate of reaction, sometimes even stopping 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.

[Portrait:Meitner]What? You think all great chemists were guys? Not so. Let us introduce Lisa Meitner. She was an Austrian chemist who worked with a guy named Otto Hahn. She worked in the new field (it was at that time) of nuclear fission. She has the head of her physics department in Europe until she had to leave during World War 2.

Since she worked with fission and heavy elements she was called a radiochemist, because there is radiation in study of fission. In 1966 she was awarded the Enrico Fermi Prize for her lifetime of work with atoms and atomic interactions.

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