What makes a chemical reaction go fast or slow? Reaction Rates What makes a chemical reaction go fast or slow?

Main Ideas Kinetic Molecular Theory Temperature Concentration Surface area Catalyst Activation energy

Kinetic Molecular Theory Chemical reactions happen because particles collide. The faster the particles move, the more likely they are to collide and react. The slower the particles move, the less likely they are to collide and react.

Kinetic Molecular Theory Add this picture into your notes:

Temperature It makes sense that the more energy we put into a reaction (heat) the faster the particles move Therefore, if the particles move faster, they collide more. If they collide more, they react faster!

Temperature The higher the temperature, the faster the reaction. This is a direct relationship (as one goes up, so does the other)

Concentration Remember that the concentration means how much solute (stuff) is dissolved in a solvent (like water). If you increase the concentration, you increase the amount of stuff (think kool-aid with a lot of sugar) If you have more stuff (higher concentration) particles will have a better chance of colliding.

Concentration Imagine you are at a crowded concert. You are lucky to have front standing room, but so many people are there you can’t move without bumping into someone.

Concentration Now imagine a side gate opens up and the space doubles. There are still the same number of people but more space to move. Now there is less chance you will bump into someone.

Concentration The higher the concentration, the more likely particles will collide, and the faster the reaction. This is a direct relationship.

Surface Area Surface area refers to the amount of surface available for contact. Think about an aspirin. Aspirin dissolves in your stomach acid (a chemical reaction). Once it dissolves it can go into the blood stream and relieve pain.

Surface Area Some people will crush their aspirin before they swallow it to help it work faster. Why? The more particles of aspirin (surface area) available the more likely it is particles of stomach acid will collide with them. Let’s draw a picture….

Surface Area The greater the surface area, the faster the reaction. This is a direct relationship.

Catalyst Remember a catalyst is something that helps a reaction go faster but doesn’t participate in the reaction. Stirring is an example of a catalyst. Stirring action moves particles and makes it more likely they will collide, but the stirring rod doesn’t actually participate in the reaction.

Catalyst Addition of a catalyst causes the reaction rate to go faster. This is a direct relationship.

Activation Energy We can see that particles collide with different energies. Not all substances need the same strength of collision to cause a reaction. Some substances will react if they just barely touch other particles, while others must slam into a particle to get going.

Activation Energy The amount of energy a substance needs to collide with to get a reaction going is called its activation energy. Having a lower activation energy means the reaction will start easier. Anything that makes activation energy lower will make a reaction happen faster.

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